Tailor Made Concrete Structures – Walraven & Stoelhorst (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-47535-8

Total precast solution for large stadium projects meet tight schedule

T.J. D’ArcyThe Consulting Engineers Group, Inc., San Antonio, TX, USA

ABSTRACT: A report on the successful application of precast prestressed concrete in large stadium projects.Framing system design and construction methods are presented and details of the construction and design ofthree major league and college stadiums are presented.

Precast prestressed concrete has proven to be anexcellent application in major stadium projects. TheConsulting Engineers Group has provided engineer-ing services for over 70 large stadium projects inNorth America. These applications include stadiumsfor National Football teams, Major League Base-ball teams, National Basketball Association teams,Olympic Stadiums and various College Stadiums.

Stadiums are designed in three manners; eitherthey are open air, enclosed or combination stadiums.Even though they are very expensive, the currenttrend in the major league is to build enclosed stadi-ums with retractable roof structures. Thereby com-bining both open air and enclosed when the weatherdemands it. The Dallas Cowboys stadium, currentlyunder construction, has a budget of one billion dollars(Fig. 1).

Stadium design and construction creates specialdemands. Among them are vibration control, durabil-ity and fast construction speed.

These requirements are a prefect fit for the appli-cation of precast prestressed concrete. Pretensioningof seat sections provides a stiff, crack free member.

Figure 1. Dallas Cowboys Stadium.

Speed can be achieved because construction can betelescoped with production in precast plant proceed-ing while job site foundations advance. In additionhigh strength precast members are more durable, andthe tolerance control of precast members produces fasterection and stadium completion (Fig. 2).

The most typical precast member to all stadiums isthe seat riser section. They have been cast one riser,two riser and three risers high and are pretensioned.The most typical shape is two risers high with lengthsup to 42 feet.

Vomitory members are also typically precast, as arewall panels and stairs. Precast concrete has been alsoemployed in raker beams, columns, ramps and con-course framing.The fixed roof or retractable roofs haveall been steel construction except for fabric roofs. Fig-ure 3 is a section through a typical precast stadiumframing plan.

As noted previously, precast seat members (risers)are the most common precast product in stadium con-struction. They are typically cast two levels high (each

Figure 2.

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Figure 3. Section of a typical concourse and seating areaframing.

Figures 4a & 4b. Section of a typical stadium riser.

1′–8′′ high) and the tread is 2′–8′′ wide and are preten-sioned with span up to 42 feet.The key design feature isto minimize excessive deflection and vibration in orderto not cause the occupants concern. Excessive vibra-tion can lead to panic in the stands. The live loading ina stadium is very dynamic and must be accounted for.Most precast prestressed standard sections are sym-metrical about their vertical axis. However, stadiumriser units are not (Fig. 4) and it is necessary to cal-culate properties about the principal axis. Strengthdesign is complex (Fig. 4b) and may be approximatedor computed graphically using automated computerprograms.

Regarding vibration control, the seating units have athree-dimensional nature and vibrate and deflect abouttheir weakest principal axis.

The minimal natural frequency of the cross-sectionis first determined which is then compared to theforcing frequencies in Hertz.

Excessive deflection in supporting beams suchas rakers must also be controlled to make sure theoccupants aren’t alarmed.

Figure 5. Match cast moment frame.

While ideal sight lines would require many variableriser heights for economical reasons, primarily formcosts, it is best to limit the numbers of different riserheights.

Raker beams with their saw toothed tops are the keymember supporting the seating units (Fig. 3). The topmost raker beam is typically a long member with asimple span and a cantilever at each end. Because ofthe heavy dynamic load these members typically havea large cross-section. They are typically cast on theirside and are pretensioned for handling and erection.Once in place and under load they are post-tension toprovide the remainder of the load capacity primarilythe negative moment at the cantilever.

These raker beams can be located very high inthe air; therefore, casting-them-place is difficult. Inaddition the tolerance control employing precast rakerbeams is much superior over cast-in-place beams.Cast-in-place raker beams frequently require a greatdeal of shimming and product trimming to get the seatsection to properly fit. While precast raker beams haveoccasionally been supported by cast-in-place columns,the more typical and efficient method is to employ pre-cast columns (Fig. 5). Since the columns are typicallyover 100 feet long they are broken into several piecesand spliced with various means. These tall columnsalso can support intermediate concourse levels orlower raker beams. These tall columns and horizontalbeams at different levels were connected by verticalpost-tensioning to create moment frames to provide

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Figure 6. Sun Coast Dome.

a very stable base for the concourse and upper rakerbeam (Fig. 5).

The beams and columns were match cast at eachlevel so that no grouting and dry packing of the jointswas required. Although an elaborate forming systemwas required for the match casting, the erector was ableto proceed at a fast rate and no expensive scaffoldingwas required to grout the joints. They were sealed withepoxy applied on the ground just prior to lifting.

These moment frames for the Citrus Bowl Stadiumin Orlando, Florida were designed for hurricane inten-sity winds and lateral loads. Similar precast frameshave been designed and constructed to resist seismicloads.

I shall now describe three entirely different projectsthat employed total precast solutions for the structuralframing.

1 SUN COAST DOME, ST. PETERSBURG, FL

While domed stadiums built prior to the Sun Coastdome had employed various precast components, theSun Coast Dome was the first domed structure inwhich all structural elements and elevated seatingare completely precast concrete (except for the cablesupported fabric roof). (Fig. 6)

The diameter of the stadium is 688′ (210 m) witha height of 225′ (69 m). The cable supported roofemploys a tension cable and compression post systemoriginated by Bucksminster Fuller termed a “tenseg-rity” system which produces an exterior compressionring and an interior tension ring (Fig. 7).

Figure 7. A schematic of stadium showing the basic roofframing system.

The original design of the structure called for acast-in-place concrete framing system which was overbudget with a poor construction schedule. The Con-sulting Engineers Group working with a precasteroffered a total precast concrete design alternate whichprovided savings in cost and time.

The Consulting Engineers Group was then hired tocompletely redesign the cast-in-place frame to a pre-cast system. The stadium construction consists of twobasic components, each made up of several uniqueprecast members.

The first component is the ring beam which sup-ports the cable roof system. The second component isthe concourse and seating area framing.

The ring beam system is comprised of 6′ diameterhollow column, 170′ long cast in 56′ lengths. Sittingatop the column is a steel knuckle assembly which sup-ports the twin ring beam members each 8′–6′′ high,16′′ thick and 90′ long (Fig. 8). A precast deck slabspan between the two parallel ring beams completingthe ring framing system. The seating and concourseframing was more or less conventional stadium fram-ing with precast pretension seat sections, combinationpretension and post-tensioned raker beams, precastcolumns and beams, vomitory panels and slabs, andconcourse double tees plus precast circular pedes-trian ramps. The floor member of the circular rampwas a special double tee cast in warped position toaccommodate the varying inside and outside slopes.

Special site erection requirements necessitated theuse of a precast dead men which were made up ofprecast blocks that were bolted together then as neededat another location disassembled and rebolted together.

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Figure 8. Cross-section of assembled ring beam showingpost-tensioning and mild steel reinforcement.

The stadium, containing over 7,000 precast com-ponents, was constructed on time and within budgetrepresenting considerable savings to the owners.

2 CITRUS BOWL STADIUM, ORLANDO, FL

This is another all precast structure system which wasbid as a design build project. Besides the tall, spectac-ular precast framing system, the structure was uniquein that the penalty damages if one key completion daywas missed were $24,000,000 for the one loss day.Vibration control was of extreme importance since aprevious steel frame stadium was turned down becauseof excessive vibration causing panic in the stands – theprecast solution performed extremely well in vibrationcontrol.

Several design and construction decisions weremade to speed construction and make sure the dead-line was met. The upper seating section is over170′ from the ground and the available footprintonly allowed a 28′ column spacing for the build-ing frame. To withstand hurricane wind forces, theframe components were vertically post-tensioned tothe foundation using post-tension bars. The rakerbeams were also connected to the support columns bypost-tensioning.

To speed up erection, it was decided to match castthe support columns to the beams located at variousconcourse levels. This process eliminated the need for

Figure 9. Completed Citrus Bowl.

Figure 10.

Figure 11. University of Oregon Stadium.

grouting as dry packing from scaffolding of framejoints (Fig. 5).

Precast pretension double riser tee seat sectionswere also employed.

In addition, to complete the all precast solution,the sloping ramps were constructed with load bearingprecast architectural finish panels and warped precastfloor slabs to create a constant sloping ramp.

The total precast framing system met the tightone-year construction schedule (from Bowl game toBowl game) and was completed one month before thedeadline (Fig. 9).

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3 UNIVERSITY OF OREGON STADIUM,EUGENE, OR

This stadium was also conceived to be constructedwith a cast-in-place framing system. However, costover runs and scheduling problems lead to the consid-eration of a total precast redesign. The redesign wasconducted by The Consulting Engineers Group andaccepted by the architect.

The architectural design of the primary bent sup-ports for the raker beams were large “V” shapedframes. As drawn, the frames were too large to pre-cast and ship. The legs of the “V” were then split intotwo components and match cast. The “V” componentswere reassembled in the field. The joints epoxy sealedand the section post-tensioned together (Fig. 10).

The stadium is located in the west coast, highseismic region so all precast components and theirconnections required consideration of seismic forces.

This stadium was constructed to meet a tight sched-ule related to the University football schedule (Fig. 11).

4 CONCLUSION

Precast framing solutions for major sport stadiums hasproven to be a viable economical solution and able tomeet extremely tight schedules. Careful design consid-erations of vibration characteristics of precast seatingmembers have produced structures that are vibrationstable providing security and comfort to attendees.

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