Design-Construction Feature

Precast Prestressed SystemProvides Solution forStadium Expansion

Robert A. Coleman, PEDirector of DesignEnwright Associates, Inc.Greenville, South Carolina

Construction time was cut almost in half by usingprecast prestressed double-tee floor members,handrail sections, stringer beams, seating slabs,and fascia sections for the new upper deck additionto Clemson Memorial Stadium in Clemson, SouthCarolina.

Ten thousand football fans streamedinto the new upper deck addition of

Clemson Memorial Stadium to witnessthe annual Clemson-South Carolinagame—climaxing an engineering andconstruction feat in which innovationwas exceeded only by suspense.

Uncertain soil conditions (discovery ofan old landfill) and new wind load re-quirements forced construction to a halt

for a 4-month period from December1977 through March 1978. When con-struction resumed, the structure wasnew but the deadline was the same,namely, the task of finishing the stadiumaddition prior to the big game betweentwo traditional state rivals.

The job at hand required placingabout 7000 cu yds (5355 m3) of concreteand 640 tons (580,608 kg) of reinforcing

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steel plus the erection of some 1200precast prestressed concrete compo-nents. All this work took place within a7-month period—cutting almost in halfthe original construction schedule.

The job confronting the engineers wastwo-fold: first, solving the foundationproblem and then, redesigning thestructure. To solve the first problem, aspecial rig was designed to "jack-up" theexisting footings to allow new concreteplacement and drilled pier (caisson)construction 90 ft (27.5 m) into firm soil.Then, to stabilize the structure underwind loads, five shear walls werespecified at strategic points to carry thewind. A rigorous computer analysis ofmathematical models of other frames ofbeams and columns simulated the ef-fects of the 100 mph (160 km/hr) designwind load.

With construction proceeding at anaccelerated pace and time already criti-cal, the engineers chose high-early-strength concrete, redesigned manycast-in-place members as precast sec-tions and adopted a unique design for

the main beams supporting the seats(stringer beams). The stringer beamswere changed from single cast-in-placemembers to six precast sections joinedtogether by post-tensioning after erec-tion. Believed to be the first design ofthis type, the precast beams saved con-siderable time and provided additionalstiffness for spectator loads.

The design effort gave way in October1978 to detailed quality control checks ofthe structure as sections were com-pleted. Teams of engineers and con-tractors scoured the stadium to look forpossible defects. They inspected pre-stressing operations ten stories in theair, ran tests on questionable concretesections, and x-rayed welded connec-tions.

As the game approached, each daypresented a new set of problems—rain,delivery delays, equipment failures, andother setbacks. The last concrete wasplaced 4 days before the game; the lastinspection conducted the night before.

The gates opened the next morning tothe upper deck—making way for the

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largest crowd ever to see a footballgame between Washington, D.C., andBirmingham, Alabama. The equivalent ofsix ten-story buildings had been com-pleted with a 4 months' lapse in the con-struction schedule.

Major emphasis in this article will beplaced on the precast prestressed con-

crete aspects of the project.

DiaphragmRigid frame action between the beams

and columns originally distributed thestadium wind loads to the foundation. Totransfer the higher wind loads to the newshear walls, the three floor levels of the

PCI JOURNAL/January-February 1980 15

WATERPROOFING 150-FLEX 881 SEALANT

II BOND #BREAKER 4 LAYERS OF 15CONT. FELT (TYP. AT ALL -Z

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stadium structure act as diaphragmsspanning between the walls under thenew design. Precast double-tee floormembers with a 2 or 3 in. (51 or 76 mm)topping formed the floor of these levels.

Shear-friction reinforcement in thetopping and welded connections be-tween the flanges of the double-teesprovided the shear force resistance inthe diaphragm. The flanges containedbent #4 bars and after erection the con-tractor welded an additional straight #5bar between the other two bars. Thereinforcement in the topping also pro-vided bending moment resistance.

The double-tee connection to the gird-ers (which were doweled into or cast aspart of the shear wall) completed thetransfer of the wind loads to the shearwalls. Location and detailing of expan-

sion and contraction joints maintainedthe integrity of the entire lateral systemwhile allowing movement to take place.

Design ProblemThe accelerated construction

schedule forced an early decision to bemade regarding the design of the mainstringer beams supporting the stadiumseats. The decision that only a precastscheme could be constructed in the timeavailable scrapped the original cast-in-place beams and established the fol-lowing design problem:

Design a precast sectional beamto span two 30-ft (9.15 m) baysand cantilever 20 ft (6.10 m) ateach end. The precast sectionsmust be capable of being erected

16

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FORM SIDE 101/2" 7 101/2" FORM SIDE

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by the tower crane and must con-nect to cast-in-place columns. De-sign loads include dead load, uni-form and unbalanced live load,wind load, and dynamic spectatorloading.

Preliminary AnalysisA preliminary investigation of design

alternatives focused on various combi-nations of member dimensions, pre-stressing schemes, and constructionmaterials. The 17-kip (76 kN) liftingcapacity of the tower crane and a needfor a very "constructable" system signifi-cantly shaped the preliminary designdecision. For example, lightweight con-crete was rejected because of concernsfor quality control and deflection be-havior. "Match-cast" and glued precastsections were also considered but were

18

found uneconomical for this particularjob.

The member finally selected for finalanalysis and design is shown in the ac-companying table and illustrations. Thestringer beam consisted of six separatesections—three longitudinal sections,each longitudinal section split in half.

The sections were to be spliced at twolocations and would connect to the col-umns on either side of a central column"tongue." A single post-tensioning ten-don would run continuously througheach series of three half sections to jointhem together and provide the requiredmoment resistance.

Gravity and Wind LoadAnalysis

The preliminary calculations estab-lished the critical member depths and a

DASHED LINE INDICATES LIMITOF CONC. DIAPHRAGM BETWEENSTRINGER BEAMS

TYP.

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DETAIL e STRINGER SPLICE

preliminary tendon profile to balance thedead load. The computer analysis (ICESSTRUDL) concentrated on refining thetendon profile to optimize moments anddeflection behavior under various com-binations of uniform dead load, uniformlive load, wind load, and unbalanced liveload.

The loads applied by the post-tensioning forces were represented by"equivalent uniform loads" and concen-trated reactions.

The computer analysis included ef-fects of post-tensioning loss throughcolumn bending.

Dynamic AnalysisThe static analysis produced design

forces and deflections. Final design ofthe stringer beam, however, awaited acheck of dynamic behavior under spec-tator loading. The dynamic analysis(STRUDL DYNAL) used a procedurefirst developed at Iowa State University.The dynamic spectator loading con-sisted of a combined harmonic loading(rhythmic jumping) and transient loading(rising spectators).

The analysis predicted the natural fre-quency of the stringer beams to be 2.04

PCI JOURNAUJanuary-February 1980 19

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cycles per second and the peak deflec-tion produced by the dynamic load to be0.108 in. (2.7 mm). The motion will beperceptible but was considered accept-able behavior due to the rapid dampingfrom the peak response and the lowprobability of its occurrence.

Prestressing DetailsThe engineers selected a concrete

strength of 5000 psi (34,500 kN/m 2) anda multi-strand post-tensioning systemwith mechanical wedge anchorage anda conduit size of 33/4 in. (70 mm) in di-

ameter. Each cable consisted of twelve1/2-in. (12.7 mm) diameter 270 ksi(1,863,000 kN/m2) seven-wire post-ten-sioning strands combined with additionalmild reinforcing steel in regions ofmaximum moment to satisfy ACI ulti-mate strength requirements and workingstresses.

Pretensioning of each individual sec-tion sustained erection loads and han-dling. Careful selection of the tendonprofile and post-tensioning forces al-lowed full application of the post-tensioning force to the stringer beam

M

prior to the application of the seats andother increments of dead load. Thiseliminated the need for staged post-tensioning operations.

Details for splice points, column con-nections, handrail connections, post-tensioning blockouts are shown in thesection details. Cast-in-place concretediaphragms tie sections together at

splice points and at the ends — provid-ing stability and erection flexibility.

The precast manufacturer cast thebeam sections in Charlotte, NorthCarolina, and shipped them to the jobsite via trucks. Using steam curing, thesections were cast in only 21 days. The"Z" section seating slabs were cast inMacon, Georgia.

PCI JOURNAL/January-February 1980 21

ErectionThe tower crane erected the sections

from the bottom up. Since each can-tilever section was approximately bal-anced about the column, the erectionrequired only minimum temporary shor-ing to stabilize the incomplete section.After installing the stringer beam sec-

tions, the erector applied couplings tothe post-tensioning conduit at the splicepoints, installed connection reinforcing,and cast the connection diaphragms.

Next came the installation of thepost-tensioning tendon. The erectorspulled the tendon through the conduitand anchored it at the lower end, apply-ing the jacking forces at the upper end

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and stressing each half of the section inpartial increments to avoid excessivetransverse forces. Grouting of each ten-don conduit completed the post-tensioning operation.

Installation of connection hardware forhandrails at the cantilever ends andcasting of the end diaphragms com-pleted the stringer beam sections. Thecontractor installed the "Z" section seatslabs shortly thereafter. The completeerection and installation of the beamstook 35 days—an estimated savings of84 days over the original cast-in-placesystem.

Concluding RemarksThe adoption of the precast stringer

design concept saved critical construc-tion time. Deck construction operationsunder the original cast-in-place designwould have required extensive siteformwork and skilled concretecrews — interfering with constructionprogress on the lower levels.

The contractor was able to concen-trate aH of his efforts on other portions of

the structure until the final weeks andthen devote full attention to the seating.Completion of the stadium on time wasfinal testimony to the success of theidea. Clemson University now has oneof the finest athletic stadiums in theSoutheast United States.

Credits

Design Engineer: Enwright Associates,Inc., Greenville, South Carolina.

Consulting Architects: Freeman, Wells &Major, Greenville, South Carolina.

General Contractor: Gilbert EngineeringCompany, Statesville, North Carolina.

Precast Concrete Manufacturers:Gifford-Hill, Charlotte, North Carolina.(Prestress Division now owned byExposaic Industries, Charlotte, NorthCarolina.)Macon Prestressed Concrete Com-pany, Macon, Georgia (produced theseating slabs).

Owner: Clemson University, Clemson,South Carolina.

PCI JOURNAL/January-February 1980 23