A MEDLEY OF PRECAST AND PRESTRESSED SYSTEMS

Neil F. Dunbar Ketch u m-Kon ke l-Barrett-N i cke I-A u sti n Consulting Engineers Denver, Co!oredo

Describes several structural systems using both cast-in-place prestressed and precast construction. The client's design criteria are presented in the form of key design parameters which are then discussed and a solution given. The precast and prestressed concrete system adopted for each set of design parameters is believed to be the best one for the particular project at the time it was used.

Prestressed concrete can be used in a time and are again proving their eco- vast number of structural systems to nomical and competitive worth. achieve safe, economical structures. Finding the best system to solve the Over the past few years our firm has problem is what all engineers strive for used cast-in-place prestressed and pre- and, hopefully, this paper will give cast prestressed concrete on a variety of those who read it some new alternatives projects. Some uses are perhaps new, for future designs. or unusual, and others are systems in every engineer's repertoire. POST-TENSIONED FLAT PLATE Rather than describe one system in PARKING GARAGE detail, we chose to place emphasis on ideas and present a number of problems Key design parameters (design parameters) with their solutions 1. Minimum floor to floor height re- using brief descriptions and pictures. quired to minimize difficult bed- We believe the solutions to the prob- rock excavation lems were the best ones for the particu- 2. Minimum slab thickness required lar project at the time they were used. 3. Watertight garage slab without Some of these systems have been used waterproofing many times, others only once, and 4. Large areas of repetition others are just being used for the second 5. Cost

78 Fig. 1. Stair-stepped slab construction Fig. 2 Concrete being placed for 11 in. post-tensioned slab

Due to the high level of the under- hairline cracks have occurred. The only lying bedrock, a parking structure re- cracks in the slab radiate off of con- quiring minimum slab thickness in crete walls used as supports around order to provide minimum floor-to-floor various shafts at interior slab locations. heights was required for this project. Additional mild steel reinforcement This minimized the amount of expen- placed in the slab around shaft wall sive excavation required into bedrock. supports would help to reduce this An 11 in. thick post-tensioned 4000 cracking considerably. psi sand-lightweight , on Lightweight aggregate concrete used 30 x 30 ft square bays, satisfied both in the Denver area has good deflection the minimum slab thickness and punch- properties and combined with the pre- ing shear stresses at the 24 in. diam- stressing in this flat plate, has resulted eter columns (no capitals). in very small slab deflections. This type of prestressed slab con- struction works out to be economical SINGLE TEE AND VERTICAL where stair-stepping the castings can be CONSTRUCTION TECHNIQUE done (Fig. 1). This allows a small, effi- cient crew of workmen to place rein- Key design parameters forcing steel and place two or three 60 1. Column free flexible interior floor x 60 ft segments of slab every week space (Fig. 2). Construction of this subgrade 2. Narrow site parking structure went smoothly. We 3. Zone 2 earthquake inspected all reinforcing steel before 4. Tight construction schedule concreting the slabs and found minor 5. Cost problems involving items such as: ten- The client required an economical don support chairs left out, too much column-free commercial 10-story office curvature (kinks) in the tendons, inter- building designed for Zone 2 earth- ference of tendons and mild steel rein- quake loading. forcement at the columns, and tendon Due to the relatively narrow build- sheathing torn off. These problems were ing, precast single tees, spanning 66 ft generally worked out in the field. to exterior precast T-columns (Figs. 3 Maintaining a minimum average and 4), was the most economical struc- compressive stress (P/A) of 150 psi re- tural system satisfying the design cri- sulted in watertight slabs except where teria. The cast-in-place concrete cores

PCI Journal/March-April 1973ҟ 79 Fig. 4. Sngle tee to T-column connec- tion

Fig. 3. 140 ton crane erecting T-col- umns and single tee floor slabs

were slip formed before any was set, and provided Zone 2 earthquake and erection stability. The only major delay (2 weeks) during the construction of the structure, came while the precast erectors waited for the arrival of the 140-ton crane needed Fig. 5. T-column splice to erect the heaviest T-column sections. The single tees were connected to the T-columns by straps welded to embed- each column and provided erection sta- ded plates in the top of the spandrel bility while the bars were being cad- beam and tee. The floors act as dia- welded. The space between the ends of phragms through the cast-in-place top- the columns was then filled with con- ping slabs, and tie each floor to the core crete to complete the splice. walls and spandrel beams. Three round This 10-story office building was holes were provided through each tee completed enough in 10 months from stem for mechanical distribution (Fig. the start of foundations to allow tenants 3). This resulted in a smaller ceiling to move in, and it was built for under space and less floor-to-floor height. $20 per sq ft. Column bar splices were made using cad-welds. The precast T-columns were cast laying flat end-to-end with bars BUILDING BLOCK COMPONENTS continuous through the splices; then the bars at the splices were cut, which as- Key design parameters sured a perfect fit in the field. The ends 1. Minimum floor depth of the columns at the splices were held 2. Repetition of standard components apart using a short steel W section as a 3. Architectural expression of struc- spacer (Fig. 5). This W section was ture bolted to an embedded weldment in 4. Cost

80 depth. The system consists of precast interior columns and precast exterior "ladder columns." The floor beams bear on an interior composite girder and on the "steps" of the exterior "ladder col- umns" (Fig. 6). Precast slabs were placed over the precast beams to act as lost forms (Fig. 7). Composite concrete topping was then placed over the entire precast com- ponent slab, completing the floor struc- ture. This system had some problems in placing additional top steel over the col- umns due to its shallow depth, and the reinforcement congestion that usually occurs at column-beam connections. The large number of pieces that have to Fig. 6. Exterior ladder columns sup- be handled during precasting and erec- porting interior composite floor beams tion is a cost disadvantage in this sys- tem. This is somewhat offset by the rep- etition and number of pieces, but it is an area which bears careful considera- tion before a decision is made to use this type of system.

"DIADECK" SYSTEM Key design parameters 1. Convention floor loading of 250 psf 2. Shallow floor depth at midspan to accommodate utility lines for ex- Fig. 7. Precast "lost form" slabs on hibit spaces above composite floor beams. The beams are 3. Exposed structure that was archi- supported by composite girders tecturally pleasing 4. Cost. The patented "Diadeck" was devel- oped to carry a heavy exhibition floor load of 250 psf, and a concentrated load The design of this research office of 10,000 lb at any point on the floor. building was based on providing a flex- The system consists of precast trian- ible and inexpensive structure for the gular pans which act as a composite client. It was decided that it would be a "lost-form" for a cast-in-place slab and tower of precast concrete and that the two-way post-tensioned beams on 30 ft infill between the precast concrete square bays. Three hundred (300) pre- would be painted concrete block. cast triangular pans were cast using a The structural system was a precast daily cycle with four forms. Erection composite concrete system which pro- crews averaged 1 hr per 30 x 30 ft bay vided repetitiveness and minimum floor in setting the precast pans with the steel

PCI Journal/March-April 1973 81 placing crews following closely behind. Two corners of the pan bear on the column capitals, which results in only one shore required at the center of each 30 ft square bay during construction (Fig. 8). The only additional forming required besides the precast pans, is a strip of masonite in the bottom of the beams to seal the joint between the pans. Fig. 8. Erecting precast triangular The beams along the column lines "lost form." Two corners of the form were post-tensioned while the beams on bear on the column capitals while the the diagonals were reinforced with mild third corner is shored. Only one shore steel (Fig. 9). However, the post-ten- per 900 sq ft is required sioning along the column lines results in a prestress force component in the diagonal beams which was used in their design. The system cost was 200 per sq ft less than an equivalent twin tee-girder system. Part of the reason for the lower cost is that the very heavy design loads favor the four-way haunched "Diadeck" system, and part is due to the simplicity and ease of erecting four pieces per bay, compared to six pieces per bay for the double tee-girder system. The "Diadeck" has performed ex- tremely well and has carried loads above its design load with negligible de- Fig. 9. "Diadeck" slab ready for plac- flections, which has been verified using ing concrete. Beams are haunched and levels. The four-way haunch allows post-tensioned two-way mechanical distribution at midspan, and results in an architectural- ly pleasing bottom surface (Fig. 10).

PRECAST CONSTRUCTION IN REMOTE AREAS Key design parameters 1. Rapid erection (short building sea- son) 2. Durability and low maintenance 3. Small skilled labor force 4. Architecturally exposed concrete The architect and owner wanted this mountain ski area gondola terminal to Fig. 10. Four-way haunch allows for be constructed out of concrete for ap- two-way mechanical distribution at pearance, fire resistance, and durability. midspan Being in the mountain ski area of Vail,

82 ed the structure using his own expe- rienced erection crews, which eliminat- ed the need for several different crews of skilled trades, as would have been re- quired for a cast-in-place concrete structure. This structure was not ideal for pre- cast concrete because of the many dif- ferent shapes, special corner details, varying roof elevations, many small Fig. 11. Shows excessive variety of pieces, and little repetition (Figs. 11 precast concrete columns and brackets. and 12). However, to build the same concrete building, which the architect and owner wanted, out of cast-in-place Colorado, where the building season is concrete would not have saved money short and the availability of the skilled and/or time. labor required for cast in place concrete With the great variety of different work is many times nonexistent, precast pieces required for this building, con- concrete was picked as the best build- siderably more coordination time was ing material. spent during all phases of the project. To maintain the short design-con- Also, more field time was spent solving struction schedule, the components problems involving missing connection were cast during the winter months at a plates and fitting tolerances, than is nor- Denver precast , then mally required on a precast job having hauled to the site and erected as soon as more repetition. However, precast con- the foundations were installed in the crete is still a good material to use in re- spring. The precast manufacturer erect- mote areas where skilled labor is in short supply and quality workmanship is difficult to maintain.

SPECIAL WAFFLE SLAB Key design parameters 1. Client wanted exposed waffle slab using special large pans for light- ing and acoustics 2. 5-ft module both ways 3. As much interior open space as possible 4. Cost A special waffle slab with ribs spaced at 5 ft on center was designed to satis- fy structural and architectural require- ments. The architectural requirements consisted of using the large pan spaces for recessed lighting and acoustical con- trol. Fig. 12. Illustrates the excessive num- With the square footage of floor in- ber of different precast shapes and volved, it was economically feasible to pieces in this precast concrete structure make special steel pan forms 4 1/2 x 41/z

PCI Journal/March-April 1973 83 Fig. 13. Special 4% x 4½ x 1½ ft pan Fig. 14. Detail of column and slab and prestressing tendons prestressing tendons intersection

x 11/a ft for building this 14-story gov- 3. Easy forming system ernment office building (Fig. 13). These 4. Watertight parking garage slab large pans considerably reduced the without waterproofing waffle slab dead load on bay spacings of 5. Cost 30 x 40 ft and 30 x 30 ft. Prestressing This 30 x 30 ft bay parking garage was used as the most economical rein- slab system was designed while work- forcement due to the 5 ft joist spacing ing with the contractor on cost and and relatively shallow joist depth. Pre- feasibility. The basic idea used in the stressing was helpful in controlling slab design is to balance moments by drap- deflections and increasing the slab ing the center of gravity of the concrete punching shear capacity at the columns. section and running the prestressing This special waffle slab was more eco- tendons straight the same distance from nomical than a conventional waffle slab the top of the slab. This is achieved by reinforced with mild steel. The savings forming the bottom of the slab as a hy- were mainly due to a lighter slab and perbolic paraboloidal surface which is reduced reinforcement placing costs. 24 in. deep at the columns and 7 in. When using this type of system, con- deep at the midspans. sideration must be given to the possible The forming system is no more diffi- interference of tendons at the joist in- cult to set than a conventional flat plate tersections and over columns (Fig. 14). which is the easiest of all systems to This system resulted in a pleasing ex- form (Fig. 15). Tendon pulls of up to posed modular structure. 240 ft long were made since there was little friction loss in the straight ten- HYPAR SLAB dons. This resulted in a substantial cost savings in end hardware, and no thread- Key design parameters ing of tendons is required as in a con- 1. Contractor worked with engineer ventional flat plate. This saved consid- to devise the most economical erable placing time and problems with structural system tendon interference. Also, mechanical 2. Minimize expensive end prestress- and electrical lines can be located near ing hardware by making tendon midspan where the slab depth (7 in.) pulls longer is a minimum, saving headroom.

84 Fig. 15. Plywood forming for hypar Fig. 16. Bottom surface of hypar slab slab

The finished slab has a pleasing bot- 30-story office building. Structurally, tom surface appearance (Fig. 16), and cast-in-place concrete core walls and the prestressing resulted in a watertight spandrel beams were required to carry slab. Cost savings, as well as 4 weeks of the lateral wind and Seismic Zone 1 construction time resulted in the build- loads. ing of this prestressed "hypar" slab. The core walls were cast one level We have recently used this "hypar" ahead of the precast floor erection. slab on another project to carry heavy Double tees were then set into pock- (250 psf) superimposed landscaping and ets in core walls, and on shores at ex- live loads on a plaza. This system works terior wall lines (Fig. 17). Following well for heavy loading because of its the tee erection, spandrel beams forms plate action and its ability to carry high were placed around the double tee legs. punching shears at the columns where Prior to casting the spandrel beams, the its depth is greatest. ends of the legs were wrapped with polyethylene to break the bond be-

MONOLITHICALLY TIED DOUBLE TEES Key design parameters 1. Contractor preference of struc- tural system 2. Climbing crane capacity 3. Architecturally exposed cast-in- place concrete building frame 4. Structural floor system which could provide ramped slabs for lower story parking garage and upper story office floors 5. Cost and construction time The contractor preferred to use 16 plus 2 double tees having a maximum Fig. 17. Setting double tees on core span of 42 ft for the floor slabs in this wall pockets and exterior shores

PCI Journal/March-April 1973 85 Fig. 18. Cast-in-place spandrel beam Fig. 19. Ramped parking slabs (tees and double tee leg connection bearing on pockets in core wall)

tween the tee and spandrel beam and parking garage and office floors to save allow the double tees to shorten. The forming and construction time. final tee-to-spandrel beam connection is The double tees were made using shown in Fig. 18. Later, a topping slab lightweight concrete to reduce the (2 in. minimum) was cast on the tees climbing crane capacity required to lift which acts as a diaphragm tie between the heaviest (42 ft long) tee. Using the exterior frame and central core, lightweight concrete increased the tee making them work together to carry camber and resulted in more concrete lateral loads. required in the topping slabs to make a The bottom 8 floors of the tower are level finished floor. ramped in a continuous spiral for a During the early stages of construc- parking garage (Fig. 19), and the re- tion, problems existed in scheduling maining 22 floors are level and used for crane time between the precast and office space. The precast, prestressed cast-in-place concrete trades. However, double tee floor slabs combined with once the crane scheduling was solved, the cast-in-place exterior frame and the combined precast and cast-in- core worked well on both the spiral place construction worked fine.

Discussion of this paper is invited. Please forward your discussion to PCI Headquarters by August 1, 1973, to permit publication in the September-October 1973 issue of the PCI JOURNAL.

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