ENGINEERING Fire Resistance of Flat Plate ETN-B-2-16 Voided Slab Systems

Introduction in the (see Figure 2). The void formers reduce the weight of the slab sig- Concrete and steel are noncombustible nificantly compared to a solid slab of the materials; that is, they will not ignite or burn same thickness; they are judiciously locat- when subjected to fire or heat. Because ed in zones where concrete is not needed assemblies and sys- and where the flexural strength and load tems are inherently fire resistant, no ad- transfer to supports are not compromised. ditional fire protection is required to meet common minimum fire ratings prescribed Typical site installation of a cast-in-place, in the building codes. These inherent mate- flat plate voided concrete slab is as follows rial properties reduce fire risk and require (Figure 1b): minimum ongoing maintenance over the lifetime of a building. 1. is erected and bottom rein- forcing bars are placed. Flat plate voided concrete slab systems, 2. Cage modules that contain the void which have been used for many years in formers are placed and tied perpen- Europe and other parts of the world, are dicular to the top layer of bottom rein- becoming increasingly popular in the U.S. forcing bars. because of many inherent benefits that can be provided. Descriptions of the vari- 3. Top reinforcing bars are placed directly ous types of flat plate voided concrete on and tied to the cage modules. slab systems are given in the next section. 4. First layer of concrete is cast, which Information and guidelines on how to deter- covers the bottom reinforcing bars mine the fire resistance for these systems and the lower portion of the cage are also provided as are results from fire modules. This layer of concrete es- tests. The data shows that the design of sentially anchors the cage modules flat plate voided concrete slab systems for and prevents possible upward move- fire protection is the same as that for other ment due to buoyancy when the sec- reinforced concrete structures and that a ¾ ond layer of concrete is placed. in. cover to the bottom reinforcing bars will 5. Second layer of concrete is cast to the provide a minimum 2-hour fire-resistance required overall thickness of the slab

Technical Note Technical rating, which meets minimum required fire- system. resistance ratings for floor assemblies in common occupancies. Another type of voided slab system uti- lizes a reinforced layer, Flat Plate Voided Concrete Slab which essentially replaces the horizontal Systems formwork of the cast-in-place system de- scribed above. Installation of this system is Various types of voided concrete slabs as follows (Figure 1a): have been used throughout the years, including one-way Sonotube systems 1. Cage modules with top and all of the and two-way joist (waffle slab) systems bottom structural reinforcement are that are constructed using dome forms. precast into 3-in.-thick panels. Contemporary systems, pioneered primar- 2. The panels are set on shoring that is ® ® ily by BubbleDeck and Cobiax , utilize hol- spaced 7 ft on center. low, plastic balls made out of high-density, recycled polyethylene (HDPE), which are 3. On site, a layer of concrete is cast to commonly referred to as void formers (see the required overall thickness. Figure 1). These are usually spherical or el- In applications where 30 to 50 foot spans lipsoidal in shape and are positioned within are required, the overall depth of the slab wire support cages to create modular grids can range from 10 to 21.5 in. It is common (cage modules), which are locked between at column locations that void formers are the upper and lower reinforcement layers omitted so that a solid concrete section is (a) (b)

Figure 1 – Void formers (a) BubbleDeck® and (b)Cobiax USA® (photos courtesy of BubbleDeck and Cobiax USA).

available to resist two-way shear stresses (see Figure or by the methods prescribed in Section 703.3 of the 3). Where additional shear strength is required, shear re- International Building Code (IBC) (IBC 2015). inforcement, such as headed shear stud reinforcement, can easily be accommodated, as shown in the Figure. Required Fire-Resistance Ratings Required fire-resistance ratings for elements in build- Flat plate voided concrete slabs systems are designed ings are given in Table 601 of the IBC based on the and detailed in accordance with requirements of ACI 318 construction type (I though V). Types I and II are types (ACI 2014) just like any other two-way slab system. It is of construction where the building elements are of important to note that the void formers do not contribute noncombustible materials, which includes reinforced to the nominal flexural and shear strengths of the slab concrete. The minimum fire-resistance rating for floor system; their only role is to provide voids in the slab. elements in Type I construction is 2 hours. Stiffness and shear modification factors are provided for deflection and shear strength calculations, respectively, Test Methods to Determine Fire-Resistance Ratings which are used in design to take into account the pres- ence of the voids within the slab. No modification factors IBC Section 703.2 permits the test procedures in are required for flexure because the voids do not have an ASTM E119 (ASTM 2016a) and UL 263 (UL 2015) for de- impact on flexural strength; nominal flexural strength is termining fire-resistance ratings of building elements, based on compression in the concrete and tension in components, and assemblies. Standard fire tests are the reinforcing bars, just as in any other two-way slab conducted by placing an assembly in a furnace and system. Additional design and detailing information, in- subjecting it to a fire that follows a standard time-tem- cluding a worked-out design example, can be found in perature curve. Fire-resistance rating of an assembly is CRSI (2014). determined by the duration of the test until one of the following end-points is reached: In addition to reduction in dead load, there are other noteworthy benefits that can be realized by utilizing a • Fire passage end-point (Walls, partitions, floors, and voided slab system, including economical long spans roofs) without forming and casting of beams, low floor-to-floor Cotton waste ignites as a result of flames or hot heights, and vibration resistance. See CRSI (2014) and gases passing through holes, cracks, or fissures in manufacturers’ literature for more information. the assembly. Fire-Resistance Ratings • Heat transmission end-point (Walls, partitions, floors, and roofs) In general, fire-resistance rating (or, fire rating), is the Temperature of the unexposed surface of the as- period of time (usually expressed in hours) a building ele- sembly rises an average of 250°F above its initial ment, component, or assembly maintains the ability to temperature. contain a fire, continues to perform a given structural function, or both. Fire ratings are determined by tests

2 Fire Resistance of Flat Plate Voided Concrete Slab Systems [ETN-B-2-16] Figure 2 – Flat plate voided concrete slab system.

• Structural end-point (All assemblies and members) Test specimen is unable to sustain the applied load- ing (collapse). Figure 3 – Void formers omitted at column locations (photo cour- tesy of Cobiax Technologies AG). The results from fire tests on flat plate voided concrete slabs systems are summarized below.

Reinforced concrete meets the requirements of being as specified in Section 20.6 easily satisfy minimum fire a noncombustible material that are set forth in ASTM resistance requirements. E136 (ASTM 2016b), and reinforced concrete assem- blies are generally classified as restrained assemblies The minimum slab thicknesses provided in IBC Table in accordance with ASTM E119. Restrained assemblies 722.2.2.1 are for slabs with a uniform thickness, such as perform better during a fire compared to assemblies that a typical one-way or two-way slab system. A method to are unrestrained. determine the thickness of slabs with ribbed or undulat- ing soffits to be used for fire-resistance ratings is given Calculations to Determine Fire-Resistance Ratings in IBC Section 722.2.2.1.3. This method is applicable for Section 703.3 of the IBC permits the use of calcula- systems with the cross-sectional profiles illustrated in tions performed in accordance with Section 722 as one IBC Figure 722.2.2.1.3 (see Figure 4). of the acceptable ways of determining fire-resistance The slab thickness for this type of construction to be ratings for structural members. The fire-resistance rat- used in determining the fire resistance ratings in accor- ings provided in that section are based on ASTM E119 dance with IBC Table 722.2.2.1 is determined as follows: fire tests. For reinforced concrete, values of minimum member thickness/size and minimum For s ≥ 4t, thickness = t over reinforcement are provided for various fire-resis- For s ≤ 2t, thickness = te tance ratings. Thus, in order to satisfy a required fire- For 4t > s > 2t, thickness = t + (4t/s-1)(t -t) resistance rating based on Type I or II construction, e reinforced concrete thickness/size and cover to the re- where inforcement must be specified that are at least equal to s = spacing of ribs or undulations the values in the appropriate tables in Section 722. t = minimum thickness t = equivalent thickness of the slab calculated as the Requirements for minimum thickness of reinforced e net area of the slab divided by the width, in which concrete floor and roof slabs and for minimum con- the maximum thickness used in the calculation crete cover over reinforcement in slabs are given in IBC shall not exceed 2t. Sections 722.2.2 and 722.2.3.1, respectively. Generally, reinforced concrete slab systems that are proportioned In the case of hollow core prestressed slabs where in accordance with the provisions in Chapter 8 of ACI the cores are of constant cross-section throughout the 318 and that have minimum cover to the reinforcement

CRSI Technical Note 3 t d

(a) s s s

Calculation Per 1 Square Grid of Void Formers

Center To Center Measures:

(b) Floor Area = s2

Figure 4 – Equivalent thickness for (a) ribbed construction and Solid Slab Volume = t × s2 (b) undulating construction. 4 d 3 d 3 Void Former Volume = = 3 2 6 length, the equivalent thickness shall be permitted to be obtained by dividing the net cross-sectional area of 3 2 d the slab including grout in the joints, by its width (see ts 3 d IBC Section 722.2.2.1.1). Equivalent Thickness = 6 =t s2 6s2 In a similar fashion, an equivalent thickness for a voided slab system can be determined by dividing the net volume of concrete by the respective floor area. Samples: Figure 5 shows examples of such calculations for both VOID FORMER ID BubbleDeck® and Cobiax® systems with spherical void VOID FORMER formers. The overall slab thicknesses t in the figure are DIMENSIONS BUBBLEDECK COBIAX the minimum available for both systems. Equivalent (in.) BD 230 CBM-E-225 thicknesses obtained by this method are greater than 5 t 9.00 13.5 in., which is the minimum slab thickness that provides a d 7.08 8.875 2-hour fire-resistance rating for concrete mixes with si- liceous aggregate (see IBC Table 722.2.2.1). Equivalent s 7.87 9.875 thicknesses for voided slab systems with overall slab Equivalent Thickness 5.97 9.75 thicknesses greater than the minimum thicknesses considered here are greater than the corresponding values in the figure. It is shown below that concrete Figure 5 – Sample calculations for equivalent thickness of voided cover to the reinforcing bars on the the fire side is the slabs. controlling parameter in the determination of the fire resistance for voided slab systems. The calculations presented here support those findings. Fire-Resistance Tests on Flat Plate Voided The minimum thickness of concrete cover to the posi- Concrete Slab Systems tive flexural reinforcement of reinforced concrete slabs with flat undersurfaces is given in IBC Table 722.2.3(1). Numerous fire tests have been performed on ® ® This table is applicable for (1) solid or hollow core one- BubbleDeck systems and Cobiax systems in accor- way slabs or two-way slabs and (2) slabs that are either dance with the provisions in DIN 4102-02 (DIN 1977). cast-in-place or precast. It is evident from the table that The time-temperature curve used to test specimens in a typical concrete cover of ¾ in. provides a fire resis- the DIN requirements is the same as that prescribed tance of 4 hours for restrained assemblies regardless in ISO 834 (ISO 1999). A comparison of the ISO 834 of the unit weight of the concrete or the type of aggre- and ASTM E119 time-temperature curves is given in gate used in the concrete mixture. Figure 6. It is evident from this figure that there are some differences in the curves. However, it has been determined that the differences in severity between the two tests are negligible (Harmathy 1987). Criteria to determine the fire-resistance rating are also basically

4 Fire Resistance of Flat Plate Voided Concrete Slab Systems [ETN-B-2-16] Figure 6 – Comparison of ISO 834 and ASTM E119 time-temperature curves.

the same. Therefore, it follows that the results obtained Summary from specimens tested in accordance with the DIN re- quirements would essentially be the same as those that It is evident from the findings of fire tests that con- would be obtained if the specimens were tested in ac- crete cover to the reinforcing bars on the side of the cordance with ASTM E119 requirements. fire is the controlling parameter in the determination of the fire resistance for flat plate voided concrete slab sys- The fire tests have revealed that the concrete cover tems. A ¾ in. cover to the bottom reinforcing bars will to the reinforcing bars on the fire side is the controlling provide a minimum 2-hour fire-resistance rating, which parameter in the determination of the fire resistance in meets minimum required fire-resistance ratings for floor all of the tests. It was found that the voids act as a ther- assemblies in common occupancies. mal isolator: the heat from the fire is dammed below the void. This leads to slightly higher temperatures in the reinforcing bars positioned below the voids. A cover of ¾ in. to the main flexural reinforcing bars resulted in a fire resistance rating of at least 2 hours. The fire ratings obtained from the Cobiax® tests have been verified by finite element analyses.

The void formers were found to be intact after the fire tests. The internal temperature remained below the melting temperature of the HDPE, which is approximate- ly between 200 and 300 degrees Fahrenheit.

CRSI Technical Note 5 References ACI (American Concrete Institute). 2014. “Building Code Require- ments for Structural Concrete and Commentary.” ACI 318-14, Farmington Hills, Michigan. ASTM (American Society for Testing and Materials). 2016a. “Stan- dard Test Method for Fire Tests of Building Construction and Ma- terials.” ASTM E119 – 16, West Conshohocken, Pennsylvania. ASTM (American Society for Testing and Materials). 2016b. “Standard Test Method for Behavior of Materials in a Vertical Tube Furnace at 750°C.” ASTM E136 – 16, West Conshohocken, Pennsylvania. CRSI (Concrete Reinforcing Steel Institute). 2014. Design Guide for Voided Concrete Slabs, Schaumburg, Illinois. DIN (Deutsches Institut für Normung). 1977. “Fire Behavior of Building Materials and Building Components; Building Compo- nents; Definitions, Requirements and Tests.” DIN 4102-2, German Institute for Standardization, Berlin, Germany. Harmathy, T.Z., Sultan, M.A., and MacLaurin, J.W. 1987. “Com- parison of Severity of Exposure in ASTM E119 and ISO 834 Fire resistance Test,” Journal of Testing and Evaluation, ASTM, 15(6), 371-375, West Conshohocken, Pennsylvania. ICC (International Code Council). 2015. International Building Code, ICC, Washington, D.C. ISO (International Organization for Standardization). 1999 (Amended 2012). “Fire-resistance Tests - Elements of Building Construction - Part 1: General Requirements.” ISO 834-1, Geneva, Switzerland. UL (Underwriters Laboratories). 2015. “Standard for Fire Tests of Building Construction and Materials.” UL 263, Northbrook, Illinois.

Contributors: The primary contributors to this publication are: David A. Fanella, PE, SE, FACI, FASCE and Mike Mota, PhD, PE, FACI, FASCE

Keywords: empirical design, fire rating, fire resistance, fire test, non-combustible, rational design, restrained, structural integrity, unrestrained.

Reference: Concrete Reinforcing Steel Institute – CRSI [2016], “Fire Resistance of Flat Plate Voided 933 North Plum Grove Rd. Concrete Slab Systems,” CRSI Technical Note ETN-M-9-16, Schaumburg, Illinois, 4 pp. Schaumburg, IL 60173-4758 Historical: None p. 847-517-1200 • f. 847-517-1206 www.crsi.org Note: This publication is intended for the use of professionals competent to evaluate the significance and limitations of its contents and who will accept responsibility for the application of the material it Regional Offices Nationwide contains. The Concrete Reinforcing Steel Institute reports the foregoing material as a matter of infor- mation and, therefore, disclaims any and all responsibility for application of the stated principles or for A Service of the Concrete Reinforcing Steel Institute the accuracy of the sources other than material developed by the Institute. ©2016 This publication, or any part thereof, may not be reproduced without the expressed written consent of CRSI.