CRSI-Design Guide for Voided Concrete Slabs

Addendum – Design Guide for Voided Concrete Slabs

Supplemental Information for the First Edition Printing Featuring New and Updated Voided Slab Design Considerations, and Project Case Studies.

Concrete Reinforcing Steel Institute 2019 Founded in 1924, the Concrete Reinforcing Steel Institute (CRSI) is a technical institute and an ANSI-accredited Standards Developing Organization (SDO) that stands as the authoritative resource for information related to steel reinforced concrete construction. Serving the needs of engineers, architects and construction professionals, CRSI offers many industry-trusted technical publications, standards documents, design aids, reference materials and educational opportunities. CRSI Industry members include manufacturers, fabricators, material suppliers and placers of steel reinforcing bars and related products. Our Professional members are involved in the research, design, and construction of steel reinforced concrete. CRSI also has a broad Region Manager network that supports both members and industry professionals and creates awareness among the design/construction community through outreach activities. Together, they form a complete network of industry information and support. Design Guide for Voided Concrete Slabs – Addendum

Overview This addendum contains revised content to the first edition of this Design Guide. In particular, the following items are new orhave been updated (denoted in bold text on the Contents page): 1. New information and design aids on vibration analysis in Section 3.6.3. 2. Updated information on fire resistance in Section 3.7. 3. The example in Section 3.9 has been updated to the provisions of ACI 318-14, and now includes headed shear stud design. 4. Updated information on concrete placement in Section 4.1. 5. Updated specifications in Section 5.2. 6. New feature projects in Chapter 6.

Concrete Reinforcing Steel Institute i Design Guide for Voided Concrete Slabs – Addendum

Contents

Overview i Chapter 5 Design Tools 5-1 Chapter 3 Design Concepts and Requirements 3-1 5.1 RC Concept Voided Slab Module 5-1 5.2 Construction Documents 5-3 3.1 Meeting the Building Codes 3-1 5.2.1 Specifications 5-3 3.2 Steps of the Structural Design Procedure 3-1 5.2.2 General Notes 5-8 3.3 Parameters Used in Structural Analysis and Design 3-2 5.2.3 Construction Drawings 5-8 3.3.1 Stiffness Modification 3-2 5.2.4 Field Placement Guidelines 5-8 3.3.2 Flexural Strength 3-2 5.2.5 Placement/Shop Drawings 5-9 3.3.3 Shear Capacity 3-2 3.3.4 Punching Shear 3-3 Chapter 6 3.4 The Impact of Self-weight Reduction 3-3 Featured Projects 6-1 3.5 Examples of Slab Weight Reduction Influenced 3-4 by the Void Former Parameters 6.1 Jorge M. Pérez Art Museum of Miami – 6-1 3.6 Structural Engineering Design Considerations 3-5 Dade County 3.6.1 Diaphragm Performance 3-5 6.2 Teaching and Learning Building, 6-1 Harvey Mudd College, Claremont, California 3.6.2 Horizontal Construction Joints 3-5 3.6.3 Serviceability Checks 3-6 6.3 Neuroscience Engineering Collaboration 6-2 3.6.4 Analytical Models 3-7 Building, Wright State University, Dayton, Ohio 3.6.5 Reinforcement Requirements 3-7 6.4 Roy and Diana Vagelos Education Center, 6-4 3.7 Fire Resistance 3-7 Columbia University Medical Center, 3.8 Sound Insulation 3-9 New York, New York 3.9 Design Example 3-10 6.5 Visual Arts Building, University of Iowa, 6-6 Iowa City, Iowa Chapter 4 Construction Considerations 4-1 6.6 Reach Expansion, John F. Kennedy Center 6-8 for the Performing Arts, Washington, DC 4.1 Concrete Placement 4-1 6.7 Schneck Professional Building, 6-9 4.2 Horizontal Construction Joints 4-1 Seymour, Indiana 4.3 Inter-Panel Joints for the Semi-Precast 4-1 Components 4.4 Reinforcing Bar Placement 4-1 4.5 Installation of Embedded Items 4-2 4.6 Drilling into the Plastic Void Formers 4-2 4.7 Attachment to Slabs with Post-Installed Anchors 4-2 4.8 Transportation and Handling of Void Formers and 4-3 Precast Panels 4.9 Time to Completion 4-3

Concrete Reinforcing Steel Institute ii Design Guide for Voided Concrete Slabs – Addendum

CHAPTER 3 Design Concepts and Requirements

3.1 Meeting the Building Codes sumptions should be made to determine the input similar to Reinforced concrete slabs constructed with the modern the design of a solid flat slab floor system. This should include: voided slab systems meet many of the prescriptive require- • Geometry of slab (thickness, horizontal contours with ments and intents of the International Building Code (IBC) supports and openings).2 model building code (IBC 2012). Concrete slabs containing • Load conditions. sacrificial void formers designed for the strength and service- • Material parameters for reinforcing and concrete (unit ability provisions of ACI 318 will meet or exceed building code weights, modulus of elasticity, strength, concrete cover requirements. The leave-in void formers in use today behave over the reinforcing, long-term deflection modifiers). much the same way as hollow-clay tiles or other insert materials (e.g., polystyrene) that have been successfully used for • Design criteria (deflections, stress or force limits). a century in the construction of voided concrete floor slabs. B. Establish adjustment factors specific to the voided slab Furthermore, air-filled cavities are present in other concrete system. systems, including but not limited to precast hollow core The following design parameters are typically adjusted as planks and the box-shaped cross sections of bridge girders. As straightforward scalar multipliers that compare the voided with any unique system, design, detailing, and construction of system to the characteristics of solid slabs: voided slab systems have system-specific requirements. • Stiffness correction factor (for practical reasons, in The assessment of code compliance is the purview of the lieu of altering the moment of inertia, E, the modulus regulatory agency official having jurisdiction over the project. of elasticity, is modified). Should the building official require, there is an “alternate • Dead load reduction. means of compliance” provision in Section 104.11 of the IBC1. Manufacturers of void formers are typically able to furnish • Shear capacity. testing data to substantiate key parameters used in the struc- C. Creation of negative dead load pattern. tural design calculations, such as reduction in shear capacity The weight of voided slab systems is reduced compared to and stiffness. Most of the testing data available to date is a solid slab in those areas where the void formers will be from reputable European testing laboratories. Some of these present. An initial estimate of average dead load reduction results are quoted here for reference. Applicability of any such (typically on the order of 25-30%) is adopted and assumed data is to be determined by the responsible design profes- to act uniformly (“smeared”) throughout the entire floor sional of the specific project. plate. To facilitate the computations, it is practical to repre- It is noted that various countries (e.g., Germany, United sent this reduced self-weight as an additional load pattern Kingdom, Netherlands, Denmark) with longer track records applied over the entire area as negative surface load (acting of voided slab applications have formally recognized these upward). Where the designer can define more precisely the systems. In certain cases it has been incorporated explicitly in floor regions where voiding is planned, the reduced self- their codes, standards, or related documents. weight can be represented in the same manner in those areas only. In this case, the reduced slab weight can be 3.2 Steps of the Structural Design Procedure more closely approximated. The design of voided slabs can be accomplished using gener- D. Perform initial analysis. al-purpose analysis programs incorporating the Finite Element Analysis should be conducted considering various code Method (FEM). While not readily available in the U.S. at the requirements for load patterns and combinations incorporat- writing of this Guide, some structural engineering software ing the above listed negative dead load. Most often, the slab producers are now incorporating design modules specific to design is governed by deflection criteria, including long- voided slab components tied to void former catalog informa- term deformations, which the software tools may or may tion. The following steps describe the iterative design process not directly account for. The design slab thickness might be applicable for all codes and software. repeatedly revised to meet the deflection limits, or strength A. Defining the computational model and parameters. that can be provided with reasonable reinforcement. Simi- larly to conventional slabs, analytical methods should be Upon selection of the computational tool, initial design as- used to predict the extent of cracking of the slabs due to shrinkage and temperature volume changes. 1 IBC allows conﬁrming code compliance via evaluation reports. At present, none of the voided slab manufacturers in the U.S. have such reports from evaluation agen- cies (e.g., ICC-ES, IAPMO-ES, etc.) or other entities accredited by the American National Standards Institute (ANSI) or the International Accreditation Service 2 Because the average dead load reduction is about 25 to 30 percent, an initial (IAS) under ISO/IEC Guide 65, General Requirements for Bodies Operating estimate of the overall slab thickness can be taken as Cn/36 where Cn is the clear Product Certiﬁcation Systems. span length.

Concrete Reinforcing Steel Institute 3-1 Design Guide for Voided Concrete Slabs – Addendum

E. Shear analysis to establish solid zones. tions, the flexural stiffness modifiers, expressed as the ratio Upon establishing the slab thickness that satisfies the de- of effective inertia to gross inertia of the uncracked solid slab, flection and flexural criteria, the entire floor area is examined were reported in the neighborhood of 90-92%. for shear induced by gravity forces. This step is to identify contours within which the reduced shear capacity of the 3.3.2 Flexural Strength voided system would not meet the shear demand. These Typical voided slab systems can be designed for flexure using shear-critical zones should be designed without void form- the same principles as customary for conventional solid slabs. ers. In addition, there might be some other considerations The usual geometry of the cross section and loading result in that may preclude some areas as host to the voids, such as flexural strains and stresses that utilize only a thin top segment in-plane force transfer issues for diaphragms, etc. As with of compressed concrete. The neutral axis is commonly located conventional slabs, the option to augment the shear capac- above the void formers, making the behavior the same as a ity of the solid slab zones of the voided slab, for example, at solid slab. Should the neutral axis fall below the contour of the supports with special shear reinforcement or drop panels is void formers, it is likely to represent only a negligible calculation available to the designers. inaccuracy of the position of the resisting internal compres- sions force considering the rounded shape of the interspaced F. Refinement iterations. cavities. Also note, that in multispan configurations, maximum The previous steps provide sufficient accuracy for prelimi- moments typically occur in the vicinity of the supports where nary or schematic design. Final design, often accomplished void formers are omitted for punching shear considerations. with a FEM analysis program, necessitates a more accurate representation, as it influences the design of the slab as These common features often allow engineers to omit the well as that of the supporting columns and foundations. At presence of the void formers placed at about mid-height this stage of the design process, the dead load pattern is of the slab in the flexural calculations. Thus, for customary adjusted to depict the actual layout of the void formers. This design scenarios, the moment resistance can be determined is accomplished by determining the void type, size, and the based on usual methods with the rare exceptions associated corresponding concrete volume reduction at specific areas. with unusual forces or column layouts. Where unique condi- With these refinements, the designer can determine the tions of high demand (e.g., presence of in-plane loads) occur dead load reduction in areas designed with voids. within a small area of the floor plate, design is simplified by omitting the void formers in those zones. Linear elastic G. Flexural design. finite element models may yield to singularities under certain Upon satisfying the shear and deflection criteria, flexural conditions, such as concave corners and at point loads. These considerations should be addressed. Based on the moment conditions should be judiciously analyzed by the designer. distribution obtained from the analysis, steel reinforcement is designed to resist flexure and satisfy other strength require- 3.3.3 Shear Capacity ments. The flow of stress trajectories in voided slabs is significantly impacted by the presence of voids. The combined effect of the 3.3 Parameters Used in Structural loss of materials compared to solid slabs and the disturbances Analysis and Design in the flow of stresses markedly reduces the shear capacity of the voided slabs. However, the smooth rounded shape of the 3.3.1 Stiffness Modification cavities allows for redistribution of stresses and the avoidance The flexural and shear rigidities of the voided slab system are of singularities. This ultimately results in a reduced shear capac- only slightly reduced compared to those of the solid flat slabs ity that is usually taken on the order of 50-65% of the solid as the result of void formers placed at mid-height. The pres- slab. This is a conservative estimate based on numerous labora- ence of localized and repeated cavities in the solid continuum is tory tests that have shown shear capacities well above 65% of typically accounted for by using an effective moment of inertia that of the solid slab with the same thickness. reflecting the specific three-dimensional arrangement of voids. These include the thickness of the slab, the size, shape and Some voided slab contractors suggest an approach, accepted spacing of the void formers, and their vertical position within by the German Institute of Building Technology, to substanti- the slab. Manufacturer guidelines may provide related data ate the assumed shear capacities. This concept is based on based on testing and/or computational models. However, as the assumption that shear strength is developed as the sum the geometrical variation of spacing and vertical positioning is of capacities attributed to: in the hands of the designer, considering the variety of shapes • Resistance of the uncracked compression zone. and distributions, it is not possible to provide a generalized • Aggregate interlock along the cracked surface in rein- formula for changes in stiffness. Furthermore, even within the forced sections. same single floor plate there could be a great variation of any of • Resistance provided by the tension reinforcement. the above parameters depending on the floor layout, supports, load conditions, etc. For many common voided slab applica- The presence of void formers at mid-depth of the cross sec-

Concrete Reinforcing Steel Institute 3-2 Design Guide for Voided Concrete Slabs – Addendum

tions largely reduces the aggregate interlock contribution at concentrated loads in most scenarios. Again, if the magnitude many sections. Various theoretical and laboratory studies also and application surface of the concentrated load warrants, void conclude that the presence of light reinforcing positioning cag- formers may be omitted in the vicinity of such loading conditions. es augment the shear capacity of the system. While current If needed, shear reinforcement may augment the punching shear typical slab design practices may not take direct consideration capacity of those solid slab segments. of this beneficial effect, ACI 318-11 Chapter 11 does allow welded wire cages to be considered as shear reinforcement. 3.4 The Impact of Self-weight Reduction The relatively heavier weight of the floor concrete systems 3.3.4 Punching Shear has been perceived as a major impediment for their use in One of the most important and common considerations in many applications. Experiences from actual projects with con- buildings constructed with flat slabs is the possibility of punch- crete voided slabs show that self-weight reduction of the floor ing shear around the columns where higher shear stress levels on the order of 25% to 35% can be achieved with the use of exist. Unless the slab shear forces are low, the void formers modern void formers. This reduces the weight of the concrete are typically omitted in the vicinity of columns to design a solid floor system in line with structural steel designs. The various slab zone to resist shear demand. If needed, headed shear stud benefits of lighter floor systems, as shown diagrammatically reinforcement can be used to supplement the punching shear in Fig. 3.1, are discussed below, focusing on the qualitative capacity as commonly done with conventional flat slabs. ramifications for structural analysis and design. Another area of potential punching shear concerns is at the loca- Slab thickness design is commonly governed by deflection cri- tion of concentrated point loads. Typical voided slabs have a high teria, with reinforcement design typically governed by strength resistance in this regard. Even though there are typically only considerations. In addition to the direct relationship dead load a few inches of solid concrete present at the top of the slabs, has on the instantaneous deflections, reduced self-weight plays the rounded top of the void formers allow for an arching effect an important role in reducing long-term deflections since self- that leads to a dispersion (redistribution) of the concentrated weight is a dominant part of the long-term loading. force. Test results have verified that punching shear capacities Where superimposed loads are light, the relatively lighter dead in excess of 30-40 kips are achieved in many common building load of the voided slab system may result in a thinner slab design slab configurations, eliminating many concerns associated with

Fig. 3.1 Building Framing Constructed with Conventionally Reinforced Flat Slabs Versus Using Voided Slab System.

Concrete Reinforcing Steel Institute 3-3 Design Guide for Voided Concrete Slabs – Addendum compared to a conventional solid concrete slab solution. With the 3.5 Examples of Slab Weight Reduction voided slab system, as concrete mass is eliminated in the mid- Influenced by the Void Former height, mid-span zone of the slab where materials are used with Parameters lesser efficacy, stiffness is typically reduced by about 10% where The geometry of void formers varies depending on the a 25-35% reduction in self-weight has been designed. manufacturers. Void formers are typically hollow plastic shapes The lighter slab not only allows the construction of longer created as either a single unit or as matched shapes that can spans and/or the ability to carry heavier live loads of the floor be combined to form the designed void. The most common system, but also contributes less dead load to the supporting shapes are spheres or flattened spheres, but a variety of systems, such as columns, walls, and foundations. In particu- shapes can be achieved. Manufacturers and suppliers of the lar, the number and size of foundation elements may have systems provide tabulated geometrical details in their catalogs. significant implications on construction time and cost associ- A few examples are summarized here in the interest of devel- ated with the excavation and material use. oping a sense of how the geometry affects weight reduction. The actual amount of dead load reduction is determined by the These examples are for demonstration purposes; actual varia- void former layout. In addition to the variable size and shape tions are at the discretion of the designer. of the individual voids by different manufacturers, the spac- For shorter spans and light load with a 10-in thick slab, then ing of these cavities, and the locations where voids are not 1 3 shallow, 5 /2-in high by 12 /8-in wide elliptical void formers present are the main factors influencing the magnitude of the 1 can be used, leaving 2 /4-in thick continuous concrete layers self-weight reduction. Typically, void formers are omitted when 3 below and above the void formers. A 1 /8-in spacing between shear considerations (near columns or collector lines), local 3 the plastic formers results in a 13 /4-in centerline spacing be- presence of embedded conduits, openings, or similar unique tween the void formers, allowing 0.76 plastic ellipsoids to be conditions exist. Thus, voids are present at mid-bays of slab placed per each square-foot of floor area. Depending on the systems in both directions. From the perspective of flexural de- shape of the elliptical former, about 0.25 ft3 of concrete is re- mand, this is an ideal situation. Mid-bay gravity loads contribute placed in each square foot of slab resulting in a 37 psf reduc- more to bending moments in slabs compared to gravity loads tion of the self-weight for an assumed normal-weight concrete near the supports. slab. This means that, in contrast to a dead load of 125 psf for The impact on the seismic mass of the building is also a criti- the solid slab, the voided slab will weigh 88 psf, correspond- cal consideration. As the floor self-weight is the predominant ing to roughly a 30% reduction. Also note, that a group of 7 contributor of the mass at each level, it is estimated that a elliptical balls tied together with roughly a 7-ft long and 6-in tall reduction of 15-25% can be achieved in the total floor mass, positioning cage module, commonly used in practice for an all- when all quasi-permanent building components (including cast-in-place process, will cover a floor area of 9.25 sf. vertical building systems) are considered at a given level. This For a longer span with a 21-in thick slab, consider 16-in diameter decrease is reflected almost proportionally in the inertia forces 1 spherical balls, allowing 2 /2-in of a continuous concrete layer generated at each level, due to the Newtonian relationship: 3 below and above the plastic void formers. A 1 /4-in spacing force = mass times acceleration. 3 between the formers results in a 17 /4-in centerline spacing In addition to reducing the demands on the vertical compo- between the balls allowing 0.46 void formers to be placed in nents of the lateral force resisting system, the floor inertia each square-foot of floor area. This translates to about 0.56 forces impact numerous other considerations of the earth- ft3 of concrete is being replaced per each square foot of slab, quake design. From the standpoint of in-plane flexural effects resulting in an 84 psf reduction of the self-weight for an as- on the diaphragms, the lesser floor inertia forces result in a sumed normal-weight concrete slab. In contrast to the 263 psf similarly reduced demand for chord reinforcement. The drop in solid slab, the voided slab will weight 185 psf, corresponding the seismic mass also results in a minor offset in the natural to about a 30% reduction. Similarly to the previous example, a period of the building. However, shortening of significant group of 5 round balls tied together with an about 8-ft long and 1 vibration periods is not likely to exceed 10%. Such shifts rarely 16 /4-in tall positioning cage module, placed on the bottom layer pose dramatic changes in the building responses. of orthogonal slab reinforcing, will cover a floor area of 10.87 sf. During design, the implications of self-weight reduction have The reader is referred to the manufacturer literature for the currently to be considered both from the perspective of the individual available void formers, noting that custom sizes may be produced floor bays and the entire floor plate. It requires engineering if the cost associated with creating molds is economical. Also, the judgment whether the actual layout of the voids, considering various geometrical parameters, such as spacing between void local solid areas, should be considered in the analytical model. formers and thickness of the continuous concrete slab above and Uniform loading, with averaged-out (“smeared”) self-weight, below are at the discretion of the designers. Nevertheless, the mini- is often assumed for the overall seismic analyses of buildings. mum concrete cover requirements of the code are intended to pro- In contrast, separate in-bay segments are usually modeled vide corrosion- and fire-protection. Also, adequate placement space more precisely to account for local effects in the gravity load of concrete has to be assured in between void formers, reinforcing analyses and design of floor components. steel, positioning cages, and any miscellaneous embedded items.

Concrete Reinforcing Steel Institute 3-4 Design Guide for Voided Concrete Slabs – Addendum

1 3.6 Structural Engineering Design Sections 21.11.4 and 21.11.10, which refers to /4 in. roughness Considerations via 11.6.9). In contrast, where the precast surface preparation requirements do not meet the above criteria, Section 21.11.6 3.6.1 Diaphragm Performance permits that the minimum thickness can be increased to 2.5 Void formers are designed to be located at about mid-height inches and the topping can be used as the diaphragm. The of the slab, creating uninterrupted top and bottom concrete code is silent about voids present in the slab, thus the conser- layers. This facilitates creation of a slab system with adequate vative interpretation is to only consider the “net thickness” of in-plane strength and stiffness to act as a diaphragm. Thus, the uninterrupted top and bottom layers of the concrete slab voided slabs can be used effectively to transfer forces in- system. As substantial concrete segments are present be- plane, as is customary in buildings subjected to lateral loads tween the void formers, the webs essentially connect these due to earthquakes, wind, earth pressures, sloped gravity two horizontal layers, making the argument that the thickness load resisting elements, shrinkage and temperature effects, of the two uninterrupted layers and the webs can be com- or other lateral force producing load, be it externally applied or bined to form a diaphragm. from time-dependent actions. The minimum reinforcement requirements for floor and roof In the absence of comprehensive test programs on diaphragm diaphragms are those set forth for conventionally reinforced behavior, the voided-slab diaphragm can be treated as an slabs in Section 7.12. The intent of the provision is to pro- equivalent thickness of solid slab. The usual assumption is to vide minimum temperature and shrinkage reinforcement. calculate the diaphragm thickness as the sum of the top and ACI 21.11.7.1 supplements this with a maximum spacing for bottom continuous concrete layers. While these layers are non-post-tensioned slabs. The maximum spacing for con- only a few inches thick, the intermittent presence of substan- ventional reinforcement is set at 18 inches. While the other tive concrete segments between the top and bottom layers reinforcement-related requirements of ACI 21.11.7 are not does provide stability, very significant added stiffness, and unique, the collector-related provisions in Section 21.11.7.5 some increase in the in-plane strength that generally is not need clarification, due to the wording of the ACI 318 Com- considered. For conditions where local stress risers result mentary where the designer is instructed to use gross cross in very high demands, such as collectors, distributors, or sectional parameters to gauge the stress levels in the flexing locations where direct force transfer takes place (e.g., slab to diaphragm. The word “gross” is intended here to refer to the shear wall or moment frame connections), the void formers 100% uncracked section as opposed to a fraction of that rep- can be omitted to provide the capacity of the full solid rein- resenting a cracked cross section to simulate a linearly elastic forced concrete slab. model. Thus, for voided slabs the presence of the cavities has to be incorporated in the assessment. As these calculations The in-plane stiffness modeling of voided slab systems pres- establish the extent of confinement needed for the collectors, ents a challenge due to the scarcity of testing data. With the designers may be prompted to investigate the in-plane stiff- latest development of building analysis tools, where semi-rigid ness of members with voids present in the diaphragm. Unless floor diaphragm elements can be incorporated, the analyst is straightforward conservative calculations can readily show currently left with either making an estimate for voided slab low flexural stresses, considering with a lesser cross section, or facing the additional hurdle of creating a detailed Finite Ele- the analyst might need to create a refined model (e.g. FEM) ment Model to depict the complex 3D geometry of the voided of the voided slab to establish the extent of the confinement slab bays. Fortunately, most concrete structures can be ad- reinforcement along the collector. equately modeled with rigid floor elements and voided slabs are appropriately approximated by rigid diaphragm action. In computing the shear capacity of the diaphragm, governed by Equation 21-10 of ACI 318, the gross area of the cross sec- ACI Section 21.113 specifies structural diaphragm require- tion needs to be considered. In that formula, again, the gross ments for projects in areas of high seismic risk, that is with cross section A should reflect the deduction for the voids. Seismic Design Category (SDC) D, E, or F. For SDC A, B, and cv Care should be exercised in using the respective values for C, the remaining code provisions are applicable. the steel ratio (l) as the section areas change at each cross ACI Section 21.11.6 establishes the minimum thickness section. Also, note that Section 21.11.9.2 limits the maximum required for concrete diaphragms, distinguishing the two shear capacity and it is also tied to Acv. This may necessitate scenarios where the cast-in-place topping is composite with the removal of the void formers in highly stressed areas, or the precast component. A minimum thickness of 2 inches is the decrease in the void size, or increase of the uninterrupted required where sufficient bond is achieved to make the top- top or bottom concrete thickness between plastic void form- ping slab composite. To ensure composite action, the con- ers and surfaces. crete topping should be placed on a “clean, free of laitance, and intentionally roughened” precast surface (as described in 3.6.2 Horizontal Construction Joints The typical voided slab construction method involves two separate concrete placements unless the void formers are 3 All references in this Guide to "Building Code Requirements for Sructural Concrete (ACI 318-11)" are given as "ACI" followed by the appropriate Section number. directly anchored to the slab formwork. Separate concrete

Concrete Reinforcing Steel Institute 3-5 Design Guide for Voided Concrete Slabs – Addendum

placements are done with the intent that the first, thin bottom rience with solid slabs, these thickness to span ratios are relied layer of concrete is to place enough concrete to counteract on by many practitioners. Uncracked voided slabs typically have the buoyancy forces that is created when the void formers are a slightly less (about 87-90%) flexural stiffness than solid slabs embedded in concrete. After the first concrete lift is placed, with the same thickness. Given the close to linear relationship the second lift can be poured prior to a cold joint forming at between the slab deflection and the flexural stiffness, these the Interface. The second lift can be placed after the first layer code tables with an adjustment for the reduced voided slab stiff- has set, and a horizontal concrete cold joint is formed at the ness may serve as a good predictor of deflection performance. bottom of the voids. The constructability and design ramifica- Thus, such processes may be considered for preliminary as- tions of construction joints are codified in ACI 6.4. sessment, but should not be used for final design. Potential negative effects of this horizontal cold joint/plane are Based on compressive test data from manufacturers, it is recom- mitigated by several factors. Foremost, the horizontal contact mended that detailed deflection checks should be performed, as surface is very large, with the majority of slab self-weight (the suggested by ACI 9.5.3.4. The use of the effective inertia value second thicker placement) located on the top. Thus, there is formulation (ACI 318-11, Equation 9-8) recognized by reference in a substantial downward load to produce frictional resistance 9.5.3.4 has to be substantiated, as that formula was developed without special surface preparation. In addition, there are nu- and validated using tests on mostly solid concrete sections. merous segments of wire crossing the plane of the cold joint. The presence of the relatively large voids does influence crack These are vertical or slanted wire components of the position- formation and crack patterns and may impact stiffness values ing cages, which act as bonded dowels or ties, and work as when determining the initial deflections. The cracking moment shear friction reinforcement across the cold joint. at the location of void formers is expected to be smaller and in proportion of the reduction of the cross sectional moment of For composite members such as voided slabs made in two inertia, typically on the order of 80% of the full solid cross sec- concrete lifts, ACI 17.5 provides horizontal shear strength tion. As indicated in ACI 9.5.3.4, “Deflections shall be computed requirements. This provision simply checks an average shear taking into account size and shape of the panel, conditions of stress level, formulated based on the vertical shear demand. support, and nature of restraints at the panel edges.” Deflection Checks performed on voided slab designs using Equation 17-1 analysis is often performed with two-dimensional finite element of ACI 318 typically work without accounting for any steel ties, programs, some with the ability to do a more detailed analysis that is, wires crossing the interface and sufficiently developed of reinforced concrete behavior. One approach is to use flat slab into both two concrete masses, that act to increase in-plane modeling tools to analyze voided slabs with elements that are shear resistance. Delamination problems, associated with the solid and full depth, but account for the stiffness reduction by presence of cold joints, are unlikely due to large slab interface, adjusting the modulus of elasticity. The reduction in cracking and have not been reported for the many voided slab projects moment is captured by reducing the cracking strength of the that have been constructed around the globe. concrete. Nevertheless, ACI 6.4.3 mandates that the construction joints The short term or initial deflections continue to increase under shall be made and located not to impair the strength of the sustained load, due to shrinkage and creep. These time-de- structure. The specific provisions for shear transfer area are pendent factors are influenced mainly by the long-term effect found in Section 11.6.9. These provisions allow shear-friction of humidity, temperature, age at the time of loading, curing, theory to be used for two phased concrete placements in quantity of compression reinforcement, and magnitude of voided slabs. This is similar to the check that is performed the sustained load. Due to the complexity of this phenom- on the interface of composite precast construction where enon, ACI 9.5.2.5 allows a simplified determination of these, single or double tees are topped with cast-in-place concrete. unless a more comprehensive analysis is performed. The Section 11.6.9 identifies1 / -in surface roughness amplitude 4 additional long-term deflection for both normal and lightweight as the threshold for the use of higher coefficient of friction, concrete flexural members can be obtained by multiplying 1.0 versus 0.6, by referencing Section 11.6.4.3. In formula- the immediate deflection caused by sustained loading by a tions where contact surface area of the construction joint is factor established earlier in this section, and also applicable to considered, the portion of the area taken by the void formers voided slabs. The obtained immediate and long-term deflec- must be deducted. tion values should be compared to limits set forth in ACI 318- 3.6.3 Serviceability Checks 11 Table 9.5(b) or other documents that identify restrictions in deflection due to sensitive items such as unique interior Deflections partitions, exterior enclosure, operation of machinery, or other The preliminary design of conventionally reinforced slabs usu- unique attributes of the project. Creep and shrinkage have ally starts with establishing a thickness to span ratio that is in been shown in voided slab tests by manufacturers’ tests to be compliance with Tables 9.5(a) for one-way and 9.5(c) for two- only marginally higher than a solid slab of similar dimension. way slab systems. Meeting the limits in these empirical tables Due to the approximate nature of serviceability calculations allows the designer to forego a detailed deflection analysis. As this small difference is usually ignored. the tabulated ratios were developed based on decades of expe-

Concrete Reinforcing Steel Institute 3-6 Design Guide for Voided Concrete Slabs – Addendum

100 Shock-Induced Vibrations in Buildings (1 to 80 Hz), ISO 2631-2, 1989] and have been successfully implemented in a wide vari- Unacceptable Zone: Areas above the curves ety of situations. Limits for different occupancies are provided for each structure type in terms of root-mean-square (rms) accelerations as multiples of a baseline curve. Suggested peak accelerations based on these multipliers are given in Figure 3.1.1. 10 Rhythmic Flat plate voided concrete slab systems designed in accor- Activities dance with the minimum serviceability criteria discussed pre-

Shopping viously readily satisfy vibration acceptance criteria for human Malls,Dining comfort under typical service conditions and occupancies. The and Dancing acceptance criteria for walking excitations are easily satisfied

1 by providing an overall slab thickness based on deflection Offices/ requirements only. Minimum overall slab thicknesses that Residential satisfy acceptance criteria for walking excitations as a function of span length are given in Figure 3.1.2. This figure is based on

Peak Acceleration Acceleration Peak %g normalweight concrete with a compressive strength of 4,000 psi and mild reinforcement with a yield strength of 60,000 psi.

0.1 Superimposed loads customarily specified in commercial and ISO Baseline residential occupancies were used in the analysis. As an ex- Curve ample, if 30-ft spans are required, a flat plate voided concrete Acceptable Zone: Areas below the curves slab with a minimum thickness of 10 in. must be provided to satisfy the acceptance criteria for walking (maximum span length is equal to 30.5 ft from the figure).

0.01 1 10 100 Acceptance criteria for rhythmic excitations are directly related Frequency cps to the fundamental frequency of a floor system. Minimum Fig. 3.1.1 Recommended Peak Acceleration for Human Comfort (ISO overall slab thicknesses that satisfy the acceptance criteria 1989). for various types of rhythmic excitations as a function of span length are given in Figure 3.1.3 based on the same material and load assumptions used for walking excitations. A 13.5-in.- Crack Widths thick flat plate voided concrete slab with a maximum span length of 30.8 ft must be used if 30-ft spans are required for Other serviceability considerations also include crack-width jumping exercises and aerobics (these activities have the control, related to durability (corrosion control) and aesthetic most stringent acceptance criteria of all the considered rhyth- needs, which rarely represents a concern for two-way sys- mic excitations). tems. While ACI 318 is silent on this issue with respect to two-way slabs, Section 10.6.4 mandates spacing limits for flexural reinforcement when used for one-way load transfer, a relatively rare application of voided slabs. Vibrations Floor vibration is another type of serviceability issue that may need to be addressed. Vibration is often not a problem for reinforced concrete floor systems because of inherent mass and stiffness. However, a vibration analysis should be performed where a floor system must support rhythmic vibration sources or sensitive equipment to ensure all pertinent acceptance criteria are satisfied. Acceptance criteria for vibrations are available for human comfort and sensitive equipment. Recommended acceleration limits for human comfort (walking and rhythmic excitations) due to specific activities were developed by the International Organization of Standard- ization [Evaluation of Human Exposure to Whole-Body Vibration – Part 2: Human Exposure to Continuous and Fig. 3.1.2 Minimum Slab Thickness / Maximum Span Lengths for Walking Excitations.

Concrete Reinforcing Steel Institute 3-7 Design Guide for Voided Concrete Slabs – Addendum

designs achieved by the “yield line” method, “strip” method, or the “optimal” analyses that evolved in the 1950s to 1980s. In the recent decades, advances in computing have allowed complex models to be built for these highly indeterminate structural components with finite element methods that may also incorpo- rate non-linear characteristics. Often simplified methods, such as those presented in Chapter 13 of ACI 318, are deemed more suitable for practical design of slabs, in particular, for very regular and rectangular floor layouts. Section 13.1.3 does allow a variety of systems to use both the direct design method and the equivalent frame method, as it spells out “Solid slabs and slabs with recesses or pockets made Fig. 3.1.3 — Minimum Slab Thickness / Maximum Span Lengths for Rhythmic Excitations. by permanent or removable fillers between ribs or joists in two For sensitive equipment, the expected maximum velocity due directions are included within the to walking-induced vibrations is inversely proportional to the scope of the Chapter 13.” Nevertheless, the authors anticipate stiffness of a floor system; thus, to satisfy the acceptance that the majority of the voided slab systems will be designed criteria in such cases, floor systems must be provided that with Finite Element tools in the years to come. For preliminary are very stiff. Minimum overall slab thicknesses that are to design purposes CRSI developed a website-based estimating be used to satisfy acceptance criteria as a function of span software RC Concept that is introduced in Chapter 6. length and walking pace based on the same material and load assumptions noted above are given in Figure 3.1.4. Results 3.6.5 Reinforcement Requirements are presented for a maximum velocity, V, of 8,000, 1,000, As with all structural components, ACI 318 contains rules for and 130 μin./sec. These velocities correspond to, for example, reinforcing steel that are not directly derived from the struc- residences and computer systems; micro, eye, and neuro tural analysis. Section 13.3 describes those rules for minimum surgery; and, manufacturing highly-sensitive microelectronic reinforcing ratios, reinforcing bar spacing, edge and corner equipment, respectively. A thicker (stiffer) slab is required to conditions, and minimum extensions at support lines. satisfy the acceptance criteria for faster walking paces. Slab Section 13.3.1 sets the minimum reinforcing steel ratio to thicknesses that are not provided for certain walking paces address shrinkage and temperature volume change relate and span lengths means that a flat plate voided concrete slab effects, as provided per Section 7.12.2.1. For minimum spacing system is unable to satisfy the acceptance criteria in those of reinforcing bars at critical sections, the limitation to two cases. times thickness for solid slabs does not appear to be applicable, Additional information on vibration of flat plate voided con- rather the limit would be the smaller of 5 times the thickness crete slab systems (and other reinforced concrete floor sys- or 18 inches per ACI 13.3.2 and 7.12. As voided slabs are rarely tems), including approximate methods to determine natural thinner than 9 inches, the 18-in reinforcement spacing limit frequency and worked-out design examples, can be found in appears to be the code-mandated practical limit. the CRSI publication Design Guide for Vibrations of Reinforced ACI 13.3.4 through 13.3.6 address extensions of positive and Concrete Floor Systems. negative reinforcing steel at discontinuous edges. Section 13.3.6 indicates added reinforcing should be used to resist 3.6.4 Analytical Models moments and cracking at unrestrained slab corners that tend Two-way slab design has been approached with a variety to lift up. Section 13.3.7 relates to assumptions allowed in re- of analytical approaches in the last century. Depending on inforcing calculations at drop panels. Section 13.3.8 describes the application, solutions ranged from linearly elastic mod- the rules for the reinforcement curtailment and splicing. els presented in the early part of the 20th century to plastic

Concrete Reinforcing Steel Institute 3-8 Design Guide for Voided Concrete Slabs

Fig. 3.1.4 Minimum Slab Thickness / Maximum Span Lengths for Sensitive Equipment.

Concrete Reinforcing Steel Institute 3-9 Design Guide for Voided Concrete Slabs – Addendum

3.7 Fire Resistance Fire-resistance rating (or, fire rating), is the period of time (usually expressed in hours) a building element, component, or assembly maintains the ability to contain a fire, continues to perform a given structural function, or both. Fire ratings are determined by tests or by the methods prescribed in Section 703.3 of the 2018 IBC. Required fire-resistance ratings for elements in buildings are given in Table 601 of the IBC based on the construction type (I though V). Types I and II are types of construction where the building elements are of noncombustible materials, which includes reinforced concrete. The minimum fire-resistance rating for floor elements in Type I construction is 2 hours. IBC Section 703.2 permits the test procedures in ASTM E119 [American Society for Testing and Materials, 2018a, “Stan- dard Test Method for Fire Tests of Building Construction and Materials,” ASTM E119-16a, West Conshohocken, PA] and UL 263 [Underwriters Laboratories, 2018, “Standard for Fire Tests of Building Construction and Materials,” UL 263, Northbrook, IL] for determining fire-resistance ratings of building elements, components, and assemblies. Standard fire tests are conducted by placing an assembly in a furnace and subjecting it to a fire that follows a standard time-temperature curve. Numerous fire tests have been performed on BubbleDeck® systems and Cobiax® systems in accordance with the provisions in DIN 4102-02 [Deutsches Institut für Normung, 1977, “Fire Behavior of Building Materials and Building Compo- Figure 3.2.1 Layout of void formers in test assembly. nents; Building Components; Definitions, Requirements and Tests,” DIN 4102-2, German Institute for Standardization, Void formers were found to be intact after the fire tests; the Berlin, Germany]. The time-temperature curve used to test internal temperature remained below the melting temperature specimens in the DIN requirements is essentially the same of the HDPE, which is approximately between 200 and 300º F. as that prescribed in ISO 834 [International Organization for Standardization, 1999 (Amended 2012), Fire-resistance Tests A fire test to supplement those performed previously was - Elements of Building Construction - Part 1: General Require- conducted in June of 2017 at the Fire Testing Laboratory of ments, ISO 834-1]. ISO 834 and ASTM E119 time-temperature NGC Testing Services in Buffalo, NY. The test assembly con- curves are also essentially the same, and it has been shown sisted of an 8-in.-thick concrete slab that was14 ft by 18 ft (the the differences in severity between the two tests are negli- plan dimensions were limited by the size of the furnace test gible [Harmathy, T.Z., Sultan, M.A., and MacLaurin, J.W, 1987, frame). The slab contained 4-in.-thick ellipsoidal void formers 3 “Comparison of Severity of Exposure in ASTM E119 and ISO with a diameter of 12 /8 in. that were spaced 13¾ in. on center 834 Fire Resistance Test,” Journal of Testing and Evaluation, within the cage modules. A total of 140 void formers were ASTM, 15(6), 371-375]. Therefore, it follows that the results used in the slab. Normalweight concrete with siliceous ag- obtained from specimens tested in accordance with the DIN gregate and a design compressive strength of 5,000 psi was requirements would essentially be the same as those that specified. Slab reinforcement consisted of ASTM A615 Grade would be obtained if the specimens were tested in accor- 60 #4 bars spaced 12 in. on center in both primary directions dance with ASTM E119 requirements. at the top and bottom of the slab with a ¾-in. clear cover. The cage modules were tied to these reinforcing bars, and a Fire tests in accordance with DIN 4102 have revealed that the 13-in.-wide strip of solid concrete was provided around the concrete cover to the reinforcing bars on the side of the fire perimeter of the slab. Construction of the assembly followed is the controlling parameter in the determination of the fire typical construction procedures used in the field for these resistance for flat plate voided concrete slab systems. It was types of systems. The layout of the void formers in the slab found that the voids act as thermal isolators, that is, the heat prior to casting of the concrete is shown in Figure 3.2.1. from the fire is dammed below the voids. This leads to slightly higher temperatures in the reinforcing bars that are positioned The assembly was tested in accordance with the require- below the voids. A cover of ¾ in. to the main flexural reinforc- ments of ASTM E119. The edges of the slab on all 4 sides ing bars resulted in a fire-resistance rating of at least 2 hours. were supported vertically by the test frame; no restraint was

Concrete Reinforcing Steel Institute 3-10 Design Guide for Voided Concrete Slabs – Addendum

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Figure 3.2.2 Underside of test assembly after ﬁre testing ݏ ൌ ͳ͵Ǥ͹ͷԣ provided for thermal expansion or rotation. As such, the assembly was unrestrained during the duration of the fire test, which is conservative for cast-in-place concrete slab systems.

The assembly was loaded during the entire time of the test ൌͺǤ͸ʹͷԣ ݐൌͳ͵Ǥͷԣ ௖௫ with a uniformly distributed load of 80 psf using water-filled ܦ steel tanks. The vertical deflection of the assembly was measured throughout the test with 3 plumb bobs located at the center and quarter points of the slab. ݀ ൌ ͳʹǤ͵͹ͷԣ At 2 hours and 52 minutes, the test was terminated because Τ Τሻ െ ሺ ݐ ൌ ሺ the temperature recorded at one of the thermocouples at ௘௤ the unexposed surface exceeded the ASTM E119 individual ͳ͵Ǥͷ ൌ െ ͲǤ͵͹͹ ൌ ͲǤ͹Ͷͺᇱ ൌ ͺǤͻͺԣ limiting temperature rise of 325 degrees F. Throughout the du- ͳʹ ration of the test, the assembly supported the applied loading ǦʹʹͲ with no signs of collapse. The underside of the assembly after the fire test is shown in Figure 3.2.2. Although the concrete Fig. 3.2.3 Sample Calculations of Equivalent Thickness had spalled at various locations, the exposed reinforcing bars exhibited minor to no damage due to the fire. An equivalent thickness of 5.9 in. corresponds to a 2-hour IBC Table 722.2.2.1 provides minimum slab thickness of fire-resistance rating for a normalweight concrete mix with reinforced concrete floor and roof assemblies to achieve siliceous aggregate (according to Table 722.2.2.1, a 5-in. thick- fire-resistance ratings based on aggregate type. Flat plate ness is required for a 2-hour rating and a 6.2-in. thickness is voided concrete slab systems are similar to slabs with ribbed required for a 3-hour rating). The fire rating based on equiva- or undulating soffits, so an equivalent slab thickness must be lent thickness is essentially the same as that determined from calculated for use in Table 722.2.2.1. the fire test. In general, the equivalent thickness of a flat plate voided con- Based on the results from the fire test, it can be concluded crete system is equal to the net volume of concrete divided that the flat plate voided concrete assembly constructed of by the floor area. The net volume of concrete is equal to the the materials and in the manner described previously achieved volume of concrete of a solid slab minus the average concrete a 2-hour unrestrained assembly rating when exposed to fire displaced by the void formers: in accordance with the test method prescribed in ASTM E119. Volume of solid 8 in. slab = 8/12 = 0.67 cu ft/sq ft This test confirms the appropriateness of the application of Appendix X3 of ASTM E119 and Section 703.3 of the 2018 IBC For the void formers used in the test assembly, average con- for flat plate voided concrete slabs. crete displaced = 0.18 cu ft/sq ft Fig. 3.2.3 shows examples of equivalent thickness calcula- Net volume of concrete = 0.67 – 0.18 = 0.49 cu ft/sq ft tions for spherical and ellipsoidal void formers (see Tables 3.2 Thus, and 3.3 for system properties). In both systems, the calculat- the equivalent thickness of the test assembly = 0.49 ft = 5.9 in. ed equivalent thickness well exceeds the often required 4.6-in.

Concrete Reinforcing Steel Institute 3-11 Design Guide for Voided Concrete Slabs – Addendum

solid-slab thickness associated with carbonate aggregate for a Sound insulation tests have been performed on the Bub- 2 hour fire-resistance rating. bleDeck® and Cobiax® systems in the laboratory and in completed buildings. For the thinnest concrete slabs (8 and Laboratory tests have indicated that the voids created in the 9 in.) without any floor coverings or ceilings, airborne sound concrete have a slight benefit in retarding the potential of reduction indices (similar to STC) of 55 and 56 dB were concrete spalling on the fire-exposed surfaces. It is believed obtained, respectively. These indices are calculated in ac- that the presence of the voids reduces the internal steam cordance with ISO standards. Impact sound pressure levels pressure build-up originating from the moisture present in (similar to IIC) of 76 dB were also obtained. The fundamental the concrete that would otherwise act on the concrete cover methods for the actual measurements and the mathematical zone. In a solid slab, the steam build-up would act on the calculations behind the ASTM and ISO acoustic standards are concrete cover. similar; however, they are significantly different in the details In addition to verifying the typical concrete cover and cross- of tests and how the numerical results are processed. It is sectional dimensions to increase or decrease fire rating, attention recommended to contact the manufacturers for the latest should be paid to avoid unique conditions that compromise the information on sound control for the various systems. same. Details for reveals, embeds, conduits, and openings should be checked as well as any joints with other materials or systems, 3.9 Example such as skylights, metal hatches, etc. IBC Section 714 for pen- The following example illustrates the design of a flat plate etrations, Section 715 for fire resistant joint systems, and Section voided concrete slab system for a typical interior exposure 716 for opening protection are as applicable to voided slab sys- application. As noted in Section 3.3.2 of this publication, these tems as they are to conventional flat slabs. As with all slabs, utility systems can be designed using the same methods for con- service lines and other penetrations will create a condition where ventional solid slabs. The provisions in Chapter 8 of ACI 318-14 penetrations should be fire-sealed both on the underside and the for two-way slabs are used to design the slab system in this topside of the slab to limit a potential fire breach. example and reference to appropriate section numbers of that Furthermore, the presence of a sprinkler system cannot be document are provided throughout the example. Design data ignored in protecting against the spread of fire. Conventional are given in Table 3.1. sprinkler systems keep the temperature of the exposed soffit Table 3.1 Design Data for Example above an active fire to significantly lower levels (less than Compressive strength f ' 4,000 psi about 250°F for standard response sprinklers and less than Concrete c 150 pcf about 200°F for quick-response sprinkler heads). These values Density wc are less than the 560°F ignition temperature of polypropylene 60,000 psi Reinforcing steel Yield strength fy and polyethylene void former materials. Superimposed dead load 20 psf Loads 100 psf 3.8 Sound Insulation Live load Spans Typical bay: 40 ft by 20 ft There are a variety of methods, models, and ratings used to 18 in. by 18 in. evaluate the transmission characteristic of floor elements for both Columns air-borne and structure-borne (e.g., impact footsteps or dropped objects) sound effects. Depending on the building functions, 1. Determine minimum slab thickness for serviceability different tolerable sound levels are desired. Currently, there are building code-mandated sound insulation limits instituted for ACI 8.3.1 contains minimum slab thickness requirements floors in the U.S. in Section 1207 of the IBC that are limited only for slabs without beams, which pertain to serviceability only. for interior floors separating adjacent dwelling units or separating These requirements are used as a guide in determining the dwelling units and adjacent public areas, such as halls, corridors, overall slab thickness for a flat plate voided slab system. stairs, or service areas. For air-borne sound, a sound transmission The governing slab thickness is determined in the 40-ft direc- class (STC) of not less than 50 (45 if field-tested) is required when tion based on the recommendation given in Part A of Section tested in accordance with ASTM E 90. For structure-borne sound, 3.2 of this publication: an impact insulation class (IIC) rating of not less than 50 (45 if field-tested) is required when tested according to ASTM E 492. Usually, the average mass per unit area is the parameter that drives the sound transmission characteristics, similar to solid slabs. For typical buildings, the sound attenuation is the combined 2. Determine slab thickness based on characteristics effect of the structural slab, the ceiling and flooring systems, if of available voided slab systems any. As voided slabs have a lighter mass, the overall sound insula- The actual overall thickness is determined using data from tion properties are slightly offset from those of the solid slabs. In proprietary flat plate voided slab systems and the information addition, the presence of voids within the concrete continuum determined in Part 1. In this example, a Cobiax Slim-Line (Type does alter the sound wave propagation patterns in slabs. S) system is specified, which utilizes ellipsoidal void formers.

Concrete Reinforcing Steel Institute 3-12 Design Guide for Voided Concrete Slabs – Addendum

Table 3.2 contains the product parameters for various slab A reduction in slab dead load occurs in the areas where there depths and void formers (the information in Table 3.2 is from are void formers. The dead load reduction corresponds to the Cobiax USA Inc.). A similar design can be performed using the average reduction in slab dead load based on the average BubbleDeck system (see Table 3.3). volume of voids in the slab and is equal to the weight of the solid slab minus the slab weight in the area of the slab that Using Table 3.2, a 14.0-in. slab depth with Type S-220 void contains void formers. In this example, the dead load reduc- formers is initially selected because it is slightly greater than tion is equal to 56 psf for the 14-in. slab with Type S-220 void the 12.8-in. slab thickness determined in Part 1. formers (see Table 3.2). 3. Determine the reduced and average dead loads The average dead load of the slab takes into consideration the A typical flat plate voided slab system usually consists of the solid areas of the slab that are required around the columns following areas: (1) voided slab areas, which contain the uni- and at the perimeter of the floor plate. At this point in the formly spaced void assemblies, (2) solid head areas, which oc- design, the solid slab areas around the column have yet to cur around the columns and do not contain any void formers, be determined because they are governed by two-way shear and (3) solid strip areas, which occur around the perimeter of requirements. However, an estimate can be obtained by as- the floor and do not contain any void formers. suming an average void area in the slab, which is typically in the range of 70 to 80%. For preliminary design, it is recom-

Table 3.2 Product Parameters for Cobiax Slim-Line System Type S Void Former [-] 100 120 140 160 180 200 220 240 260 Slab Depth [inch] 8.0 8.5 10.0 11.0 12.0 12.5 14.0 15.0 15.5 Load Reduction* [psf] 26 31 37 42 47 52 56 61 66 Stiffness Factor [-] 0.95 0.93 0.93 0.92 0.90 0.89 0.89 0.89 0.87 Shear Factor [-] 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 Void Former Height [inch] 3.9 4.7 5.5 6.3 7.1 7.9 8.7 9.4 10.2 3 3 3 3 3 3 3 3 3 Void Former Diameter [inch] 12 /8 12 /8 12 /8 12 /8 12 /8 12 /8 12 /8 12 /8 12 /8 3 3 3 3 3 3 3 3 3 Void Former Centerline Spacing [inch] 13 /4 13 /4 13 /4 13 /4 13 /4 13 /4 13 /4 13 /4 13 /4 Void Formers [per sq ft] 0.76 0.76 0.76 0.76 0.76 0.76 0.76 0.76 0.76 Voided Volume [cu ft/sq ft] 0.173 0.210 0.247 0.281 0.315 0.346 0.377 0.409 0.442 *based on normalweight concrete with a density of 150 pcf Courtesy of Cobiax Technologies.

Table 3.3(b) Product Parameters for BubbleDeck Cage Only and Precast Panel

Slab Depth [inch] 9.0 11.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 30.0 Load Reduction* [psf] 30 41 46 52 66 73 82 94 103 122 Stiffness Factor [-] 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 Shear Factor [-] 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 7 3 5 3 1 3 1 Ball Diameter [inch] 7 8 /8 9 /4 10 /8 12 /8 14 /8 16 17 /4 19 /2 23 3 1 1 3 5 5 3 Ball Centerline Spacing [inch] 8 /4 10 /2 11 /2 12 13 /4 16 18 19 /8 21 /8 25 /4 CAGE ONLY Nominal Cage Size [ft] 4 = 84 = 84 = 84 = 8 6 = 87 = 88 = 88 = 88 = 88 = 8 Balls per Cage [-] 55 48 32 32 35 30 25 16 16 16 PRECAST PANEL Precast Size Maximum [ft] 11 = 40 11 = 40 11 = 40 11 = 40 11 = 40 11 = 40 11 = 40 11 = 40 11 = 40 11 = 40 Nominal Balls per Panel [-] 810 540 450 440 315 240 190 145 130 110 1 1 1 1 Panel Thickness [inch] 2 /2 2 /2 2 /2 2 /2 333333 *based on normalweight concrete with a density of 150 pcf Courtesy of BubbleDeck.

Concrete Reinforcing Steel Institute 3-13 Design Guide for Voided Concrete Slabs – Addendum

mended to use a value of 70%. Thus, the average dead load Because all the limitations are satisfied, the Direct Design is equal to the weight of the solid slab based on the total as- Method can be used to determine the bending moments at sumed slab thickness minus the percentage of the dead load the critical negative and positive sections in the column strips reduction based on the assumed average void area in the slab: and middle strips along the span. The bending moments are determined by calculating the total static moment in each direction and then applying the The average dead load is combined with the other applicable appropriate coefficients given in ACI 8.10.5 and 8.10.6 for loads that are required in the design of the slab system.. the column and middle strips, respectively. Calculations are provided for an interior design strip with the 40-ft long spans 4. Determine the bending moments at the critical in the direction of analysis. sections for a typical interior bay As noted previously, the flat plate voided slab system is ana- Step 1: Determine the total factored static moment lyzed as a flat plate. Two approximate methods of analysis−the in each span. Direct Design Method and the Equivalent Frame Method−are The total factored moment Mo is determined by ACI Equation given in Chapter 8 of ACI 318-14. Check if the limitations of (8.10.3.2): the Direct Design Method in ACI 8.10 are satisfied: q 2 M = u2n • There shall be a minimum of three continuous spans in each o 8 direction. Assuming that there are 5 spans in both directions, this limitation is satisfied. Total factored uniform load (ACI Table 5.3.1): • Panels shall be rectangular, with a ratio of longer to shorter span center-to-center of supports within a panel not greater than 2. Ratio of longer to shorter center-to-center spans in Clear span this example is equal to 2, so this limitation is satisfied. Therefore, • Successive span lengths center-to-center of supports in each direction shall not differ by more than one-third the longer span. Successive span lengths in each direction are equal, so this limitation is satisfied. • Offset of columns by a maximum of 10 percent of the span (in Step 2: Distribute the total factored static moment each direction of offset) from either axis between centerlines into negative and positive bending moments in each of successive columns shall be permitted. Assuming that span. there are no column offsets in this example, this limita- The moment Mo is divided into negative and positive tion is satisfied. moments in accordance with distribution factors given in • All loads shall be due to gravity only and uniformly distributed ACI 8.10.4. A summary of the total design strip moments at over an entire panel. The unfactored live load shall not exceed the critical sections of this flat plate is given in Table 3.4. two times the unfactored dead load. In this example, all of Step 3: Distribute the total negative and positive the loads are due to gravity. Also, the ratio of the unfac- bending moments in the design strip to the column tored live load to the unfactored dead load is equal to strip and middle strip. 100.0 / (135.8 + 20.0) = 0.6 < 2.0. Thus, this limitation is satisfied. The percentages of the negative and positive bending moments at the critical sections that are to be assigned to the • For a panel with beams between supports on all sides, column strips and middle strips are given in ACI 8.10.5 and ACI Eq. (8.10.2.7b) shall be satisfied for beams in the two 8.10.6. A summary of the factored bending moments at the perpendicular directions. Because there are no beams in critical sections is given in Table 3.5. this example, this limitation is not applicable.

Table 3.4 Summary of Total Design Strip Moments (ft-kips) for a Typical Interior Design Strip

End Span Interior Span

Exterior Negative Positive Interior Negative Positive Interior Negative

0.26 334.3 0.52 668.7 0.70 900.1 0.35 450.1 0.65 835.8 Mo Mo Mo Mo Mo

Concrete Reinforcing Steel Institute 3-14 Design Guide for Voided Concrete Slabs – Addendum

Table 3.5 Summary of Factored Bending Moments (ft-kips) at the Critical Sections for a Typical Interior Design Strip

End Span Interior Span

Exterior Negative Positive Interior Negative Positive Interior Negative

Column 0.26M 334.3 0.31M 398.6 0.53M 681.5 0.21M 270.0 0.49M 630.0 Strip o o o o o Middle 0 0.21M 270.0 0.17M 218.6 0.14M 180.0 0.16M 205.7 Strip o o o o

5. Determine the required flexural reinforcement at The required flexural reinforcement at the critical sections is sum- the critical sections for a typical interior bay marized in Table 3.6. An average The required flexural reinforcement at the critical sections can is used in the calculations. It can be shown that the neutral be obtained using the strength design methods in ACI Chap- axis depth at all critical sections is less ter 22 for tension-controlled , rectangular sections than the solid slab thickness above and below the voids, with a single layer of tension reinforcement: which is equal to (14.0 – 8.625) / 2 = 2.7 in. in this example. Thus, the voided slab behaves like any other rectangular reinforced concrete section where the width is equal to the width of the column strip or middle strip. In uncommon cases where is greater than the solid slab thickness, the section behaves like a T-section and the required area of steel must be determined appropriately. All the critical sections are tension-controlled because the The required area of flexural reinforcementA s is determined at the critical sections in the column strip and middle strip using required area of flexural reinforcement is less than the area of reinforcement corresponding to a tension-controlled section the appropriate factored bending moments Mu in Table 3.5. (this equation is applicable to 4,000-psi con- Because the width of the column crete and Grade 60 reinforcing bars). strip in this interior design strip is equal to 20.0 / 2 = 10.0 ft

(see ACI 8.4.1.5). Therefore, the width of the middle strip is Sample calculations are provided for the required area of steel equal to 20.0 – 10.0 = 10.0 ft. in the column strip at the first interior support in an end span.

Table 3.6 Summary of Required Slab Reinforcement at the Critical Sections for a Typical Interior Design Strip

M b R A Location u n s Reinforcement* (ft-kips) (in.) (in.) (in.2) Exterior Negative 334.3 120 229 60.4 14-#6 Column Strip Positive 398.6 120 272 7.25 17-#6 Interior Negative 681.5 120 466 12.83 30-#6 End Span Exterior Negative 0.0 120 0 3.02 7-#6 Middle Strip Positive 270.0 120 185 4.84 11-#6 Interior Negative 218.6 120 149 3.90 9-#6 Positive 270.0 120 185 4.84 11-#6 Column Strip Negative 630.0 120 431 11.78 27-#6 Interior Span Positive 180.0 120 123 3.20 8-#6 Middle Strip Negative 205.7 120 141 3.66 9-#6

* 2 As,min = 0.0018bh = 0.0018 x 120.0 x 14.0 = 3.02 in. Max. spacing = lesser of (2h, 18.0 in.) = 18.0 in. Min. number of bars = 120.0 / 18.0 = 6.7, say 7 bars 2 As,t = 0.018bd = 0.018 x 120.0 x 12.75 = 27.5 in.

Concrete Reinforcing Steel Institute 3-15 Design Guide for Voided Concrete Slabs – Addendum

Step 1: Assume tension-controlled section. Step 6: Choose size and spacing of reinforcing bars. 2 Sections of flexural members, including two-way slabs, should The required area of reinforcement is 12.83 in. be tension-controlled whenever possible. Thus, assume that Use 30-#6 bars the strength reduction factor q0.9 Similar calculations can be performed for the other critical sec- Step 2: Determine the nominal strength coefficient tions in the column strip and middle strip. R of resistance n. It is evident from Table 3.6 that reinforcement based on maxi- For a rectangular section, Rn is a function of the factored mum bar spacing is not required at any of the critical sections. bending moment M , which is equal to 681.5 ft-kips (see Table u Check that the flexural reinforcement at the edge column is 3.6) adequate to satisfy the moment transfer requirements of ACI 8.4.2.3.

The factored slab moment transferred to an edge column at this slab-column connection is equal to 334.3 ft-kips, Step 3: Determine the required area of reinforce- which is the total moment in the column strip (see Table 3.6). ment. A fraction of this moment must be transferred over an effective width equal to where is determined by ACI Equation (8.4.2.3.2):

where

Step 4: Determine the minimum required area of reinforcement. For two-way slabs with Grade 60 reinforcement, is determined in accordance with ACI 8.6.1.1: For edge columns bending perpendicular to the edge, the value of calculated by ACI Equation (8.4.2.3.2) may be increased to 1.0 provided that the condition in ACI Table 8.4.2.3.4 is satisfied. No adjustment to is made in this example. This minimum reinforcement is applicable to both the column and middle strips in this direction of analysis. Therefore,

Step 5: Determine the area of reinforcement corresponding to tension-controlled sections. The required area of reinforcement to resist this moment in The area of reinforcement corresponding to tension-controlled the 60.0-in. wide strip is equal to 3.84 in.2, which is equivalent sections is equal to the following: to 9-#6 bars.

Provide the 9-#6 bars by concentrating 9 of the 14 column strip bars within the 60.0-in. width over the column. For sym- where , which is the strain in the tension rein- metry, add 1-#6 bar and check bar spacing: forcement corresponding to compression-controlled sections. For 9-#6 within the 60.0-in. width: bar spacing = 60.0/9 = 6.7 For Grade 60 reinforcement and in. < 18.0 in. For 6-#6 within the 120.0 – 60.0 = 60.0-in. width: 60.0/6 = 10.0 in. < 18.0 in. Similar analyses can be performed at interior columns. where for (ACI 22.2.2.4.3). Because the slab is subjected to gravity loads only, the lengths of the reinforcing bars given in ACI Figure 8.7.4.1.3a This area of reinforcement is greater than the required area of for slabs without drop panels can be used for this flat plate reinforcement, so the section is tension-controlled. voided slab system.

Concrete Reinforcing Steel Institute 3-16 Design Guide for Voided Concrete Slabs – Addendum

6. Check two-way shear requirements. Provide shear reinforcement in the form of headed shear In general, the total factored shear stress in the vicinity of the studs to satisfy two-way shear requirements. columns is the sum of the direct shear stress plus the shear Check the maximum shear strength permitted with headed stress due to the fraction of the factored slab moment trans- shear studs (ACI Table 22.6.6.2): ferred by eccentricity of shear. It is typical for the void formers to be omitted in the vicinity of the columns so that a solid section of concrete is available to counteract the shear stresses. Therefore, headed shear studs can be used as shear rein- Check the total shear stress at an edge column bending per- forcement. pendicular to the edge: Determine the design shear strength of the concrete with headed shear studs (ACI Table 22.6.6.1):

The critical section for two-way shear is located a distance from the face of the column. Maximum spacing between adjacent lines of shear studs (ACI Table 8.7.7.1.2). At an edge column, the factored shear force due to gravity loads is: Provide 2 lines of headed shear studs on each face of the column.

Assuming 5/8-in.-diameter studs and where (ACI R20.5.1), the required spacing is the following:

In this equation, is the cross-sectional area of all the headed shear studs on one peripheral line that is parallel to the perimeter of the column (see Figure 3.2.4). Thus, Because maximum spacing

Assuming a 5.0-in. spacing, check the requirements of ACI ACI 8.10.7.3 requires that 22.6.8.3:

Determine by ACI Equation (8.4.4.2.2):

The section properties of the critical section are determined Use 5/8-in.-diameter headed shear studs spaced at 5.0 in. on as follows: center.

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Figure 3.2.4 Headed shear stud reinforcement at the edge columns in the example.

Concrete Reinforcing Steel Institute 3-17 Design Guide for Voided Concrete Slabs – Addendum

Headed shear studs can be terminated where the design Total factored shear stress: strength of the concrete can resist the factored shear stress without the headed shear studs.

At the critical section located a distance from the outmost peripheral line of headed shear studs (ACI Table 22.6.6.1):

Therefore, the 5/8-in.-diameter headed shear studs spaced at 5.0 in. on center with 2 lines of headed studs on each face of Assume that 13 headed shear studs are provided in each line the column are adequate. parallel to each face of the column and check if the maximum factored shear stress at the critical section located a distance The extent of the solid area of concrete around the column of from the centerline of the outermost peripheral line is dependent on the location where the shear strength of of headed studs is less than or equal to . the voided area can resist the total shear stress alone. The shear factor accounts for the reduced shear strength in the Section properties of the polygonal critical section: voided area of the slab. For this system, the shear factor is equal to 0.5 (see Table 3.2). Considering the extent over which headed shear studs are required (see Figure 3.2.4), assume the solid area of concrete is 8.0 ft by 14.0 ft, and check the factored shear stress conservatively assuming : Segment 1:

Segment 2:

Segment 3:

Therefore, the 8.0 ft. by 14.0 ft. solid area of concrete around the column is adequate for two-way shear in the voided area of the slab.

The transfer moment is essentially zero at this critical section, but for calculation purposes, is conservatively taken as .

Concrete Reinforcing Steel Institute 3-18 Design Guide for Voided Concrete Slabs – Addendum

CHAPTER 4 Construction Considerations 4.1 Concrete Placement 4.2 Horizontal Construction Joints Concrete in voided slab systems is usually placed in two To assure the integrity of the two phased slab construction, separate lifts unless the void formers are directly tied to the other than the commonly used ACI-318 provisions, there are slab formwork. Typically, the first lift is intended to lock in the no code-specific requirements for voided slabs cautioning void formers and positioning cages (also called lattice girders), about the creation of the horizontal construction joint between thereby securing the plastic balls against the buoyant forces the two concrete lifts. The following code sections are ad- developed during the initial fluid-like state of concrete at the dress such construction joint conditions: time of placement. For two-stage concreting, there are two • Section 6.4.1 – indicates that surface of concrete con- common and distinct methods of building with void formers, struction joints shall be cleaned and free of laitance. either semi-precast or all cast-in-place approach. Thus, some of the ensuing comments may relate to the work of the cast- • Section 6.4.2 – articulates that immediately before new in-place concrete subcontractors while other notes address concrete is placed, all construction joints shall be wetted the tasks of the precast-panel manufacturers. and standing water removed. • Section 6.4.3 – invokes strength related provisions of If the voided slab is constructed using the cast-in-place approach, 11.6.9 about intentionally making the concrete interface the flowability of the concrete mixture forming the first, bottom rough to achieve higher coefficient of friction, as elabo- layer is particularly important. The placed concrete has to flow in rated in 11.6.4.3. between and beneath the void formers and be consolidated. In addition, concrete has to flow between the positioning cages/ 4.3 Inter-Panel Joints for the Semi-Precast lattice girders, the bottom (and possibly top) flexural reinforcing Components of the slab, and any conduits or miscellaneous other embedded items. The flat-soffit, formed by this first concrete placement The large precast panel modules used in many projects have typi- operation, may be left exposed during the service life of the cal horizontal dimensions typically on the order of about 8 ft by 35 structure depending on the interior design of the building and ft. Nevertheless, panels as large as 11 ft by 40 ft have been con- therefore has to be fully consolidated for architectural reasons. structed in North America. The size of the precast slab element depends upon the specifics of the building configuration and the It is noted that the grid of the void formers does provide for availability of adequate cranes and trucks. In order to provide for a convenient visual indication for the operator of any needle 1 1 an aesthetical finish, usually, a /4 in. to /2 in. bottom bevel/fillet is vibrator to follow the concrete flowing beneath the void formed along the edges of adjacent large panels. formers making sure that all areas of the slab are consistently 1 consolidated. It has also been observed that the rounded The resulting /2 in. to 1 in. wide grooves can be taped, filled plastic shapes actually facilitate the placement of the concrete with silicone caulk, or left intact as the precast components in voided slabs as the plastic concrete mixture readily flows are made with geometrically precise fit. This joint is generally around the curved surfaces. Direct prolonged vibration of the not conducive to wet cement paste leakage. Should the erec- void formers or the reinforcing should be avoided as it may tion result in wider gaps, backing rod and silicone caulk might result in a segregation of the aggregates. be considered to close the opening and to allow cement paste to fill the gap from above. The same abutting edges of In some past projects, the location of the void formers is adjacent panels can be formed with upper chamfers (creating reflected on the soffit surface of the freshly stripped concrete. an inch-wide, 45-degree bevel), as those slanted surfaces may This “mirroring effect” is most likely attributable to the vari- facilitate lowering the panels into position by allowing easier able 1-3 inches thick concrete cover over the voids. The differ- sliding along the formed edges for the crane crews. ent thicknesses of concrete have slightly different placement characteristics and drying pattern, compared to the slab cross 4.4 Reinforcing Bar Placement sections with full-depth concrete. The presence of added plastic void formers may lead to the As concrete is often placed in one solid lift at areas without voids, false impression that the reinforcing bar placement is made e.g., in the vicinity of columns and other zones of high shear particularly cumbersome. However, these plastic components demand, the transitions to the two phased slab areas have to be also may act as reinforcing bar supports resulting in fairly evaluated for construction efficiency. These zones of single place- straightforward reinforcing steel placement during the field ment can be accomplished together with either the casting of placing operations. Furthermore, some void formers are made bottom (first phase) or the top (second phase) layer of the voided with surface deformations to facilitate the proper location of zones but that is at the discretion of the concrete subcontractor. reinforcing bar. In addition, the fact that voided slab systems lend themselves to flat soffit construction brings a tremen- dous savings on the reinforcing bar field placement-related

Concrete Reinforcing Steel Institute 4-1 Design Guide for Voided Concrete Slabs – Addendum labor cost. By avoiding the beam and girder installations, the 4.6 Drilling into the Plastic Void Formers complexity of reinforcing bar work is substantially reduced. The location of voids may facilitate the placement of em- Flat slab formwork requires placing operations in downhand bedded items or the intentional perforation of the slabs to position, but the ironworkers do not have to be on their knees accommodate MEP pipes or conduits connecting the floor and bending down from the deck to tie reinforcing bars in the below and above. The presence of void formers may facilitate downturn beam or girder. the core drilling and even allow the placement of items such For obvious construction reasons, the positioning of reinforc- as light fixtures. Nevertheless, the perforations should be ing bar cages/lattice girders should be oriented such that their properly sealed per fire regulations. For roof slabs, additional continuous top and bottom wires are placed perpendicular to the waterproofing layers or flashing should be detailed and ap- reinforcing steel layer upon which they rest on at the bottom of plied. Similar damp-proofing procedures should be adhered to the slab. This implies that the directionality of the two orthogonal in wet areas of bathrooms and kitchens. reinforcing bar layers at the slab bottom should be set such that Also, care should be exercised when unintentionally drilling the one closer to the slab soffit should ideally be parallel to the into the hollow plastic balls. Rainwater may collect in the void continuous horizontal framing cage/lattice girder component. formers through drilled holes and during cold weather and The top wire of the wire-cage/lattice girder should be perpen- freeze, causing damage to the surrounding concrete. There- dicular to the first layer of top reinforcing steel, sparing the fore, in such circumstances a small hole should be drilled cost and labor associated with standees. Thus, out of the two through the concrete cover at the underside of the balls to orthogonal reinforcing bar layers at the top of the slab, the allow water to drain out or seal hole. lower one should be perpendicular to the orientation of the horizontal positioning-cage steel. To facilitate efficient con- 4.7 Attachment to Slabs with Post- struction, a typical detail, depicting this arrangement, is sug- Installed Anchors gested in the construction documents and should be reflected One of the most often articulated concerns with voided slabs on the reinforcement placing drawings. is the reliability of any anchorage to the already built slab sof- Edge conditions of voided slabs should be protected for several fit. Depending on the weight attached, different options can reasons. Boundary forces occur, often unaccounted by engineers, be considered to minimize interference issues with the void particularly near supports (like columns and walls) and at corners. formers. At any of these installation locations, the foremost In addition, permanent rails for curtain walls are often secured by item is the identification of void positions. anchoring to the edge of the slabs. Horizontal hairpin (U-shaped) The horizontal location of the void formers may be found bars are commonly added along the boundary. Another option is based on visual observation of the soffit, construction, and to install steel embed plates along the edge that can be anchored design documents (floor-plans and photos), or nondestruc- to other reinforcement. These plates serve as leave-in formwork tive testing methods. In addition, the minimum thickness of pieces that allow fastening other building parts to. concrete cover at the underside of the plastic balls should be established using nondestructive testing techniques. It 4.5 Installation of Embedded Items is important to recognize that for spherical void formers, the One of the common concerns raised about the voided slab minimum cover thickness is only applicable to the center point system is whether the presence of void formers poses much of each void. A few inches removed from those points there is impediment in accommodating various embedded items. While substantial increase of the concrete cover. In contrast, ellipti- the plastic void formers indeed prompt a need for a higher level cal void formers may have a broader area where the concrete of coordination, the large number of successful construction would be at the minimum slab thickness. projects proved that a variety of such needs could be accommo- The other consideration is the accurate assessment of the dated for in voided concrete slabs. This is with the understanding support capacity needs, as a gross overestimation of the that advance coordination is warranted should the other embed- force demand may result in an unnecessarily complicated and ded items necessitate the realignment of the void formers and costly solution. Similarly, the need for specific suspension significantly alter the flow of forces. locations ought to be assessed, given the fact that even a few Whereas small electrical juncture boxes and pipes less than about inch movement of a horizontal anchor point translates to a 3 inches in diameter can be installed without much interference, qualitatively better solution. larger items such as MEP conduits running in the slab may re- In addition, the various novel types of attachment systems quire that the regular pattern of void formers be interrupted, some should be carefully studied, as advances in adhesive and plastic balls either omitted or shifted. Typically, a limited number of mechanical anchors now present a variety of options. Typically, such reconfiguration can be accommodated without detrimental Evaluation Reports provide strength values for each anchor effects. Nevertheless, the structural engineer of record should be depending on embedment length and edge distance. The spe- consulted for any instances where the void formers are moved cific requirements for anchor installation, in accordance with from the position indicated on the plans. the anchor manufacturer’s instructions, should be carefully

Concrete Reinforcing Steel Institute 4-2 Design Guide for Voided Concrete Slabs – Addendum

adhered to as this is not only a matter of code compliance but and in the planning phases throughout North America. Nev- any deviation may impair the capacity of the attachment. ertheless, based on the limited domestic experience and the more extensive foreign experience, the voided slab building After considering all these factors, often short embedment method has demonstrated that it is a viable solution as it of- screw type of anchors are found effective to suspend low load fers potential substantial time-savings on site and other clearly items. If the points of thin concrete cover can be straddled, favorable attributes. Floor erection cycles may be reduced as and several anchor points are allowed, this solution may rep- the combined advent of gains in building the formwork, rein- resent a cost-effective solution with rapid installation process. forcement, and placement of concrete and craning. For larger loads, longer embedment mechanical or adhesive anchors are typically used in between the void formers. How- In concrete construction, the cost of formwork represents a ever, the relationship of void contours and the concrete break- very significant component of the total cost and time. Any out cones has to be scrutinized. An alternative option is the simplification in the forming and shoring process may result installation of the anchors into the voids in combination with in substantial reduction of complexity, translating to savings in filling part of the void with adhesive or mortar. Yet another op- time, material, and labor cost. With the larger spans and more tion, for very heavy loads, is through bolting to the slab. While significant load carrying capacity inherent of the voided slab this solution allows the engagement of a fairly large segment method, the otherwise time-consuming and costly construc- of the slab, the “punch-through” concrete surface may be tion of support girders and beams is eliminated or reduced. markedly weakened by the presence of cavities. The benefit of building flat soffits isways al considerable and often architecturally desirable. 4.8 Transportation and Handling of Void Formers and Precast Panels For the semi-precast method, the lack of site-built slab formwork is an added advantage. However, working with a local As the void formers and the precast panels are relatively delicate, precaster to manufacture, transport, and erect the bottom pan- construction crews have to be cautious not to damage these els is adding to the logistics. In the overall scheme, this hybrid items during handling and placing. The high density plastic void method can be as competitive as the all cast-in-place ways of formers have a very thin shell, which is resilient to the extent of building, proven by the many hundreds of successful projects normal construction processes. However, they do not tolerate ex- built with the various voided slab methods around the word. cessive loads, concentrated forces, or other forms of gross mis- handling, such as piling materials directly on them. Should any Ultimately, just like with any new construction techniques, this of the void formers be crumpled while already cast into the first method requires attention to several details and items that layer of slab, they cannot be easily replaced. Those voids with in- may result in time-delays or added labor cost. Some are more dentations can be filled with resilient foam to restore the original apparent and significant for all cast-in-place, some for the shape. Transportation, lifting, and storage of the bundled plastic semi-precast method. A few of these design and construction balls should be per manufacturer’s instruction. Care should be ex- issues are associated with: ercised not to soil, puncture, or deform the void formers. The pre- • Excessive number of geometrically varied bay sizes. cast elements should be kept raised off the ground. Similarly, the • Presence of odd supports (beams, girders, walls). precast elements should be lifted at multiple points, as directed by the precaster, using the lattice girders that provide stiffness • Recesses in soffits or other unusual geometric conditions. and point of attachment. Precast elements, while they may be • Field alignment of void-former modules. stacked on flatbed trucks or at the storage site, should be rested • Transition zones from areas of voids to solid slabs. on full width battens, wooden boards, or similar cribbing at each support point per the instruction of the precast manufacturer. The lifting devices should remain accessible and undamaged. The precast elements in their final position should be supported on flat and even surfaces. Temporary shoring should be provided to allow the system to gain sufficient strength to be self-supporting as the concrete in the second placement hardens. None of the components of the lattice girder should be cut without the supplier’s consent. If these bars are cut, extra propping should be provided to prevent the bending of the element during the cast-in-place concrete placement.

4.9 Time to Completion As with all new construction technologies, there is a learning curve for the industry. At the writing of this Addendum, there are at least 15 completed projects and others in construction

Concrete Reinforcing Steel Institute 4-3

Design Guide for Voided Concrete Slabs – Addendum

CHAPTER 5 Design Tools Providers of void formers offer a variety of assistance tools As the program is focused on horizontal framing systems, upon that are specific to their product. These include product cata- entering the basic identifiers, the user is prompted to select logs with detailed recommendations of design parameters from the available systems, with voided slabs being one of the used for their specific variation of void formers. In addition, selections. In addition, the current version features flat slabs, flat research reports featuring individual products, various guide- plates, slabs supported by one-way joists, wide module one way lines, sample construction documents, estimating tables, and joists, or two-way joists (waffle). Future versions may include ad- software are the typical resources provided. This section pro- ditional systems, such as post-tensioned slab systems. vides a generic overview, with the understanding that design and construction professionals need to review the specific information for the selected system.

5.1 RC Concept Voided Slab Module To assist design professionals with preliminary estimates in selecting reinforced concrete floor systems, CRSI has recently developed a web-based interactive tool. Reinforced Concrete Concept (or RC Concept as it is commonly referred to) is a quick and easy program that does not require an extensive technical background. Yet, it is an efficient software for devising floor schemes at preliminary design, or to perform rudimentary checks for existing designs.

Upon selecting the voided slab system, the user is prompted to indicate some basic design parameters, such as the load criteria (superimposed dead load and live load), the strength of concrete and reinforcing steel, and the unit weight of concrete. The software also incorporates detailed cost figures, aimed at pricing the selected structural system. The data entries are specific and relevant to the floor system being analyzed. For example, the cost of the void formers is included for the voided slabs.

This free utility program can be accessed at concept.crsi.org after a quick registration. The user may create a database of estimates for a given project with an assortment of structural configurations. The various saved design concepts can be recalled and compared.

The geometry of the system is entered via the graphical user interface. At least 3 span lengths have to be defined in both directions. The software automatically checks whether the code requirements, such as the aspect ratio of the slabs and ratio of the adjacent span lengths, for the applicability of the calculation method (direct design) is met. In addition to the individual span lengths, the program requires inputting the dimensions of the typical interior, edge, and corner columns.

Concrete Reinforcing Steel Institute 5-1 Design Guide for Voided Concrete Slabs – Addendum

Another important section is focused on the size of the void formers, their impact on the calculation parameters using estimates for stiffness, and shear capacity reduction. The user is also guided through considerations such as the weight reduction estimate of the slab system. The system addresses issues that may seem to be nuances at first but nevertheless impact the preliminary design, such as the desired bar sizes, additional reinforcement not reflected by the flexural calculations, and the usual presence of the “void-free” solid regions.

After meeting all the requirements, the interface provides summary sheets of the calculation information that may serve the user well for the next steps. Along with the summary of select input, the load criteria is recast with the indication of factored loads according to Chapter 9 of the ACI 318 Code and the amount of concrete displaced by voids.

The program immediately performs extensive checks on the prescriptive limits of the entered data, such as size limits, to eliminate unnecessary steps. After those criteria are met, the program guides the user through any significant problems that may deem the selected preliminary designs insufficient. The software not only identifies the code requirements that are not met, but offers suggestions to improve the initial design. In order to obtain results, two-way shear requirements must be satisfied. The program does not include an option for shear reinforcement, so column sizes must be increased for preliminary design purposes. The larger column sizes need not be provided if headed shear studs are used instead.

The result summary page concludes with details on the types and quantities of materials to be used to achieve a code compliant floor system solution. Along with the quantities, financial figures are presented, to allow the design and construction team to compare various floor systems. The dollar figures are broken down into easy to follow cost components so the estimator can adjust for accuracy. This system, developed in 2012, was intended to provide only preliminary estimates. Nevertheless, it is a powerful tool to establish approximate design parameters and costs at early phases of the design. In the years to come, CRSI expects to embellish the program with new features and additional floor types.

Concrete Reinforcing Steel Institute 5-2 Design Guide for Voided Concrete Slabs – Addendum

5.2 Construction Documents

5.2.1 Specifications The following paragraphs contain samples of text that may be considered in developing guide specifications for the void former nda related framing cages, lattice girder, or other ancillary devices needed to successfully build with the variety of voided slab systems available. Note that certain articles are applicable only to the specific technology uses (all cast-in-place vs. hybrid precast, or one- vs. two- stage concrete placement). Specific articles below may or may not be relevant depending on whether the voided slab engineer is or is not responsible for the entire slab design.

PART 1 GENERAL 1.1 Summary A. Section Includes: 1. Voided concrete floor and roof slab systems consisting of structural reinforced concrete containing hollow spherical and ellipsoidal concrete saving elements (formers), where concrete is placed in one or two stages. 1 2. Where Precast Filigree Elements are used, those elements consist of 2 /2 to 3 inches of filigree concrete, top and bottom welded wire reinforcement, and void formers. B. Related Sections: 1. Applicable provisions of Division 01 shall govern all work under this Section. 2. [Division 3] [Section 03 10 00 – Concrete Forming and Accessories.] 3. [Division 3] [Section 03 20 00 – Concrete Reinforcing.] 4. [Division 3] [Section 03 31 00 – Structural Concrete: Structural concrete [site poured topping slab] 5. [Division 3] [Section 03 38 00 – Post-Tensioned Concrete.] 6. [Division 07 – Thermal and Moisture Protection] [Section 07 84 00 - Firestopping]: Firestopping materials. 7. [Division 07 – Thermal and Moisture Protection] [Section 07 90 00 – Joint Protection]: Caulking of butt joints of precast filigree panels at exposed underside of floor members 8. [Division 3] [Section [______-______]: Interior applied finish.] 9. [Division 09 – Finishes] [Section [______-______]: Anchorage devices for ceiling suspension] 10. [Division 22 – Plumbing] [Section [______-______]: Anchorage devices for plumbing equipment and piping hangers.] 11. [Division 23 – Heating, Venting and Air Conditioning] [Section [______-______]: Anchorage devices for HVAC equipment and piping hangers][______].] 12. [Division 26 – Electrical] [Section [______-______]: Anchorage devices for electrical equipment and piping hangers] [______].

1.2 References Unless noted otherwise, all references are to the current version of the documents listed below. A. American Concrete Institute (ACI): 1. ACI 301 – Specifications for Structural Concrete. 2. ACI 302.1R – Guide to Concrete Floor and Slab Construction. 3. ACI 315 – Details and Detailing of Concrete Reinforcement. 4. ACI 318 – Building Code Requirements for Structural Concrete and Commentary. 5. ACI 347 – Guide to Formwork for Concrete. 6. ACI SP-66 – ACI Detailing Manual. B. American Society for Testing and Materials (ASTM International): 1. ASTM A108 – Specification for Steel Bar, Carbon and Alloy, Cold-Finished 2. ASTM A615 – Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement. 3. ASTM A706 – Specification for Low-Alloy Steel Deformed and Plain Bars for Concrete Reinforcement. 4. ASTM A775 – Specification for Epoxy-Coated Steel Reinforcing Bars. 5. ASTM A884 – Specification for Epoxy-Coated Steel Wire and Welded Wire Reinforcement. 6. ASTM A934 – Specification for Epoxy-Coated Prefabricated Steel Reinforcing Bars. 7. ASTM A955 – Specification for Deformed and Plain Stainless Steel Bars for Concrete Reinforcement.

Concrete Reinforcing Steel Institute 5-3 Design Guide for Voided Concrete Slabs – Addendum

8. ASTM A996 – Specification for Rail-Steel and Axle-Steel Deformed Bars for Concrete Reinforcement. 9. ASTM A1035 – Specification for Deformed and Plain, Low Carbon, Chromium, Steel Bars for Concrete Reinforcement. 10. ASTM A1064 – Specification for Carbon-Steel Wire and Welded Wire Reinforcement, Plain and Deformed, for Concrete. C. American Welding Society (AWS): 1. AWS B2.1 – Base Metal Grouping for Welding Procedure and Performance Qualification. 2. AWS D1.1 – Structural Welding Code - Steel. 3. AWS D1.4 – Structural Welding Code - Reinforcing Steel. D. Precast/Prestressed Concrete Institute (PCI): 1. PCI MNL-116 – Manual for Quality Control for Plants and Production of Structural Precast Concrete Products. 2. PCI MNL-120 – PCI Design Handbook - Precast and Prestressed Concrete. E. International Code Council (ICC) 1. IBC 2012 – 2012 International Building Code.

1.3 Design Requirements A. Design precast filigree elements to withstand initial and handling stresses. B. Fire Resistance: Provide designs acceptable to code requirements of authorities having jurisdiction to achieve hourly ratings as follows: 1. In accordance with equivalent thickness concept of IBC 2012 721.2.2.1.1. 2. Cover thickness for reinforced concrete floor or roof slabs in accordance with IBC 2012 Table 720.1.(1). C. [Design precast filigree elements to withstand dead loads and live loads in [restrained] [unrestrained] condition: 1. Roof Assembly: [______] lb/sq ft live load. 2. Floor Assembly: [______] lb/sq ft live load. 3. As indicated on drawings 4. Concentrated loads as indicated on drawings. D. Maximum Allowable Deflection of Voided Concrete Slab Roofs: [1/180] [1/240] [______] span, including effect of long-term creep, [cambered to achieve slope to drain.] [______.] E. Maximum Allowable Deflection of Voided Concrete Slab Floors: [1/240] [1/360] [______] span, including effect of long-term creep, [cambered to achieve flat surface under dead load.] F. The steel framing cages used to contain the voids are to be dimensioned such that the spacing and position of the voids is maintained and of sufficient strength to support normal foot traffic and reinforcing bars. G. The framing steel reinforcing girder used should possess enough strength and stiffness to allow the transportation, lifting, and handling of the precast panel. H. The steel anchor attachment devices used should possess adequate strength and be spaced sufficiently to secure the position of the void formers and the slab reinforcing in place while counteracting the buoyancy forces of the plastic concrete.

1.4 Submittals A. Shop Drawings: Indicate panel layout, panel thickness, layout of lattice girder, layout of top and bottom welded wire reinforcement, side lap bars, void former layout, layout of control strips [unit identification marks], connection details, edge conditions, bearing requirements, support conditions, dimensions, openings, [openings intended to be field cut], and relationship to adjacent materials. The shop drawings shall include the dimensioned locations, spacing requirements and quantities for each void type shown on the structural drawings. The quantity of concrete to be displaced by the voids shall be indicated for each area or concrete placement shown on each drawing. The shop drawings shall include installation instructions for the handling and placement of the void cages as well as instructions to be followed for the concreting operation. B. Shop Drawings shall be submitted and reviewed by the [Engineer] [Engineer/Architect]. C. Layout drawing of field-placed negative reinforcement.

1.5 Sustainable Design Submittals A. Section - Sustainable Design Requirements. Requirements for sustainable design submittals.

Concrete Reinforcing Steel Institute 5-4 Design Guide for Voided Concrete Slabs – Addendum

B. Manufacturer’s Certificate: Certify products meet or exceed specified sustainable design requirements. 1. Materials Resources Certificates: a. Certify recycled material content for recycled content products. b. Certify source for local and regional materials and distance from Project site. C. Product Cost Data: Submit cost of products to verify compliance with Project sustainable design requirements. Exclude cost of labor and equipment to install products. 1. Provide cost data for the following products: a. Products with recycled material content. b. Local and regional products. c. [______]

1.6 Quality Assurance A. Design precast filigree panels in accordance with requirements of: 1. ACI 318. 2. ACI 301. B. Sustainable Design Requirements: 1. Recycled Content Materials: Furnish materials with recycled content.

1.7 Qualifications A. Fabricator: Precast concrete fabricator shall be certified by the Precast/Prestressed Concrete Institute (PCI) Plant Certification Program. B. Welder: Qualified within previous 12 months in accordance with AWS B2.1. C. Design panels under direct supervision of [Professional] [Structural] Engineer experienced in design of this Work and licensed [in State of [______].]

1.8 Pre-Installation Meetings A. Slab pre-construction meetings: At least 20 days prior to placing first voided concrete slab, Contractor shall hold a meeting to review detailed requirements for preparing final concrete design mixes and to establish procedures for placing, finishing, curing, and protecting concrete to meet required quality under anticipated conditions. B. Contractor shall request responsible representative of each party concerned with concrete work to attend meeting, including but not limited to following: 1. Contractor’s Superintendent. 2. Testing Laboratory responsible for field quality control. 3. Concrete Subcontractor’s Project Manager. 4. [Ready-mix Concrete Supplier.] 5. [Concrete Pumping Equipment Supplier.] 6. [Resident Project Representative.] 7. Voided Concrete Slab Representative. 8. Structural Engineer of Record. C. Discuss anchor and weld plate locations, sleeve locations, and cautions regarding cutting or core drilling. D. Minutes of the meeting shall be recorded, typed, reproduced, and distributed by Contractor to all parties concerned within five working days of meeting. E. Minutes shall include a statement by admixture manufacturer(s) indicating that proposed mix design and placing can produce concrete quality required by this Section. F. [During construction, additional meetings may be held to review and modify procedures and materials established to assure attainment of required quality level.]

1.9 Delivery, Storage, And Handling A. [Section [01 60 00] [01 60 01]] [Division 01] - Product Requirements: Product storage and handling requirements.

Concrete Reinforcing Steel Institute 5-5 Design Guide for Voided Concrete Slabs – Addendum

B. Lifting or Handling Devices: Capable of supporting member in positions anticipated during manufacture, storage, transportation, and erection. C. Number each member to show location according to layout drawings. D. Store all units off ground. E. Separate stacked members by battens across full width of each bearing point. F. Stack so that lifting devices are accessible and undamaged.

PART 2 PRODUCTS 2.1 Voided Slab System Suppliers A. Voided slab system shall be manufactured by [______]. Any substitutions need to be approved by the architect and structural engineer or record.

2.2 Materials A. Voided Slab Site Poured Concrete Materials: As specified in Section 03 31 00 – Structural Concrete; the max aggregate size 3 is ¾ inch diameter for all slabs greater than or equal to 11 inch total slab thickness. The max aggregate size is /8 inch diameter for all slabs less than 11 inch total slab thickness. B. Reinforcing Steel: ASTM A615, Grade [60] [___], deformed steel bars. C. Deformed Welded Wire Reinforcement: ASTM A1064, and as follows: 1. WWR Yield Strength: Provide minimum yield strength of 80,000 psi. 2. Wire Spacing and Size: Provide wire spacing and size, as calculated to maintain the specified area of steel as indicated on the contract drawings. D. Void formers: High-density polyethylene, with color as chosen by the manufacturer, with the following properties: 1. Must not react chemically with concrete and all reinforcing steel and welded wire reinforcement. 2. Must be nonporous. 3. Shall be of sufficient strength and stiffness to carry loads induced by construction activities safely in the phases before and during the placement of concrete. 4. Safety characteristics a. Decomposition temperatures > … deg F b. Flashpoint > …. deg F c. Spontaneous ignition temperature > … deg F 5. The material used to produce the spherical or ellipsoidal void formers is to be 100% post-consumer/post-industrial recycled HDPE with no fragrance and no fillers. E. Lattice Girders: Bars with the yield strength of …ksi and tensile strength of … ksi consisting of a top chord, a bottom chord, and a diagonal. F. Shear Rail Reinforcement: Plain, ASTM A108, yield strength 50 ksi, tensile strength 60 ksi.

2.3 Fabrication A. Filigree Precast Panels: Plant cast. B. Fabricate openings required by other sections, at locations indicated. C. Panel manufacturer shall provide for openings 8 inches round, square or larger as indicated on structural drawings. D. All other openings shall be located and field drilled or cut by contractor requiring such work after precast filigree panels have been erected. E. Openings shall be approved by Engineer/Architect and manufacturer before drilling or cutting. F. Finishes: Bottom surface shall be flat and uniform without major chips, spalls, or imperfections. G. Patching: Will be acceptable providing the structural adequacy of the system, as confirmed by the voided slab engineer, is not impaired. H. Plant Finish: Finish members to PCI MNL-116S Finish A Grade.

Concrete Reinforcing Steel Institute 5-6 Design Guide for Voided Concrete Slabs – Addendum

2.4 Fabrication Tolerances A. Conform to PCI MNL-116. 3 B. Length of precast filigree elements shall not vary from design dimensions by more than plus or minus /16 inch. C. Deviations from straight lines shall not exceed 1/8 inch in 30 feet.

D. Precast filigree elements shall not vary by more than plus or minus1 /8 inch from true overall cross sectional shape as measured by difference in diagonal dimensions.

PART 3 EXECUTION 3.1 PREPARATION A. Prepare support devices for erection procedure and temporary bracing.

3.2 ERECTION OF SEMI-PRECAST SYSTEMS A. Install voided slab system in accordance with supplier’s requirements. B. Contractor shall be responsible for providing suitable access to the building, proper drainage and firm, level bearing for the hauling and erection equipment to operate under their own power. C. Contractor shall be responsible for providing true, level-bearing surfaces on all field-placed bearing walls and other field-placed supporting members. D. Contractor shall be responsible for placement and accurate alignment of anchor bolts, plates, or dowels in columns and walls. E. Erect members without damage to structural capacity, shape, or finish. Replace or repair damaged members. F. Align and maintain uniform horizontal and end joints, as erection progresses. G. Maintain temporary bracing in place until final connections are made. Protect members from staining. H. Adjust differential elevation between precast filigree panels to tolerance before pouring of site concrete. I. [Tape seal underside of] [Install sealant backer rod in] [Provide bead of silicone sealant in] recast joints to prevent concrete leakage when joints between precast filigree panels have not been closely butted.

3.3 INSTALLATION OF CAST-IN-PLACE SYSTEMS A. The containment cages shall be tied to the bottom reinforcement to prevent displacement and to counteract initial buoyancy of the voids during the first layer of concreting. B. The concreting operation, in the areas containing the voids, will be done in two stages with sufficient time (approximately 2-3 hours) between stages to allow the first stage to attain an initial set to counteract the buoyancy effect of the voids. The first stage of concrete shall engage the bottom reinforcing bars and be approximately 1" above the bottom of the voids.

3.4 TECHNICAL ASSISTANCE A. The supplier of the void form system shall be available for a pre-construction meeting to review the particulars of the system as they apply to the project and offer any additional recommendations. B. The supplier of the form system shall have a qualified representative visit the site during the initial placement of the voids prior to the first placement of concrete in the voided slab.

Concrete Reinforcing Steel Institute 5-7 Design Guide for Voided Concrete Slabs – Addendum

5.2.2 General Notes Corrective methods addressing cosmetic surface General notes, typically placed prominently on the construc- defects in concrete that may develop at manufacturing, tion drawings, should reflect many unique aspects to facilitate transportation, and handling. the proper execution of the building process. A few items to be highlighted by these narratives should include: 5.2.3 Construction Drawings • Slab plans depicting reinforcement and void formers, Drawings should identify design intent in showing: should prompt referenced architectural and MEP draw- • Horizontal and vertical controls should be provided to estab- ings for openings and building components/systems lish all void former locations on floor plans and elevations on framing into the slabs. details. • Should alert the construction team to get approval from • Transition area between voided and solid concrete zones. and coordinate with the structural engineer of record These details should show how concrete is terminated any relocation or omission of void formers, associated to delineate the two-phase concrete placement to areas products (e.g., positioning cages, supports), construction where one solid placement was applied. In addition, joints, and areas field forming. areas without void formers may need reinforcing steel • The plastic void formers are easy to damage. In concert supports for the top layer of flexural slab bars. with the manufacturer, criteria or guideline should be set • Placement of embedded items, such as post-installed to prevent inadvertent loads, and a procedure to replace. anchors should be detailed to provide guidance to allow • Notes on the sequencing of construction has to identify secure position limitations and placement methodology. for the two stage cast-in-place approach that the void • To avoid a potential duplication in bid assumptions, it is formers have to be secured in position by the bottom desirable to define whether certain construction materi- layer concrete sufficiently set to avoid buoyancy issues. als are supplied by a specific subcontractor (e.g., splicing • The role of the positioning wire cages, if relied on as reinforcing bars typically provided by the precast manu- reinforcement support, should be spelled out. facturer). • Provisions for the spacing of void formers may be spelled • For the precast hybrid system, details should show how slab out, and instructions for potentially cutting of wire cages joints and closing strips are to be executed and the continuity between void formers may be allowed. of reinforcement is maintained. Similarly, conditions where • Cleaning of the horizontal concrete contact surface be- slabs are framing into beams, walls and intersected by col- tween the two layers of concrete should be called for. umns or any other structural component should be detailed. • Similarly, attention should be brought to properly vibrating 5.2.4 Field Placement Guidelines the concrete during placement. Void modules are delivered to the site in bundles to facilitate • Temporary holes drilled into concrete and void former for being lifted with a crane. The voids may be pre-fixed in a dowels supporting props are to be sealed appropriately. wire cage for simple installation. The void modules are to be Unintentional filling of void former with water is to be installed in accordance with supplier’s installation guidelines avoided. for quality control and an efficient construction progress. Void • Estimates of displaced concrete volume might facilitate modules act as displacement bodies only. In this case the contractors’ scheduling of the ready-mix concrete place- cage also performs as supports for the top reinforcement. ment, and may serve as a record that identifies slab First the bottom reinforcement has to be placed. In the weight for future purposes. second step, void modules are placed and wired to the bot- • For the precast-hybrid system, notes should indicate: tom reinforcement. Subsequently the top reinforcement is Orientation and maximum spacing of temporary hori- installed. Void modules are provided in lengths of 8 feet and zontal supports. can be cut to fit to the slab geometry. The wire cage should Dimensional tolerances of panel placement, in particu- be cut between the vertical legs that are in place between lar with respect to supporting structures (girders, walls, every single void. Do not just remove the void former from columns). the cage but keep the entire remnant of cage and void intact. The remnants can be reused. Field and transportation handling of the panels requiring proper support and temporary storage area. The spacing between center lines of void modules as deter- Maximum unsupported length of the panel for cantile- mined on the shop drawings can be held by using a simple ver-like conditions. wooden installation aid. It is to be placed between the top longitudinal cage wires and the bottom wires of the cages, Pre-designated hoisting locations. then tied to the bottom bars with 6 ties per cage to secure. Minimum compressive strength of concrete at the time of lifting and shipping.

Concrete Reinforcing Steel Institute 5-8 Design Guide for Voided Concrete Slabs – Addendum

In areas of heavy worker traffic, prior to the top reinforcement lems with the void formers. Therefore, along these complex being installed, it may be advisable to lay plywood strips or edge lines the void formers are often omitted. Similarly, the wood planks on top of the cages to eliminate accidental dam- locations of block-outs for HVAC, plumbing, and electrical age to the voids. runs (only those of consequential size) should be indicated on the shop drawings and the void formers omitted where they Holes drilled in the slab for temporary use during the con- occur. Commonly, these items are not specified at the time struction phase (e.g. for dowels fixing props) are to be sealed design drawings are created, and may be established only appropriately after use. Damaged voids are to be replaced at the shop drawing production phase. Along with the shop before installation. After the installation the void former must drawings, general notes and specifications should be followed not be damaged. These provisions are made to avoid the unin- as part of the instructions addressing the installers, and in tentional filling of void formers with water which might lead to particular, should highlight if the concrete placement should damages caused by frost bursting. be done in two stages. As the role of the first concrete layer A buoyancy force occurs during the concrete pour because of is to secure against buoyancy, it should be clearly indicated the concrete displacement. The void modules need to be held to what extent the first lift is to encase the bottom longitu- down with appropriate provisions to avoid the buoyancy of the dinal bars of the positioning cages in addition to the bottom void formers. To avoid any fixing to the formwork or weighting slab reinforcing layers. Additional notes and details need to of void former modules, the concrete pour is executed in two address alignment, spacing, and recommendations to ensure layers. It is important to maintain the required embedment the protection of the void formers in areas of high construc- depths of the longitudinal lower cage module wire during the tion traffic at the site. first concrete layer pour. Local concrete accumulations have to Similar suggestions apply for semi-precast panels where the be avoided when pouring the first layer of concrete. It is rec- shop drawings must be coordinated with MEP openings, ommended to check the void module’s vertical position after penetrations and the installation within the precast of items, pouring of the first layer. Once the first layer of concrete has such as, J-boxes and embeds. These shop drawings must be reached its initial set, the cage modules will be fixed and kept created for each produced panel and clearly show the bottom in their vertical position. The surface of the first concrete layer and top reinforcement, the position of lattice girders, and the needs to be kept free of debris. Solid areas without voids can location of the individual plastic void formers. Also, it is es- be poured in one single layer. sential to unmistakably identify the concrete edge detail and The concrete is to be supplied, placed and compacted ac- reinforcing steel directions, as the semi-precast system locks cording to the structural engineers’ requirements. Maximum the voids into a fixed position prior to the erection of the sys- 3 aggregate size of /4 in. is recommended for smaller void for- tem. The thin bottom precast layer secures the embedment mer types and generally is defined by the structural engineer. of these components early on to assure no movement during Reinforcing bars and void former modules must not be moved transportation, erection, or during the on-site placement of during the concrete pour. The concrete needs to be placed the top concrete layer. and compacted accurately to allow a tight concrete coat on Typically, the connectivity between the components in the the reinforcement and void formers. It may be required to semi-precast system is established via development (stich) vibrate in between each module. reinforcing bars, running perpendicular to the precast span, located in the cast-in-place concrete layer. Often, these are 5.2.5 Placement/Shop Drawings temporarily secured to the precast panel with tie wire until One of the most important steps in the quality assurance those panels are erected into their final positions. To provide process is the production of shop drawings. When using site- added protection against progressive collapse, development installed void formers, the presence of those and the posi- bars from the precast panels should pass through the column tioning cages (or lattice girders for the semi-precast system) cages. introduces a large number of objects placed in the concrete. Thus, it is imperative to depict and coordinate all items in detail. Beyond the usual items of the conventional reinforced concrete slab placing drawings, the added objects require heightened attention to detect potential conflicts and congest- ed areas that would preclude proper concrete placement and ultimately compromise quality. It is often the shop drawing level of detailing that brings to light constructability issues, such as proper orientation of the orthogonal flexural reinforcing layers and the positioning wire cage/lattice girder modules. Embeds, inserts, post-installed anchors, and attachments that often occur along the exterior slab edges and interior openings may pose interference prob-

Concrete Reinforcing Steel Institute 5-9

Design Guide for Voided Concrete Slabs – Addendum

CHAPTER 6 Feature Projects

6.1 Pérez Art Museum Miami, Miami, Florida thickness by as much as 6 in., The new Pérez Miami Art Museum Miami represents the first adding slab dead load that was voided slab project built in the United States with both slab con- of paramount concern given the crete placement stages cast in place. The masterfully crafted and long spans. The structural floor unique reinforced concrete building was designed by the Swiss system that offered the most architectural firm Herzog & de Meuron and New York-based economical solution while solv- Handel Architects. A rendering of the museum is shown in Fig. ing the problem of the added dead load from the recessed 6.1. The structural design was engineered by the New York City Fig. 6.2 Aerial View of Concrete office of Arup Inc. General contractor John Moriarty & slab consisted of a steel-rein- Framing System Near Completion. Associates of Florida was charged with building the 200,000 forced concrete voided slab. square feet of exhibit space. The museum opened in December The reinforced concrete system was constructed by subcontrac- 2013 at a cost of $200 million. The building was designed to tors Baker Concrete and Titon Builders America, the latter sup- achieve a Leadership in Energy and Environmental Design (LEED) plying and installing the reinforcing steel and voided slab system. silver certification from the U.S. Green Building Council. Tarmac Concrete and Gancedo Rebar Services provided the 17,500 cubic yard ready-mix concrete and 3,000 tons of reinforcing steel, respectively. There is about 80,000 square feet of voided slab area. It was estimated that the voided slab removed about 935 cubic yards of concrete, translating into significant savings of material, truck traffic, CO2 emission and loads on the supporting system columns and pile foundations.

Fig. 6.1 Rendering of the Museum Front Overlooking Biscayne Bay.

The building is located on the west side of Miami’s downtown, facing Biscayne Bay. In addition to traditional exhibit spaces, it houses an educational complex with a library, auditorium, classrooms and workshops (studios), as well as a café and museum store. The Museum is dedicated to collecting and exhibiting international art of the 20th and 21st centuries with an emphasis on the cultures of the Atlantic Rim – the Americas, Europe and Africa – countries Fig. 6.3 Placement of Void Formers and Upturn Beam Reinforcement. representing the origins of a vast majority of Miami residents.

The design for the building’s structural system grew out of its functional parameters. The museum itself rises from the basic Depending on the load criteria and spans, spherical and ellipsoi- parking area grid where the columns are placed at regular dal voids were used with the depth of plastic balls ranging from intervals across the site. Falling both inside and outside the mu- 7 inches to 18 inches (Fig. 6.3). Considering the vicinity of the seum building envelope, these columns support the two-way marine environment and the expanse of the exposed concrete load carrying platform, the upper levels of the museum and the surfaces, corrosion protection measures included the use of roof as depicted in the aerial view photo shown in Fig. 6.2. shrinkage reducing admixtures and increased concrete cover over the reinforcing steel (2 inches in the slabs and 2.5 inches in the The architectural design called for large flat soffited galleria spaces columns and beams.) Other special construction measures used with open spans ranging between 50 and 100 feet. In order to included slag aggregates to achieve lighter concrete unit weight in achieve these clear spans without sacrificing floor to ceiling height select areas and high surface quality forming systems. clearances, the design team evaluated more than 40 different structural floor system schemes. The most economical structural solution 6.2 Teaching and Learning Building, Harvey employed a system of conventionally reinforced voided floor slabs Mudd College, Claremont, California with upturned beams that were cast together with walls to create a The voided slab building method was introduced to Los Angeles box-like structural system. This “structural box” requires only three area at the Harvey Mudd College (Claremont, CA). The technol- wall supports to be stable and in turn opened up one elevation for ogy-oriented institution constructed a new 80,000 square foot the full height, offering stunning views of Biscayne Bay. instructional facility which opened in 2013. The four-story Teaching In addition, the architectural design called for recesses on the and Learning Building, renderings shown in Fig. 6.4, houses lec- underside of the slab to house lights and sprinkler systems. These ture halls, specialty classrooms, offices and large public spaces, soffit recesses, also called “rebates,” increased the overall floor all to accommodate a growing school population.

Concrete Reinforcing Steel Institute 6-1 Design Guide for Voided Concrete Slabs – Addendum

Fig. 6.4 Renderings of The Teaching and Learning Building.

The $43 million building is slated to receive a LEED Platinum Certification and is the first academic facility constructed on this campus in 20 years. It is the third voided slab project to be built in the United States, preceded by the new Pérez Art Museum Miami in Florida and the LeBahn Hockey Arena in Fig. 6.6 Aerial View of the Slab Construction Showing Expanses of Void Madison, WI. Formers and Solid Areas Around Columns. The Portland, Oregon-based design team of Boora Architects and KPFF Structural Engineers partnered with Los Angeles contrac- The game-changing technology in the voided-slab system is the tor Matt Construction to bring this new solution to fruition. Help use of recycled plastic void formers to create voids at mid-height of also came from BubbleDeck Inc. (the supplier of the voided slab the concrete slabs. The hollow, thin-shelled spheres are used in the system), Graef Engineering Inc. (the structural engineering firm tens of thousands in this project, as shown in Fig. 6.6, efficiently devising the slab system) and the San Diego precast plant of U.S. displacing concrete at the locations where it was not needed to Concrete Inc. (manufacturer of the precast soffit panels). establish full continuity between the solid top and bottom layers. The nearly 30% reduction of self-weight provided building owners Reinforced concrete floor systems are preferred by most with flat-soffit concrete slab and the traditional multi-benefits of developers, including the college’s, due to its long-term value concrete systems while lesser seismic mass, an important con- stemming from its recognized durability, strength and stiff- sideration for all projects in the Los Angeles Basin. The project was ness. Attributes of resilience to extreme events (fire, blast, estimated to save about 750 cubic yard concrete and the associ- earthquakes and high winds), and environmental effects (mold/ ated ready-mix concrete truck trips to the site. Savings in the slab mildew, termites, rodents, etc.) were broadened in this project weight also translated to smaller column loads, ultimately reducing with more efficiency in material use and other green factors. the sizes of the lateral system, columns, and footings. The architectural design called for large instructional spaces The structural slabs in the Teaching and Learning Building were exceeding 35 foot spans. The collaborative engineering effort of built with the hybrid version of voided slab system, which entails a KPFF and Graef identified the voided slab system as the most ef- combination of precast slab panels and a second, in situ, placement ficient structural scheme for the project, the BIM model of which of concrete. The blow-molded plastic balls were fastened on top is shown on Fig. 6.5. Despite many advantages of other concrete of the large reinforcing concrete plates manufactured at a precast floor systems, the inherent issue of heavy self-weight often dis- plant. Upon transporting to the construction site, these slab soffit suades design and construction teams from considering concrete elements were hoisted floors for long spans and/or heavy loads, such as for instructional/ into their final position (as laboratory space found in university facilities. depicted in Fig. 6.7) and additional reinforcing bars, similarly to a traditional cast- in-place flat plate system, were added for continuity prior to placing the second Fig. 6.7 Precast Panels in Place, Prior to lift of concrete. Establishing Continuity. The precast panels served as a stay-in-place formwork for the cast-in-place portion of the concrete slab. This hybrid-slab system had a variable total thickness ranging from 9 inches to 20 inches, depending upon the span and use of the floor. The production of the precast panels provided an opportunity to create a very high quality exposed soffit, thus eliminating the cost and labor associate with slab formwork. The end product is a cost-efficient construction solution that is expected to propel this building, with concrete slabs without Fig. 6.5 BIM Structural Model. beams, well into the 21st century.

Concrete Reinforcing Steel Institute 6-2 Design Guide for Voided Concrete Slabs – Addendum

6.3 Neuroscience Engineering Collabora- was selected because it provided the required stiffness to tion Building, Wright State University, satisfy the acceptance criteria for vibration of the long spans, Dayton, Ohio while at the same time providing a floor system without beams that weighed significantly less than other comparable Completed in April of 2015, the Neuroscience Engineering floor systems. In short, the vibration-resistant floor system Collaboration Building at Wright State University provides an enables the laboratory to be nationally competitive in micro- environment where neurological researchers, engineers, fac- electronics and nanoelectronics. ulty, and students can work together on pioneering research and medical breakthroughs (see Figure 6.8). The flat plate voided concrete slab system also had a direct impact on the cost of the foundations: Because of the The four-story, 90,000-square-foot, L-shaped building has two reduced dead load, the foundations were designed for smaller wings – one for neuroscience and one for engineering – and a loads compared to comparable floor systems, and because of central, multi-story atrium surrounded by collaboration spaces, the longer spans, fewer foundations were needed. All of this office suites, and laboratories. Perkins +Will, the architect of added up to significant cost savings. record, designed the spaces to be flexible and adaptable to facilitate teamwork and the exchange of knowledge. The build- The $37.5 million project was constructed by Messer Con- ing also houses a 105-seat auditorium for research sympo- struction and the structural engineer of record is Shell + siums. Meyer Associates, Inc., both of Dayton. Long, column-free spans were required in the laboratory spaces to allow for easy reconfiguration of equipment based 6.4 Roy and Diana Vagelos Education on project requirements (see Figure 6.9). Typical equipment Center, Columbia University Medical includes lasers, powerful microscopes, magnetic resonance Center, New York, New York imaging (MRI) scanners, and positron emission tomography Columbia University Medical Center’s new, state-of-the-art (PET) scanners, to name a few. These types of equipment medical and graduate education building, the Roy and Diana are very sensitive to vibrations and will not operate properly Vagelos Education Center, opened in August of 2016 with where the supporting floor system does not meet stringent vibration requirements. A flat plate voided concrete slab system

Fig. 6.8 The Neuroscience Engineering Collaboration Building, Wright State University, Dayton, Ohio.

Fig. 6.9 Typical ﬂoor construction of the Neuroscience Engineering Col- Fig. 6.10 The Roy and Diana Vagelos Education Center, Columbia Uni- laboration Building. versity Medical Center.

Concrete Reinforcing Steel Institute 6-3 Design Guide for Voided Concrete Slabs – Addendum

LEED Gold Certification (see Figure 6.10). Designed by Diller Scofidio + Renfro of New York, in collaboration with Gensler as executive architect, the 15-story, 100,000-square-foot building houses classrooms, collaboration spaces, and a simulation center, all of which facilitates the development of skills vital for modern medical practice. Also included are specialized spaces for mock examinations rooms, clinics, and operating rooms and a 275-seat multi-purpose auditorium that will be used for campus-wide events such as lectures, screenings, and concerts. The building layout allows key activities to be centralized, thereby providing an excellent learning environment for students. A stair, dubbed the “study cascade”, ascends up the south side of the building; it expands and contracts along its height to create atrium-like spaces, which include areas of tiered seating, glass-enclosed meeting rooms, open gathering spaces, and outdoor terraces where students can socialize, study, or relax. A number of structural challenges were encountered by the structural engineer, Leslie E. Robertson Associates of New York. One of the main tasks was to provide long, open floor spans of minimal structural depth that were, among other things, sufficiently stiff to accommodate the stringent deflection requirements of the glass façade (which is typically not parallel to the edges of the slab). Also, long, cantilevered slabs were required for the “Study Cascade”, some of which are 26-ft long (there are no perimeter columns in this part of the structure; see Figure 6.11). A flat plate voided concrete system was chosen to meet these challenges (see Figure 6.12). The reduced self-weight of the slab in combination with post-tensioning helped in achieving acceptable long-term deflection limits. The general contractor for the project was F.J. Sciame Construction Company, Inc. of New York, and the concrete Fig. 6.11 The “Study Cascade” in the Roy and Diana Vagelos Education Center (Columbia University Medical Center). subcontractor was Difama Concrete Inc.

6.5 Visual Arts Building, University of Iowa, Iowa City, Iowa The 126,000-square-foot, LEED Gold Certified Visual Arts Building at the University of Iowa, designed by Steven Holl Architects, New York, and BNIM Architects, Des Moines, opened in the fall of 2016 as a replacement for the 1936 Art Building, which was damaged by a flood (see Figure 6.13). The building houses studios for ceramics, sculpture, metals, photography, print making, 3D design, intermedia, and graphic design; space is also provided for graduate student studios, faculty and staff studios and offices, and gallery space. A punched cast-in-place concrete frame at the exterior of the building is utilized, which harnesses the benefits of the inherent thermal mass of concrete. Interconnection of all the departments, which is of fundamental importance in a modern art school, was achieved by large, open floor plates. A flat plate voided concrete system was selected because it allowed greater spans with fewer columns Fig. 6.12 Typical ﬂoor construction of the Roy and Diana Vagelos Educa- and no beams, resulting in greater flexibility in space utiliza- tion Center (Columbia University Medical Center).

Concrete Reinforcing Steel Institute 6-4 Design Guide for Voided Concrete Slabs – Addendum

landings are provided at some locations with tables and chairs while others open into lounge spaces with sofas. The structural engineers for the project were Buro Happold Engineering, Chicago, and Structural Engineering Associates, Kansas City, MO. The general contractor was Miron Construc- tion Company, Inc., Cedar Rapids.

6.6 Reach Expansion, John F. Kennedy Center for the Performing Arts, Washing- ton, DC The three-story, 120,000-square-foot Reach Expansion will provide visitors to the Kennedy Center for Performing Arts a unique opportunity to fully engage and interact with the Fig. 6.13 The Visual Arts Building, University of Iowa. activities at the Center. It will transform the campus from a traditional performing arts center into a living theater (see Figure 6.16). Located south of the existing facility, the $28 million project, which was designed by Steven Holl Architects, includes space for rehearsal rooms and presentation rooms for private showings and an expansion of the existing parking garage. Just south of the new building is a 40,000-square-foot space that includes an area for buses to bring patrons and honored guests to visit the Center.

Fig. 6.14 Typical ﬂoor construction of the Visual Arts Building.

Fig. 6.15 Stairs as horizontal meeting spaces in the Visual Arts Building.

tion on the floors and a flat soffit (see Figure 6.14). The overall structural depth of this system was found to be less than that of a structural steel floor system; this resulted in lower floor- to-floor heights and a reduced building volume, both of which translated into significant cost savings. Additional savings were realized because fewer foundations were required in conjunction with reduced self-weight of the structure. Unique cast-in-place reinforced concrete stairs encourage meeting, interaction, and discussion (see Figure 6.15). Large Fig. 6.16 The Reach, John F. Kennedy Center for the Performing Arts.

Concrete Reinforcing Steel Institute 6-5 Design Guide for Voided Concrete Slabs – Addendum

Fig. 6.19 Typical ﬂoor construction of the Schneck Professional Building.

office building and parking garage with completion expected in 2019 (see Figure 6.18). The owner of the building required an open floor plate, low floor-to-floor heights that had to match the floor elevations of the adjacent building, the capability to make future modifications to the structure, and a long-term, low maintenance structure. A flat plate voided concrete system was selected, which satisfies these objectives and then some. Fig. 6.17 Typical ﬂoor construction of The Reach. The typical bay size in the building is 30 by 28 ft (see Figure 6.19). A 10-in.-thick flat plate voided concrete slab was utilized by the structural engineer for the project, Lynch, Harrison & Long spans with large, open spaces that achieved maximum Brumleve of Indianapolis, which satisfied both strength and flexibility in architectural programming, durability and resil- serviceability requirements (including the additional benefit of ience, and cost savings were all reasons why a flat plate satisfactory vibration performance for the long spans) while voided concrete system was chosen for this project (see maintaining a flat soffit without beams. Figure 6.17). Because of the reduced self-weight of the structure, columns Robert Silman Associates was the structural engineer, Whit- and foundations were smaller. The lower floor-to-floor heights ing-Turner Contracting Company was the general contractor, resulted in significant cost savings attributed to the overall and Lane Construction was the concrete contractor. smaller volume of the building. Matching the floor elevations of the adjacent structure would have been very costly with 6.7 Schneck Professional Building, Sey- a structural steel floor system. Modifications to a flat plate mour, Indiana voided concrete slab system can be made just like any other Work is underway on the newest addition to Schneck Medi- two-way reinforced concrete slab system. Because void cal Center, the Schneck Professional Building. This major formers decrease the self-weight of the slab only and do not expansion, designed by the architectural firm arcDESIGN in contribute to strength or serviceability, cutting through them Indianapolis, features a five-story, 80,000-square-foot medical does not have a detrimental effect on the structure. F.A Wilhelm Construction Co., Inc., the concrete contractor for the project along with Pepper Construc- tion, the general contractor, visited The Reach Expansion, John F. Ken- nedy Center for the Performing Arts, Washington, DC, while it was under construction with same type of floor system to witness construction practices and to learn from the contractors, Whiting-Turner Contracting Fig. 6.18 The Schneck Professional Building. Company and Lane Construction.

Concrete Reinforcing Steel Institute 6-6

Description of Publication This design guide addendum presents state-of-the-art practices in voided slab construction. The concept centers on removing concrete mass from the areas of the slab where it is not structurally efficient; thus reducing the dead load by as much as 35%. This floor system allows for large, clear spans and efficient overall slab thicknesses. Inno- vative structural slab construction practices have taken the efficiency of traditional slab systems to new heights.

Concrete Reinforcing Steel Institute

933 North Plum Grove Road [ Schaumburg, IL 60173 [ Tel. 847.517.1200 [ www.crsi.org 10-DG-VOIDED-ADDENEDUM