Trust

Precast Vierendeel TrussesProvide Unique StructuralFacade for Parking Structure

Pradeep H. ShahChief EngineerConrad Associates EastChicago, Illinois

Howard R. MayPresident

Conrad Associates EastChicago, Illinois

A major feature of an all-precast parking structurefor the city of Rock Island, Illinois, is the use ofprecast prestressed concrete Vierendeel trusses.

This article describes the planning and designconsiderations in the prefabrication andprestressing of the trusses and briefly discussessome special structural considerations that areimportant in the design of parking structures.

One major challenge encounteredin the design of a parking struc-

ture is achieving an architecturalcharacter through treatment of itsfacade.

Unlike other commercial buildings,

use of ornamental skin, to cover themajor structural elements, makes thedesign of a parking structure heavy,expensive and unjustifiable.

Therefore, in most parking gara-ges, it is usually a necessity that

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Fig. 1. The Rock Island parking structure, Rock Island, Illinois. Overall view ofstructure from south-east corner.

the "structure become architecture."To meet this requirement, large

precast prestressed concrete Vieren-deel trusses were used effectively inthe design of a parking structure forthe city of Rock Island, Illinois.Trusses held together with other pre-cast components created a uniquestructural facade which unified esthe-tic expression, function and economy(see Figs. 1 and 2).

Description of Structure

The parking deck is a three-levelstructure with overall plan dimen-sions of 236 x 290 ft. Four bays of 59-ftclear spans provide column-free space

Fig. 2. Close-up of the structural facade.

for parking and easy maneuvering ofvehicles. The perimeter of the struc-ture is kept level, while the centraltwo bays slope in the same direc-tion—permitting one-way traffic withangle parking.

Precast prestressed concrete doubletees, 8 ft wide x 24 in. deep, span 59 ftto form the parking floors. In the inte-rior bays, the tees are supported on6-ft high single ledge precast beams atsloping floors and 2 ft 3 in. deep pre-cast inverted tee beams at cross-overs.These beams span between the col-umns spaced at 32 ft.

On the exterior, 16 in. wide x 11 ft10 in. high precast prestressed Vie-rendeel trusses span 32 ft between thecolumns. Bottom and top chords, 16

in. wide x 22 in. deep, of the trussessupport the double tees at the secondlevel and roof, respectively.

Precast spandrels, 4 ft 4 in. high,around the perimeter of the structureact as guardrails. Fig. 3 shows the typ-ical floor framing plan identifying the

different structural precast members.Fig. 4 shows the typical cross sectionsthrough the exterior and interior ofthe structure.

This article discusses the effectiveuse of precast prestressed Vierendeeltrusses, reasons for their use, consid-

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26

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Fig. 4. Cross section through exterior (left) and interior (right) of structure.

erations in determining the physicaldimensions and structural design,how they are prefabricated and whatrole they play in an all-precast systemused in the design of a modern park-ing facility. However, before proceed-ing with this discussion, it is appro-priate to understand the backgroundand the reasoning that went intoselecting the design criteria.

Background

Rock Island, Illinois, is a typicalmiddle sized midwestern city whichis going through an organized pro-

gram of new developments as well asrevitalization within the city's centralbusiness district.

An earlier comprehensive planstudy had recommended constructionof "certain larger scale, visually prom-inent and functionally importantstructures" in the downtown area.This included a 550-car capacity park-ing structure, to be built within aone-block area bounded by 3rd Av-enue (N), 4th Avenue (S), 16th Street(W) and 17th Street (E)—across fromthe city hall.

After the decision was made to goahead with the parking structure, thedesign criteria were laid out by the

PCI JOURNAL/July-August 1977ҟ 27

city officials. The structure was notgoing to be just another anonymousservice building, but, it had to be de-signed to play an important rolewithin the city's revitalization pro-gram. Besides being functional andeconomical, it had to be contemporaryto attract visual interest. In short, thechallenge was to produce a uniquestructure at low cost.

Selection ofStructural System

From the functional parking layout,it was clear that a structural systemwith long span capabilities would bemost economical and desirable. Pre-stressed concrete was the obviouschoice.

A comparison made between thetwo systems using cast-in-place post-tensioned concrete and precast pre-tensioned concrete proved to be al-most even. However, to achieve anesthetic expression, the facade had tobe developed and treated. To makethe facade economically feasible, theuse of nonfunctional ornamental ele-ments was ruled out.

After a careful consideration of allthe factors involved, it was concludedthat if the structural components wereexpressed properly, the desired ar-chitectural effects could be achieved.The interplay between the structuralcomponents could then be enhancedby judicious use of color and textureof exposed concrete surfaces.

The above requirements of interest-ing structural forms, better qualitycontrol and uniform finish coupledwith the speed of erection and econ-omy dictated the use of a precast con-crete system.

Why Vierendeel Trusses?

Two principal reasons for using theVierendeel trusses were as follows:

• Expression of the total structuralsystem to achieve character: Largeopenings formed between the verticalmembers and top and bottom chordsof the trusses allowed full expressionof the double-tee floor system. The in-teresting arrangement of the basicfour structural components (doubletees, spandrels, trusses and columns),made possible only by Vierendeeltrusses, created an attractive facadesatisfying one of the critical designrequirements.• Structural efficiency: Because of itsfloor-to-floor structural depth, oneVierendeel truss carried two floors,thus reducing the number of perime-ter beams (that would be otherwiserequired) in half. Also, supporting thedouble tees on top and bottom chordswithout any eccentricity (whichwould be the case in a conventionalsingle-ledge beam) resulted in elimi-nation of extra reinforcement and spe-cial connections to resist torsion.

Thus, overall economy, not only inmaterials but also in transportationand field erection, was achieved byusing Vierendeel trusses.

Design Considerationsof Vierendeel Trusses

• Physical dimensions: The govern-ing factors in determining the physi-cal dimensions of the trusses weretransportation, and repeated use of thesame forms for varying truss spans.Structurally, it was advantageous touse the maximum height possible.However, precast manufacturers fromthree states, namely, Illinois, Iowaand Wisconsin were interested inbidding on the job. Clearance underthe bridges on-route from all threeprecasting plants to the job-site dic-tated that the maximum height oftrusses be about 12 ft.

Another significant factor thatwould affect the transportation costsubstantially was the weight limita-

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tion of 44 kips without a special per-mit. Keeping these considerations inmind, 16 x 22-in, cross-sectional di-mensions for top chord, bottom chordand verticals with an overall trussheight of 11 ft 10 in. were found to beoptimum. This arrangement alsoworked very well for a floor-to-floorheight of 10 ft for parking. The nextstep was to check the system's struc-tural adequacy.• Structural requirements: The struc-tural analysis of the Vierendeel trusswith optimum cross-sectional dimen-sions showed a definite need for pre-stressing in the top and bottom chordsas well as the verticals. This require-ment of two-way prestressing createda problem as far as pretensioning ofthe truss in the precasting bed wasconcerned.

Earlier talks with the precast man-ufacturers had indicated that somewould prefer conventional pretension-ing methods while others would pre-fer post-tensioning the trusses out ofthe precasting beds. Therefore, to ac-commodate all the structural require-ments and to facilitate production forall the three precasters, the trusseswere designed with a combination ofmild steel plus prestressed reinforce-ment.

The choice in prestressing methodswas also given—whereby trussescould be pretensioned either conven-tionally by use of strands, or after theywere cast by use of post-tensioningtendons. In the case of pretensioning,in order to develop the full force inthe strands at the ends of the mem-bers where it was structurally re-quired, hydraulically seated strandanchors were specified. In the case ofpost-tensioning the tendons, groutingwas found necessary and prescribed.

Structural Design

criteria as those used for the rest ofthe structure. For reasons discussedlater in this article, a crack control ap-proach was employed for the struc-tural design.

The tensile stresses were keptbelow 6under realistic workingload conditions. The ultimate capacitywas obtained by supplementing theprestressing steel with mild steel rein-forcement. While the analysis of theVierendeel truss was carried out usingcenter line dimensions, their designwas based on forces at the faces of themembers.

The following is a summary of thefinal design adopted:

A prestressing force of 300 kips wasprovided in the top and bottomchords. Twelve '/2-in. diameter 270-ksistrands or four 1-in. diameter 150-ksithreaded bars in each chord werespecified at the time of design. In thefinal fabrication, threaded bars wereused.

In the first interior vertical memberof the truss, a prestressing force of 228kips was provided by two 1 1/4-in. di-ameter 150-ksi threaded bars. A con-crete strength of 6000 psi at 28 daysand 3500 psi at the time of prestress-ing was specified.

Fig. 5 shows the elevation of a typi-cal 32-ft long Vierendeel truss withdimensions. Cross sections throughthe top and bottom chord and the ver-tical show the reinforcing steel de-tails.

Prefabrication ofVierendeel Trusses

The precasting of all the structuralcomponents was done in the plant ofBlakeslee-Midwest Co., Rochelle, Il-linois, located approximately 100miles from Rock Island.

The prefabrication of Vierendeeltrusses was done in the following

The structural design of the Vieren- stages:deel trusses was based on the same The complete formwork for the

PCI JOURNAL/July-August 1977ҟ 29

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longest truss was set up using steelforms. Fig. 6 shows the formwork for a37-ft span set up on a "tilt" table. Twosuch forms were used to cast all 38trusses varying in spans from 9 to 37ft.

Fig. 7 shows the prefabricated cageof mild steel reinforcement set up onanother table. Fig. 8 depicts the cagebeing lowered in position into theformwork, and Fig. 9 shows the rein-

forcing cage being tied in the final po-sition.

Next, the post-tensioning tendonswith ducts were carefully placed andaligned making sure that the anchorsat the ends were in proper location.

Concrete having a specified designstrength of 6000 psi at 28 days wascast. The fresh concrete was vibratedby externally attached form vibrators.

Fig. 10 shows the truss being tilted

Fig. 6. Formwork for a typical 37-ft long truss set up on a "tilt" table.

Fig. 7. Prefabricated cage of mild steel reinforcement set up on another table..

PCI JOURNAL/July-August 1977 31

in position for form removal and Fig.11 shows the truss being carried outwhere it was set up for the prestress-ing operations.

The trusses were then prestressedand grouted in the precaster's yard.Now, they were ready to receive thearchitectural finish.

Fig. 12 shows the truss beingsandblasted and Fig. 13 shows thefinished trusses being stored.

Design of ConnectionsThe trusses were designed as sim-

ple spans, so no fixity was required atthe top and bottom connections. Thebottom connection was made by weld-ing a "pin" (3/4 x 1½-in, bar) to thebearing plate already cast in the truss.

The top connection between thetruss and the column was provided forlateral stability only. A bolted connec-

Fig. 8. Mild steel reinforcing cage being lowered in position into formwork for truss.

Fig. 9. Reinforcing cage being tied in its final position in the formwork.

tion with a slotted hole was furnishedto permit the movement of the trussdue to live load, creep, shrinkage andtemperature changes.

Fig. 14 shows the typical exteriorcolumn (lying horizontally in the pic-ture) with a notch to receive the truss.

Fig. 15 depicts the top and bottomconnections between the truss and thecolumn.

Fig. 16 shows a typical exterior col-umn with two trusses bearing on it.

The connections between thebeams and columns in the interiorbays were made by using embeddedstructural shapes. Stability connec-tions were made by using highstrength bolts. All field connectionswere designed as bolted connectionsto reduce erection time and the cost ofthe structure.

Erection ofPrecast Components

Erection of the two interior bayswith conventional beam-column fram-ing was done first. The exterior col-

Fig. 10. Vierendeel truss tilted in positionfor stripping.

umns and the trusses were set nextwith the floor and roof double tees fol-lowing. The precast spandrel guard-rails were bolted to the perimetertrusses completing the precast erec-tion. Figs. 17 through 20 show theerection of the structure at variousstages.

In total, 733 precast pieces wereused to frame an approximate area of123,000 sq ft in the main structure,

Fig. 11. Truss being carried out in the yard for prestressing operations.

Fig. 12. Sandblasting of Vierendeel truss.

Fig: 13. Storage of the finished trusses in the yard.

Fig. 14. Typical exterior column (shown in horizontal position) with a notch forVierendeel truss.

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four stair towers and one elevatortower. Some 607 pieces consisting of38 Vierendeel trusses, 75 columns, 50single ledge beams, 35 inverted teebeams, 135 spandrel guardrails and274 double tees provided framing forthe main structure.

Erection time for the main structurewas 34 work days, averaging 18 piecesper day. In all, 126 pieces consistingof 97 stair panels, 16 stairs and 13hollow-core slabs provided framingfor the four stair towers and oneelevator tower. Erection time forthese towers was 18 work days averag-ing 7 pieces per day. The total job waserected in 52 work days with an aver-age of 14 precast pieces erected perday.

Additional Features• Architectural finish on precastcomponents: The desired architec-tural effects were achieved by varyingthe color and texture of the finishedsurfaces of the precast components.

The Vierendeel trusses and exteriorcolumns were cast with warmtonecement, limestone coarse aggregatesand a special sand. The surfaces werelightly sandblasted. Spandrel guard-rail surfaces were deep waterblastedto expose hard pebble aggregatescreating an interesting contrast. Theexposed ends of the double tees werefinished smooth with grey cement.• Stair towers: One stair tower waslocated in each corner of the structureto highlight serviceability and usersecurity.

These towers were framed with fullheight precast concrete wall panelsand glass on the front side. The panelswere cast in an "L" shape or channelshape to avoid the problem of cornermatching and to soften the visual im-pact of the masses.

The front face of the tower wasframed with glass to create a feeling of

"openness." An exposed pebbleaggregate finish on the warmtoneconcrete panels harmonized the tow-ers with the rest of the structure (seeFig. 21).

Special Considerations

Considerations that are special toparking structure design come fromthe fact that the garages are:

1. Constantly exposed to extreme

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PCI JOURNAL/July-August 1977ҟ 35

weather changes (as regards moisture,temperature, and wind).

2. Continually used under corro-sive conditions caused by such factorsas rain, ice, snow and road salts.

The preceding factors manifestthemselves as volume changes in thestructure and drainage and must beaccounted for in the structural design.

For this project, these factors weregiven careful consideration. Particularattention was paid in the structuraldesign and detailing to eliminatewater retention areas and to controlthe cracking of concrete. Volumechanges in the structure were handledby relieving the forces rather than re-sisting them. The following are someof the concepts and details used in theproject:• Limited moment capacity was pro-

Fig. 16. Exterior column and bearingof trusses.

Fig. 17. Erection of precast elements in interior bays.

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Fig. 18. Erection of inverted tee beam at crossover.

Fig. 19. Erection of double tee on bottom chord of Vierendeel truss.

PCI JOURNAL/July-August 1977ҟ 37

vided at the base of exterior precastcolumns to form a "hinge" under cer-tain loading conditions.• Precast columns were kept free ofcast-in-place foundation walls by pro-

viding a 1-in, joint around the col-umns.• Connections between precast com-ponents were designed such that theyprovided the necessary stability to the

Fig. 20. Progress photograph taken in September 1976.

Fig. 21. Stair and elevator towers in north-east corner.

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Fig. 22. Completed structure from north-east corner.

structure without making it too rigid.• Several expansion joints were pro-vided in the structure. The main ex-pansion joint ran through the middleof the building in the east-west direc-tion dividing the structure into twoparts. Stiff stair towers were also sepa-rated from the main structure.• Control joints were provided in thetopping concrete over the precastdouble-tee flanges where they butt.Tooled joints at every 8 ft on centercreated several inherently weakplanes in the decks to permit move-ment without losing structural integ-rity.• Camber of the prestressed tees atthe time of erection was consideredvery carefully in calculating the slopeof the decks for drainage. Extra top-ping (about 3 in. of concrete) was pro-vided along the perimeter and alongthe beams on the interior to insureproper drainage.• Most precast connections werepatched with concrete to avoid mois-ture penetration. However, some ex-terior connections which could not beprotected satisfactorily (such as bear-ing plates under the exposed teestems around the exterior of the struc-ture) were specified in stainless steel.

Conclusion

The objective of designing a mod-ern parking structure at low cost wasachieved by developing a uniquestructural facade.

Precast prestressed concrete Vie-rendeel trusses were used success-fully to highlight the esthetic expres-sion of the functional and economicalstructural system.

This 565-car parking facility cover-ing a total area of 189,000 sq ft wasconstructed for about $10 per sq ft inless than 9 months despite difficultfoundation conditions, inclementweather and labor strikes.

Since completion in November1976, the structure (see Fig. 22) hasperformed quite satisfactorily.

Credits

Architect-Engineer: Conrad AssociatesEast, Chicago, Illinois.

General Contractor: Priester ConstructionCo., Davenport, Iowa.

Precast Manufacturer: Blakeslee-MidwestPrestressed Concrete Co., Rochelle, Il-linois.

Owner: City of Rock Island, Illinois.

PCI JOURNAL/July-August 1977ҟ 39