Loadbearing Architectural Precast

Sidney FreedmanDirector Architectural PrecastConcrete ServicesPrecast/Prestressed Concrete InstituteChicago, Illinois

Architectural precast concrete wall panels that act as loadbearingelements in a building are both a structurally ef f icient andeconomical means of transferring floor and roof loads through thestructure and into the foundation. In many cases, this integration canalso simplify construction and reduce costs. This article presents them a ny benef its that can be derived f rom using loadbearingarchitectural precast concrete walls in buildings. Discussed herein arethe various shapes and sizes of wall panels, major designconsiderations, and when loadbearing or shear wall units should bethe first design choice. The role of connections, shear walls, and theuse of precast concrete as forms for cast-in-place concrete isexplained. In general, the design methods and techniques presentedin this article apply to buildings in both seismic and non-seismicareas. The latter part of this article shows how these design principlescan be applied in practice in a variety of buildings. These examplesillustrate the use of window wall panels, spandrels, and solid orsandwich wall panels as the loadbearing wall members. When all theadvantages of using arc h itectural precast concrete as loadbearingwalls are added up, it makes good sense to use this structural form inbuilding applications.

Loadbearing facades have both anaesthetic and structural function.In building practice, the most

economical application of architec-tural precast concrete is as loadbearingstructural elements. Loadbearing unitsbecome an integral part of the struc-ture, taking the vertical and horizontalfloor and roof loads, and/or transfer-ring horizontal loads into shear wallsor service cores. Such an arrangement

can be economical, not only from astructural design standpoint, but alsofrom the viewpoint of overall con-struction. In some cases, the loadbear-ing elements also can contribute to thehorizontal stability of the building.

Architectural precast concretecladding is noted for its diversity ofexpression, as well as its desirablethermal, acoustic and fire-resistantproperties. Commonly overlooked is

Loadbearing Architectural Precast Concrete Wall Panels

92 PCI JOURNAL

September-October 1999 93

the fact that concrete elements nor-mally used for cladding applications,such as solid wall panels, window wallor spandrel panels, have considerableinherent structural capability.

In the case of low- or mid-rise struc-tures, the amount of reinforcement re-quired to handle and erect a precastcomponent is often more than neces-sary for carrying imposed loads. Thus,with relatively few modifications,many cladding panels can function asloadbearing members. For taller build-ings, additional reinforcement may benecessary for the lower level panels.

The slight increases in loadbearingwall panel cost (due to reinforcement

and connection requirements) canusually be more than offset by theelimination of a separate perimeterstructural frame. Depending upon theapplication, the loadbearing panelsalso may reduce or eliminate a struc-tural core or interior shear walls, par-ticularily in buildings with a largeratio of wall-to-floor area. The in-crease in interior floor space gainedby eliminating columns can be sub-stantial and, depending on the floorplan, partition layout flexibility can bee n h a n c e d .

To take maximum advantage of load-bearing units, decisions as to their func-tions should be made before structural

design has progressed to a stage whererevisions become costly. Cost savingstend to be greatest in low- to mid-risestructures of three to ten stories. Aswith all precast concrete applications,further economies can be realized if thepanels are repetitive. Besides minimiz-ing the number of casting forms neces-sary, repetitive panel designs enablerepetitive connections.

Architectural loadbearing panels canbe used effectively to renovate and re-habilitate old deteriorated structures.These panels can be used not only inall-precast structures but also in struc-tural steel framed structures and cast-in-place concrete structures.

Fig. 1. Various types ofarchitecturalloadbearing wall panels.

(a) Flat, hollow-core, orinsulated panel.

(b) Vertical windowor mullion panel.

(c) Horizontal window or mullionpanel.

(d) Ribbed panel. (e) Double-tee panel. (f) Spandrel (same as “a”).

94 PCI JOURNAL

Guidance for using loadbearing ar-chitectural precast concrete wall panelscan be found in Refs. 1 and 2. Otherpertinent information on this subjectcan be found in the list of references.

SHAPES AND SIZESArchitectural load carrying compo-

nents can be provided in a variety ofcustom designed or standard sectionshapes. A wall system can be com-prised of flat or curved panels (solid,hollow-core, or insulated) (see Figs.1a and f), window or mullion panels(see Figs. 1b and c), ribbed panels(Fig. 1d), or a double-tee (see Fig. 1e).Each type of panel will readily accom-modate openings for doors and win-dows. Fig. 1b, c, d and e illustrate var-ious types of ribbed panels. The panelshown in Fig.1c is a horizontalVierendeel truss window mullionpanel, while the other panels are verti-cal window mullion panels. Fig. 1fshows an exterior horizontal spandrelas part of a column-wall system.*

In the interest of both economy andfunction, precast panels should be aslarge as practical, while consideringproduction efficiency and transporta-tion and erection limitations. By mak-ing panels as large as possible, numer-ous economies are realized t h enumber of panels needed is reduced,fewer joints (waterproofing require-ments), lower erection cost, and fewerconnections are required. Panels maybe designed for use in either verticalor horizontal positions.

For low-rise buildings, by spanningloadbearing panels vertically throughseveral stories, complex connectiondetails can be minimized, and conse-quently the economic advantages ofloadbearing wall panels are increased.

For high-rise buildings, it is normallymore practical to work with single-story horizontal panels connected ateach floor level. The elements can bemore slender, simplifying the erection.

Ref. 3 discusses the use of horizontalpanels on the 16-story United BankTower in Colorado Springs, Colorado(see Fig. 2). The single-story panels aretypically 14 ft 6 in. x 16 ft x 8 in. (4.42x 4.88 m x 203 mm) thick with 13 x 30

Fig. 2. United Bank Tower,Colorado Springs,Colorado.

Fig. 3. Single-story wallpanels.

*In most cases, when the term panel is used in thetext, it refers to all panels shown in Fig. 1.

in. (330 x 762 mm) monolithically castpilasters at the ends (see Fig. 3).

Multistory panels usually do not ex-ceed 45 ft (13.7 m) in height themaximum transportable length inmany states (see Fig. 4). Panels shouldbe designed in specific widths to suitthe building s modular planning.When such a building is designedproperly, the economic advantages ofloadbearing wall panels are signifi-cantly increased.

Panel dimensions generally are gov-erned by architectural requirements.Most shapes, textures, and surface fin-ishes commonly associated withcladding are possible, provided struc-tural integrity and other technical re-quirements can be satisfied at thesame time.

Load uniformity is one of the im-portant advantages for high-rise, load-bearing panel structures where thebearing walls also serve as shear


walls. It produces even loads on theperimeter foundations and reduces thetendency for differential settlement.The jointed nature of the facade alsomakes it more tolerant of any differen-tial settlement.

Curves are easily handled by precastconcrete. On curved panels, a continu-ous supporting ledge cast on the insideface is preferred to provide bearing forfloor/roof members and to stiffen thepanels to minimize warpage. The Po-lice Administ ration Building inPhiladelphia, Pennsylvania, made his-tory as one of the first major buildingsto utilize the inherent structural char-acteristics of architectural precast con-crete (see Fig. 5). The building is un-usual in its plan configuration,consisting of two round structuresconnected by a curving central sec-tion, demonstrating the versatility ofprecast concrete for unusual planforms. The 5 ft (1.52 m) wide, 35 ft(10.7 m) high (three-story) exteriorpanels carry the two upper floors androof (see Fig. 6).

Wall panel size and shape can be af-fected by the details and locations ofthe vertical and horizontal panel-to-panel connections. Both gravity loadtransfer between panels and gravityand axial load combinations caused bylateral loadings or size of windowopenings can become the major fac-tors influencing panel structural di-mensions and connection design. Al-though, for most precast exteriorbearing wall structures, it will befound that the gravity dead and liveload condition will control structuraldimensions.

When stemmed floor members,such as double tees, are used, thewidth of loadbearing walls or span-drels should module with the double-tee width. For example, for 12 ft (3.66m) double tees, walls should be 12,24, or 36 ft (3.66, 7.32, or 10.97 m)wide. Local precast concrete produc-ers should be contacted for their par-ticular module.

Inverted tee beams typically areused on interior spans. To minimizefloor-to-floor dimensions, double teesare frequently dapped at interior beamlines and at exterior spandrels. Dap-ping is generally not necessary on ver-tical wall panels.

Fig. 4. The brick-faced precast panels on the Barker Substation, Denver, Coloradoare 36 ft (11 m) tall.

Fig. 5. The Police Administration Building at Philadelphia, Pennsylvania.

Fig. 6. Three-story panels supporting double tees.

96 PCI JOURNAL

DESIGN CONSIDERATIONSIn recent years, tremendous ad-

vances have been made in precast con-crete structural engineering technol-ogy. Greater knowledge regardingconnections and wall panel design hasmade it possible to use architecturalloadbearing precast wall panels morecost effectively. Solid panels, or pan-els with small openings, constitutetrue bearing walls as their major

stress is in compression. Uniaxialbending from gravity loads is nor-mally only minor and incidental. Withsolid flat panels, load path locationscan be determined easily.

As openings in the wall becomelarger, loadbearing concrete panelsmay approach frames in appearanceand the concentration of load in thenarrower vertical sections increases. Inmultistory structures this load accu-mulates, generally requiring reinforce-ment of the wall section as a column(at panel ends and at mullions betweenwindows) designed for biaxial bend-ing due to load eccentricities.

Loadbearing panels and shear walls,generally, will be supported continu-ously along their lower surface. Theymay be supported by continuous foot-ings, isolated piers and grade beams ortransfer girders. The bearing wall unitscan start at an upper floor level withthe lower floors framed of beams andcolumns allowing a more open spaceon the lower levels.

When this is done, careful attentionmust be paid to the effective transferof the lateral forces to the foundation.As with a vertical irregularity in anybuilding in a seismic zone, the struc-tural engineer should make a carefulassessment of the behavior and detail-ing. In multistory bearing walls, de-sign forces are transmitted throughhigh quality grout in horizontal joints.

As in all precast construction, thetransfer of vertical load from elementto element is a major consideration.Differences in section, shape, archi-tectural features and unit stress resultin a variety of solutions and types ofc o n n e c t i o n s .

Depending on wal l section andfoundation conditions, a loadbearingwall panel can be fixed at the base(shear walls for lateral forces) with theroof elements freely supported on the

Fig. 8. Plan view of possible locations of vertical cores with respect to loadbearingwalls. (Note: Although the building core is an important element of the lateral forceresisting system, it may be insufficient to handle the torsional effects of eccentricallyapplied loads in some of these plans. Also some plans have re-entrant corners thatcreate plan irregularities.)

Fig. 7. Building layouts in which loadbearing panels can be used advantageously.(Note: Caution must be used with irregular plans in regions of moderate or highseismic risk.)

panel. Alternately, depending on theshape of the building, wall elementflexural stresses can be reduced by pinconnecting them at the foundation andproviding shear wall bracings at the

ends or across the building to ensurelateral stability.

The design and structural behaviorof exterior architectural precast con-crete bearing walls depends on the


panel shape and configuration. The de-signer should consider the following:¥ Gravity loads and the transfer of

these loads to the foundation Vertical (gravity) loads are parallelto the plane of the wall, at an eccen-tricity influenced by the geometry ofthe wall, location of load, manufac-turing and erection tolerances.

¥ Magnitude and distribution of lat-eral loads and the means for resist-ing these loads using shear wallsand floor diaphragms Loads inthe horizontal direction may be bothparallel to and perpendicular to theplane of the wall.

¥ Location of joints to control volumechange deformations due to con-crete creep, shrinkage and tempera-ture movements; influence upon de-sign for gravity and lateral loads,and effect upon non-structural com-ponents. Volume change effects canbe evaluated using methods re-viewed in Chapter 3 of the PCI De-sign Handbook.1 Particular cautionmust be exercised at load path tran-sitions, such as at the corners of abuilding where loadbearing andnon-loadbearing panels meet or atre-entrant corners.

¥ Connection concepts and types re-quired to resist the various appliedloads.

¥ Tolerances required for the structurebeing designed with regard to pro-duction and erection for both precastconcrete units and connections, in-cluding tolerances for interfacingdifferent materials.

¥ Specific requirements during theconstruction stage which may controldesigns, such as site accessibility.

Loadbearing or shear wall unitsshould be the primary design consider-ation if one or more of the followingthree conditions exist:

1. There is inherent structural capa-bility of the units due to either theirconfiguration or to sufficient panelthickness. The sculptural configura-tion of units often enables them tocarry vertical loads with only a slightincrease in reinforcement. For exam-ple, the precast concrete units mayhave ribs or projections that enablethem to function as column elementsfor the structure. Ribs may be part ofthe architectural expression, or where Fig. 9. Loadbearing spandrel.

and transfer them to the foundation(see Fig. 8). When the core creates atorsional irregularity, it should be sup-plemented by designing the perimeterpanels as part of the lateral force re-sisting system. Plan irregularities cre-ated by the extended wings of the Cand Z shapes in Fig. 7 are particularlyproblematic in moderate or high seis-mic risk areas. Because the core orbay provides the structural rigidity,panel-to-floor connections can remainrelatively simple. A typical buildingcore may contain an elevator lobby,elevators, stairways, mechanical andelectrical equipment, and space for airducts. While the core is being erectedor cast, the precaster can proceed withthe fabrication of the exterior wallunits and install them as the shear wallor core is being constructed, often re-sulting in saving construction time.

Loadbearing wall panels used toconstruct the core are connected aftererection to form composite T, L, U orbox-shaped sections in plan. The mainadvantages of precast cores versuscast-in-place cores are surface finishquality, faster construct ion, andgreater flexibility of the precast con-crete erection sequencing.

The three conditions do not precludeother situations where loadbearingpanels or shear walls may be used.

Architectural precast panel designdoes not differ from two-dimensionalframe design, once the panel is iso-

flat exposed surfaces are required, ribsmay be added to the back of panels foradditional stiffness. Projections do nothave to be continuous or straight, aslong as no weak point is createdwithin the units. Generally, there is lit-tle cost premium for sculptured panelswhen there is adequate repetition.Similarly, some flat panels (includingsandwich wall panels) may be suffi-ciently thick to carry loads with onlyminor increases in re inforcement.Structural design of panels with insu-lation placed between layers of con-crete (sandwich panels) usually ig-nores the loadbearing capacity of thenon-bearing wythe.4 If possible, thestructural wythe of a sandwich wallpanel should be kept on the tempera-ture stabilized side of the building toreduce thermal stresses due to temper-ature variation.

2. A uniform structural layout ofthe building facilitates distribution oflateral forces from wind or earth-quake loads. Plus, this uniformitylends itself to repetitive, economiccastings (see Fig. 7). This concept isdifficult to employ if the load pathsare continually changing. Cast-in-place topping on precast concretefloor units enable the floors to act asdiaphragms, dis t ributing lateralforces, reducing both individual wallunit loads and connections.

3. The building has a central core orbay designed to absorb lateral forces

98 PCI JOURNAL

lated and taken as a free body. Ac-cepted design procedures and code re-quirements apply. Perhaps the onlydesign consideration difference is rec-ognizing the role precast concretepanel production and erection play inthe overall design process. Similarly,usual accepted procedures and coderequirements apply to the design of anindividual precast concrete panel andits components.

When spanning horizontally, panelsare designed as beams; or, if they havefrequent, regularly spaced windowopenings as shown in Fig. 1c, they aredesigned as Vierendeel trusses. A hor-izontal Vierendeel truss type panellends itself to simple handling since itis shipped in its erected position, re-quires vertical load transfer connec-tions at each story level, and requiresonly minimum erection handling anderection bracing.

A two-story vertical panel requiresadditional erection handling because itneeds to be rotated during erection,and because it demands more sophisti-cated erection bracing. When the pan-els are placed vertically, they usuallyare designed as columns, and slender-ness should be considered (see Sec-tions 3.5 and 4.9 of the PCI DesignHandbook, Fifth Edition1). If a largeportion of the panel is a window open-ing, as shown in Fig. 1b, it may benecessary to analyze the member as arigid frame.

Loadbearing spandrel panels are es-sentially perimeter beams, that mayextend both above and below the floorsurface, and transfer vertical loadsfrom the floor or roof to the columns.Except for the magnitude and locationof these additional vertical loads, thedesign is the same as for a non-load-

bearing spandrel. Loadbearing span-drels are either ledged, pocketed, orhave individual or button haunches(also known as spot corbels) to sup-port floor/roof members. Steel shapesand plates may be cast in to reducehaunch height and, therefore, floor-to-floor height. Non-loadbearing (clo-sure) spandrel panels may have muchthe same cross section as loadbearingspandrels without ledges, pockets, orhaunches.

Loadbearing members loaded non-symmetrically may be subject to bothinternal and external torsion. If the re-sulting applied load is not coincidentwith the member s shear center, tor-sion will exist along the span of themember. A typical arrangement ofspandrel and supported floor is shownin Fig. 9.

Torsion due to eccentricity must beresisted by the spandrel. When torsionis resisted in this manner in the com-pleted structure, twisting of the span-drel during erection must be consid-ered. Spandrels that are pocketed toreceive stems of the double-tee flooror roof slabs decrease torsion stressesgreatly, as well as minimize twist andeccentricity during erection. If torsioncannot be removed by floor connec-tions, the spandrel panel should be de-signed for induced stresses.

Non-prestressed reinforced concretemembers subject to torsion should bedesigned in accordance with the appli-cable provisions of the ACI BuildingCode, Chapter 11.5 Prestressed mem-bers subject to torsion should be de-signed in accordance with the applica-ble provisions of the PCI DesignHandbook, Chapter 4,1 Design ofSpandrel Beams,6 and the ACI Build-ing Code.

Fig. 10. Principle of

diaphragm action inprecast floors and

roofs.

Precast building elements are com-monly reinforced with welded wirefabric, mild reinforcing steel or pre-stressing steel. Unless analysis or ex-perience indicates otherwise, bothloadbearing and non-loadbearing pan-els should be reinforced with anamount of mild reinforcing steel, asspecified in the appropriate buildingcode, and be at least equal to ρ =0.001.

Lateral loads applied normal to thewall are the result of wind or seismicforces, and are usually transmitted tovertical stiffening cores, shear walls,structural frames, or other stabilizingcomponents by roof and floor mem-bers acting as horizontal diaphragms.This reduces the load on individualwall units and their connections (seeFig. 10). The connections betweenfa ade elements and floor membersare normally designed as hinges in thedirection perpendicular to their plane.

Vertical continuity is achieved byproviding connections at horizontaljoints of vertical members. Columnsshould be braced at each levelthrough a continuous load path to thed i a p h r a g m .

SHEAR WALLSIn many structures, it is economical

to take advantage of the inherentstrength and in-plane rigidity of exte-rior wall panels by designing them toserve as the part of the lateral load re-sisting system. Walls taking horizon-tal loads from the effects of wind orearthquakes are referred to as shearwalls. Shear walls are used as themost common and economical lateralforce resisting system and have beenutilized widely in buildings up to 30s t o r i e s .

A shear wall system s effectivenessis dependent largely upon panel-to-panel connection design. A significantadvantage of jointed construction is inthe inherent ease of defining loadpaths through connections. As such, itis relatively easy to separate a precastconcrete lateral force resisting sys-tem s performance from that of thevertical loadbearing frame.

Shear walls are vertical members,which transfer lateral forces, in or par-allel to the plane of the wall, from su-perstructure to foundation. Thus, shear


walls act as vertical cantilever beams.Shear walls are placed at appropriatelocations within and around the build-ing perimeter according to the archi-tectural and functional design require-m e n t s .7 The 1 ft (0.305 m) thickpanels on the Sarasota County JudicialCenter located in Sarasota, Florida,measure 21 x 15 ft (6.40 x 4.57 m).They serve as shear walls at the cor-ners of the building (see Fig. 11). Con-tinuous steel plate connections werecast into the corner panels to permit awelded connection at the vertical cor-ner joint.

Typically, a structure incorporatesnumerous walls, which can be used toresist lateral forces in both principalaxes of the building. The portion ofthe total lateral force which each wallresists depends upon the wall s bend-ing and shear resistance capacity, theparticipation of the floor, and the char-acteristics of the foundation.

It is common practice to assume thatfloors act as rigid elements for loads inthe plane of the floor, and that the de-formations of the footings and soil canbe neglected. Thus, for most struc-tures, lateral load distribution is onlybased on the properties of the walls.Shear wall building design is per-formed in accordance with Sections3.7 and 3.10 of the PCI Design Hand-book, Fifth Edition.1

The importance of earthquake loadsvaries according to a project s geo-graphic location. Many areas of theUnited States require structural analy-sis for potential earthquake forces invarying degrees of intensity. Concretepanels have the inherent strength toperform as shear walls with little or noadditional reinforcement. It is impor-tant, however, that the connections bedesigned to transfer lateral forces, andalso accommodate thermal movementsand differential deflections (or cam-ber). The ability to transfer lateralforces may be a panel s only structuralpurpose. But, it is more often com-bined with loadbearing capabilities.

Shear walls are economical becausewalls already required by the buildinglayout [such as exterior walls, interiorwalls (see Fig. 12)8,9 or walls of the el-evator, stairway, mechanical shafts orcores] can become structural shearwalls. Load transfer from horizontal

Fig. 12. Building layout showing shear walls for a symmetrical condominiumbuilding plan (Ref. 8).

Fig. 11. Shear wall at the corners of the Sarasota County Judicial Center, Sarasota,Florida.

BUILDING BOUNDARY

100

diaphragm to shear walls, or to eleva-tor walls, stairway cores or mechani-cal shafts, can be accomplished eithervia connections or by direct bearing.Whenever possible, it is desirable todesign shear walls as loadbearing pan-els. The increased dead load acting onthe panel is an inherent advantage be-cause it increases the panel s resis-tance to uplift and overturning forcescreated by lateral forces.

The effect of cumulative loads onconnections between panels must beconsidered, since these loads becomea significant factor in determiningminimum panel dimensions. Shear

walls in precast concrete buildings canbe individual wall panels or wall pan-els which are connected together tofunction as a single unit. Connectedpanels greatly increase shear resis-tance capacity.

Connecting long lengths of wallpanels together, however, can result inan undesirable build-up of volumechange forces. Hence, it is preferableto connect only as many units as nec-essary to resist in-plane shear forcesand the overturning moment. Connect-ing as few units as necessary near themid-length of the wall will minimizethe volume change restraint forces.

In some structures, it may be desir-able to provide shear connections be-tween non-loadbearing and loadbear-ing shear walls in order to increase thedead load resistance to momentscaused by lateral loads. However, inmost cases, an exterior shear wall (orperimeter frame) system providesmore efficient and flexible floor plansthan does an interior shear wall systembecause it eliminates the need for astructural core (see Fig. 13).

Furthermore, exterior shear walls donot affect the interior traffic flow orsight lines. The exterior walls providethe vertical strength and horizontal

Fig. 13. Exterior shear wall system (or perimeter frame).

Fig. 14. Interior shear wall system.Fig. 15. Precast concrete units serving as forms for cast-in-place concrete to tie walls, beams, and floor together.

PCI JOURNAL


connections to allow the entire wall tofunction as a single unit to mobilizedead load overturning resistance. Inaddition, they eliminate the need forexterior columns and beams.

In an interior shear wall system, thelateral forces are not transferred di-rectly to the foundation. Instead, thewall panels distribute the lateral forcesto floor diaphragms, which, in turn,transfer them to a structural core or tothe interior shear walls (see Fig. 14).Frequently, the shear wall panels areconnected vertically and at the cornersto form a structural tube that can-tilevers from the foundation, creatinga stronger element than its indiviudalparts.

PRECAST CONCRETE ASFORMS FOR CIP CONCRETE

Archi tectural precast concre teunits also may serve as forms forcast-in-place concrete. This applica-tion is especially suitable for com-bining architectural (surface aesthet-ics) and st ructural funct ions inloadbearing fa ades, or for improv-ing ductility in locations of highs e i smic risk by using wet cast con-nections with high levels of rein-forcement at the joints. Continuityand ductility are achieved by castingin place the beams and columnsusing precast concrete loadbearingpanels as the exterior formwork.

The ductility of walls depends par-tially upon reinforcement locations.Ductile behavior is improved signifi-cantly if the reinforcement is locatedat the ends of the walls. This way,structurally inactive cladding can be-come a major lateral load (seismic andwind) resisting element. Seismic loadsare resisted primarily by the building scentral core and partly by the ductileconcrete exterior frame. Basically,floor slabs act as diaphragms. Fig. 15illustrates the use of cast-in-place con-crete to tie the walls, beams, and floortogether. This can be an efficient sys-tem for providing lateral resistance inprecast concrete buildings.

The four-tiered colonnade wrappingLiberty Square in Vancouver, BritishColumbia, is constructed of precastpanels that double as formwork forcast-in-place concrete (see Figs. 16and 17). The loadbearing capacity of

Fig. 16. Liberty Square, Vancouver, British Columbia.

Fig. 17. Panels serve as formsfor cast-in-placeconcrete.

the integrated precast and cast-in-place frame resulted in a substantiallystronger wall one that is struc-turally independent of the central core.

The Marathon Plaza, San Francisco,California, consists of two terraced

towers, nine and ten stories high (seeFig. 18). Precast wall panels were de-signed with their edges serving asforms for columns and spandrel beams,thus integrating the panels into a tubeto resist lateral forces (see Fig. 19).

102 PCI JOURNAL

The cast-in-place concrete structureof the 13-story Simmons BiomedicalResearch Building, Dallas, Texas (seeF i g . 20), was cast into forms consist-ing of precast concrete column andspandrel beam covers (see Fig. 21).

CONNECTIONSConnections for loadbearing wall

panels are an essential part of the struc-tural support system. The stability ofthe structure depends upon them.Loadbearing panel connections shouldbe designed and detailed in the same

manner as connections for other pre-cast structural members. It is desirableto design loadbearing precast concretestructures with connections whichallow lateral movement and rotation,and to design the structure to achievelateral stability through the use of floorand roof diaphragms and shear walls.

Connection methods include bolt-ing, welding, post-tensioning, grout-ing, or a combination of these tech-niques. The floor system may or maynot have a structural topping. Design-ers are referred to an extensive treat-ment of design methods in Refs. 1 and

Fig. 18. Construction of Marathon Plaza, San Francisco,California.

Fig. 19. Close-up of edges of panel.

Fig. 20. Simmons Biomedical Research Building, Dallas, Texas.

Fig. 21. Panels serve double duty as formwork for columns and perimeterbeams.

2. Often, loadbearing walls have hori-zontal and/or vertical joints acrosswhich forces must be transferred.

Connections must comply with localcodes whose provisions generally varyacross North America. Connectionsmay be subject to functional require-ments such as recessing for flushfloors and/or exposed ceilings. Indi-vidual manufacturers have developedspecific connections over the years be-cause they suit their particular produc-tion and/or erection techniques. How-ever, some basic connection conceptsgoverning design, performance andmaterial requirements can be formu-lated. No attempt has been made tosize or detail individual pieces, weld-ing or anchorages of the connectionsshown in this discussion. Instead, this


is an engineering task required foreach individual project.

Horizontal joints in loadbearingwall construction usually occur atfloor levels and at the transition tofoundation or transfer beams. Thesejoints may connect floors and walls orwall units only. The principal forcesto be transferred are vertical and hori-zontal loads from panels above andfrom the diaphragm action of floors l a b s .

Horizontal joint and connection de-tails of exterior bearing walls are espe-cially critical, because the floor ele-ments usually are connected at thiselevation, and since a waterproofingdetail must be incorporated. Verticaljoints may be designed so that the ad-jacent wall panels form one structuralunit (coupled), or act independently.In addition to the vertical shear forcetransfer due to lateral loads, verticaljoints also may be subject to shear

forces induced by differential loadsupon adjacent panels.

The stability of the structure duringconstruction must be considered whenplanning erection procedures. There-fore, temporary guying and/or bracingmust be provided until final structuralstability is achieved in the completedstructure. This bracing design is theresponsibility of the precast concreteerector and should be shown on abracing plan prepared by the erector.Sometimes, the bracing plan requiresreview by the engineer of record andbuilding officials.

Connections that transfer vertical orlateral loads from panel to panel maydiffer according to the particularbuilding. Gravity load transfer oftencan be achieved with simple weldplate connections because of con-crete s inherent strength in compres-sion. Mechanical reinforcement spliceconnections may be required to pro-

vide uniform load paths for tensileforces. Lateral connections can allowrotation (pin connections) or be rigid(moment connections), dependingupon the structural system selected.

Wall-to-Foundation Connections

Wall-to-foundation connections areused to tie loadbearing walls to thefoundation (see Figs. 22 to 26). Anyconnection joining a wall panel to afoundation wall or a continuous foot-ing should provide a means of levelingand aligning the panel. The attachmentmethod also should be capable of ac-cepting the base shear in any direction.In cases where an interior core carrieslateral loads, this may be accomplishedwith a simple welded connection.

Slab-to-Wall Connections

Slab-to-wall connections are madeto join precast or cast-in-place concrete

Fig. 22. Wall to Foundation (WF1). Fig. 23. Wall to Foundation (WF2).

104 PCI JOURNAL

floor or roof members to precast con-crete walls (see Figs. 27 to 30). Con-nections joining the slabs and wallsmay require load transfer or bearing,diaphragm action, and moment resis-tance. Secondary forces caused bytemperature fluctuations, long-termshrinkage and creep, and bending mo-ments induced by end restraints, areusually of minor importance.

Gravity transfer may be through acontinuous ledge (corbel) or individual(spot) corbels or connection hardware.

Blockouts in wall panels can also beused to support floor members. Suchpockets in wall panels or spandrelsgreatly decrease torsion stresses, and

also minimize twist and eccentricityduring erection. These pockets requiresubstantial draft on their sides [1/2 in.(12.7 mm) every 6 in. (152 mm)depth] and should have at least 21/2 in.(64 mm) cover to the exposed face.More cover [3 in. (76 mm) minimum]is required if the exterior surface hasan architectural finish.

In the case of a fine textured finish,there can be a light area (the approxi-mate size of the blockout) shown onthe face of the panel due to differentialdrying. This will usually be apparent,despite the uniformity of the texture.The initial cure of the 21/2 to 3 in. (64to 76 mm) of concrete versus 8 to 9 in.

(203 to 229 mm) in the surroundingarea will make the difference.

When the slab functions as a di-aphragm, the connections must trans-mit diaphragm shear and chord forcesto a structural core, thus reducing theload on individual exterior wall orspandrel units and their connections.In those instances, simple welded con-nections can be employed to join pan-els. When the wall participates moreactively in lateral or shear resistance,larger and more numerous weldedconnections are required. Flat,stemmed or hollow-core slabs may beused. When the slab is used with acomposite topping, some connections

Fig. 26. Wall to Foundation (WF5).



September-October 1999

onnection ishe degree of

a judiciouscombinationr by welding in the flooreel can also

be doweled, threaded, or welded tothe walls.

Connections to shear walls along the(non-bearing) sides of floor or roofslabs should be able to transmit lateralloads and should either allow somevertical movement to accommodate

Fig. 27. Slab to Wall (SW1).

may be necessary to achieve stabilityof the structure during erection withthe final diaphragm connectionachieved using dowels from the wallto the topping.

Most designs result in some degreeof continuity for these connections.

However, a fully fixed cgenerally not desirable. Tfixity can be controlled by use of bearing pads in with clamping forces, oto anchor plates placedmembers. Reinforcing st


105

PCI JOURNAL

Fig. 32. Wall to Wall (WW2).


106


Fig. 31. Wall to Wall (WW1).


camber and deflection of the floorunits, or be designed to develop forcesinduced by restraining the units.

Wall-to-Wall Connections

Wall-to-wall connections are pri-marily intended to position and securethe walls, although with proper designand construction, they are capable of

carrying lateral loads from shear wallor frame action as well (see Figs. 31 to33). The two locations of wall-to-wallconnections are horizontal joints (usu-ally in combination with floor con-struction) and vertical joints.

The most practical connection is onethat allows realistic tolerances and en-sures transfer of load between panels.It also is desirable that the connection

Fig. 34. Twenty-story Mutual Benefit Life, Philadelphia,Pennsylvania.

Fig. 35. Window wall panels serve as elements of Vierendeeltruss on One Hundred Washington Square office building,Minneapolis, Minnesota.

Fig. 33. Welded Alignment

(WA2).

require no grouting or at the very leasta minimum amount of field grouting.

APPLICATIONSIn the last 40 years, many tall struc-

tures have been constructed with load-bearing architectural precast concretewindow wall panels. Among them isthe 20-story Mutual Benefit Buildingin Philadelphia, Pennsylvania, built in1969 (see Fig. 34). The panels mea-sure 12 ft high and 20 ft wide (3.66 x6.10 m) and each has four openings.The mullions are designed for columnaction. Spandrels are hidden behinddark glass panels permitting an accentof vertical lines.

Loadbearing sandwich window wallpanels for the 20-story, 300 ft (91 m) tallOne Hundred Washington Square officebuilding in Minneapolis, Minnesota, are13 ft high and 10 ft wide (3.96 x 3.05 m)(see Fig. 35).1 1 , 1 2 They have a 16 in. (406mm) interior wythe, 21/2 in. (64 mm) ofinsulation and a 3 in. (76 mm) exteriorskin. The corner columns have claddingat the base and then serve as insulatedformwork for cast-in-place concrete forthe rest of the height.

108 PCI JOURNAL

The 32-story Tannen Towers condo-minium project in Atlantic City, NewJersey, completed in 1987 uses portalframes at the base, and bearing wallsin the upper levels (see Fig. 36). Thebuilding is subdivided from top to bot-tom by a central corridor. A row of 37ft (11.3 m) long bearing walls, whichare typically 8 in. (203 mm) thick,runs along either side of the corridor.The walls cantilever 11 ft (3.35 m) be-yond the face of the base structure onboth sides of the building. To stabilizethe structure, the design links pairs ofbearing walls across the corridor withsteel ties back-to-back angles rein -forced with a continuous plate.

The majority of loadbearing build-ings built in recent years, however,have been less than ten stories.

A typical application of loadbearingspandrels is in parking structures. TheBoston College parking structure inChestnut Hill, Massachusetts, is an en-tirely precast/prestressed concretebuilding comprised of precast columnsand beams with precast spandrels sup-porting double tees (see Fig. 37a).1 3

An early construction shot of the park-ing structure (see Fig. 37b) shows theerection of the precast frame.

The Barnett Bank parking facility inJacksonville, Florida, is part of a cam-pus serving as a national headquarters(see Fig. 38). The loadbearing span-drels, which are 40 ft (12.2 m) long,eliminate the need for cladding andframing and make use of the sectionproperties of the framing members.

The six-story precast concrete Com-munity Service Building Garage inWilmington, Delaware, fits in with theneighboring architectural landscape incolor, style, and ornamentation (seeFig. 39). In addition, the building isdesigned to serve as the architecturalbase for a future nine-story officebuilding. The cornice spandrel at theroof is over 4 ft (1.22 m) thick andweighs over 80,000 lbs (36290 kg).The elevation adjacent to an existingbuilding and invisible from the streetis constructed of precast concreteloadbearing shear walls.

The architect for the Interlocken Of-fice Campus in Broomfield, Colorado,innovatively used the same molds tocreate three unique all-precast officebuildings (see Fig. 40a). Architectural

Fig. 36. The 32-story Tannen

Towers, Atlantic City,New Jersey.

Fig. 37a. The Boston College parking structure in Chestnut Hill, Massachusetts.

Fig. 37b. Early construction shot of the Boston College parking structure.


Fig. 38. Barnett Bank parking facility, Jacksonville, Florida.Fig. 39. Community Service Building Garage, Wilmington,Delaware.

Fig. 40a. Interlocken OfficeCampus, Broomfield,Colorado.

Fig. 40b. Construction showinguse of columns andspandrels inInterlocken OfficeCampus.

110 PCI JOURNAL

loadbearing spandrels and columnsminimized the contractor s time andrisk in completing the core and shell(see Fig. 40b). The architectural exte-rior used an acid-etch finish with twocolors and an ashlar stone formliner.

The all-precast 10-story InternalRevenue Service office building inOklahoma City, Oklahoma, has 8 in.(203 mm) thick prestressed rectangu-

lar spandrels supporting 44 ft long x10 ft wide x 28 in. deep (13.4 m x3.05 m x 711 mm) double tees (seeFig. 41).1 4

The horizontal mass of the CrescentVIII building in Crescent Town Centerin Denver, Colorado, is broken by ex-pressed vertical pilasters (see Fig.42a). The architect wanted to maxi-mize the window space while also

Fig. 41.Internal Revenue

Service officebuilding, Oklahoma

City, Oklahoma.

Fig. 42a. The Crescent Town Center Campus in Denver Technological Center,Denver, Colorado, features a series of buildings designed around an open,park-like crescent.

Fig. 42b. The use of integrated loadbearing/architectural spandrel panelserected in 8 weeks facilitated a tight construction schedule (The CrescentTown Center Campus, Denver, Colorado).

maintaining a heavier, substantial wallform. This was achieved with the de-tailing and implication of the beam-and-column look to the precast panels(see Fig. 42b).

Shepard s/McGraw-Hil l WorldHeadquarters in Colorado Springs,Colorado, has 12 in. (305 mm) thickacid etched window box panels sup-porting 32 in. (813 mm) deep double-tee floor and roof members spanning60 ft (18.3 m), creating large column-free areas (see Figs. 43a and b).

Perhaps the single most importantfactor to influence the design of the11-story Orange County RegionalService Center in Orlando, Florida,was energy consumption (see Fig.44). To simplify construction, the ar-chitectural precast elements becamestructural, loadbearing units with abuilt-in eyebrow for sun screening.The loadbearing 12 ft wide x 12.5high ft x 3 ft deep (3.66 x 3.81 x 0.91m) window boxes, with fully opera-ble windows recessed 3 ft (0.91 m)from the building exterior surfaceprovide total shade on the glass dur-ing most of a typical summer day,when heat gain to the building inte-rior would peak.

The loadbearing window wall unitson the Aurora, Colorado MunicipalJustice Center are two stories high andweigh 20,000 lbs (9080 kg) each (seeFig. 45a). They are boldly detailed


with bullnoses, cornices, and friezes(see Fig. 45b).

The all-precast Jefferson AvenueParking Structure in Detroit, Michi-gan, features modules with punchedopenings measuring four stories highand 10 ft (3.05 m) wide. Doublingthem vertically provides the totalheight (see Fig. 46a). This small mod-ule allows the fa ade to curve in re-sponse to the shape of the nearby Re-naissance Center. The precast concretebearing wall system supports 4 ft widex 12 ft long (1.22 x 3.66 m) hollow-core slabs and 10 ft wide x 55 ft long(3.05 x 16.8 m) double tees in variousareas (see Fig. 46b).

The Denver Wastewater Manage-ment Building in Denver, Colorado, isan all-precast structure including thecore and shear walls with loadbearingwindow wall and solid panels (see Fig.47a) featuring highly articulated ArtDeco detailing (see Fig. 47b). The six-story office tower is topped by twomechanical floors with a curved pedi-ment (see Fig. 47c).

Hardened criteria for the walls androof of the Conrail Computer Technol-ogy Center in Philadelphia, Pennsyl-vania, included the ability to withstandthe impact of an irregular object at 200miles per hour (320 km/hr) and anequivalent explosive force (see Fig.48). The use of 12 in. (305 mm) thick,40 ton (36 t) precast wall panels notonly met these criteria, but in their ca-pacity as bearing walls, lessened thecost of the building.

The 10 x 30 ft (3.05 x 9.14 m) bear-ing walls of the gymnasium at theUnited States Olympic Training Cen-ter in Colorado Springs, Colorado are13 in. (330 mm) thick insulated wallpanels (see Fig. 49a). Lateral forcesare resisted by shear wall action of theexterior interconnected wall panels(see Fig. 49b).

CONCLUDINGREMARKS

Architectural precast concrete s fullpotential as loadbearing walls can berealized when the entire design or de-sign/build team architect, engineerof record, mechanical engineer, con-tractor, and precaster has the op-portunity to develop a project jointlystarting at the project s preliminary

Fig. 43a. Shepard’s/McGraw-Hill World Headquarters, Colorado Springs, Colorado.

Fig. 43b. Construction showing window box panels supporting double-tee floor androof members (Shepard’s/McGraw-Hill World Headquarters, Colorado Springs,Colorado).

Fig. 44. Loadbearing window

boxes on OrangeCounty Regional

Service Center,Orlando, Florida.

112 PCI JOURNAL

design stage. Finish types, shapes,repetitive use of efficient and econom-ical precast concrete modules, joint lo-cations, access or site restriction, erec-tion procedures and sequencing, allbecome important considerations for aproject s successful completion. Prop-erly implemented, an early and contin-uing dialogue between the designersand precaster will ensure maximumproduct quality and appearance at aminimum installed construction cost.

The following additional benefitscan be derived by using architecturalprecast concrete units as loadbearingwalls:❑Prefabrication combined with speed

of erection saves valuable overallconstruction time. Production ofprecast concrete components andsite preparation can proceed simul-taneously. On-site labor cost is min-imized, and erection is possible inall kinds of weather. Construction ismuch faster with a fully integratedstructure and skin system whereloadbearing wall panels provideboth structural support and architec-tural finish. Rapid enclosure allowsearlier access by finishing trades.Faster completion reduces interimfinancing costs and results in earliercash flows.

❑ Loadbearing wall panels becomepart of the structural framing. They

Fig. 45a. Two-story window wall units supporting double tees for Aurora, ColoradoMunicipal Justice Center, Aurora, Colorado.

Fig. 45b. Details on

loadbearing panelsfor Aurora Municipal

Justice Center,Aurora, Colorado.

Fig. 46a. Jefferson Avenue Parking Structure,Detroit, Michigan.

Fig. 46b. Hollow-core units supported by walls in walkway areas (JeffersonAvenue Parking Structure, Detroit, Michigan).


form the supporting structure forf loors and roof at the bui ldingperimeter. This generates interiorspace free of perimeter columnsand interior bearing walls, provid-ing maximum floor plan layoutflexibility. When a loadbearingwall panel building is erected, thearchitect and owner receive single-source responsibility for the build-ing shell. This reduces the numberof subcontractors and minimizestrade coordination.

❑ Elimination of separate structuralframes from exterior walls results insavings far exceeding the minimaladditional costs of increased rein-forcement and connections requiredfor loadbearing units. This savingsis most apparent in buildings with alarge ratio of wall-to-floor area.

❑ Precast concrete, manufactured infactory-controlled conditions as-sures the highest quality possible,thus ensuring a uniformly high qual-ity fa ade in the desired shapes, col -ors, and textures. Greatest economyis achieved by using an integral ar-chitectural finish for both exteriorand interior faces. Integral finishesnot only result in a savings of mate-rial and labor, but also reduces theoverall thickness of the exteriorwall. This permits maximum inte-rior space utilization. Precast con-crete panels resist weather and cor-rosion, requiring little or nomaintenance. Their aesthetic versa-tility is virtually unmatched by anyother material.

Fig. 47a. Construction of total precast Denver WastewaterManagement Building, Denver, Colorado.

Fig. 47b. Closeup of Art Deco details in DenverWastewater Management Building, Denver,Colorado.

Fig. 47c. Final erection shot ofDenver WastewaterManagementBuilding, Denver,Colorado.

❑Panels can be designed as recepta-cles and distributors for electrical,mechanical, plumbing and HVACsub-systems, thereby decreasingtrade overlap problems and elimi-nating the need for a separate wallcavity.

❑ Loadbearing window wall panelscan inherently form deeply recessed

window frames to provide a highdegree of sun shading. This canminimize air-conditioning systemcosts by reducing thermal load.Also, the thermal mass of concreteand the possibility of incorporatinginsulation into a sandwich wallpanel contribute to reducing heatingand cooling costs.

114 PCI JOURNAL

❑ Design flexibility for the precast ex-terior allows unique expressionswhile interior framing can be simpleand standard. This provides an eco-nomical solution for structures withvarying loading, fire and space plan-ning requirements. Precast con-crete s aesthetic flexibility simpli-fies changes in plane, relief, color,and texture. Wall panels can be cus-tom designed in desired shapes andsizes or may be selected from a vari-ety of standard sections depending

on the building s intended use andbudget. For walls requiring repeti-tive fenestration, precast concretecan offer sculptured architectural ef-fects with maximum simplicity ofstructural design and minimumerection cost. Final results are lim-ited only by the designer s imagina-tion.

❑ Precast concrete loadbearing wallunits, comprising structural-aes-thetic functional features, providethe opportunity to construct an eco-

Fig. 48. Conrail Computer Technology Center, Philadelphia, Pennsylvania.

Fig. 49a. U.S. Olympic Training Center, Colorado Springs,Colorado.

Fig. 49b. Erection of main roof system consisting of 401⁄2 in.(1029 mm) deep lightweight double tees spanning 111 to 140ft (34 to 43 m) from bearing wall to bearing wall (United StatesOlympic Training Center, Colorado Springs, Colorado).

nomical, attractive building. Suchstructures contribute significantly tothe development of contemporaryarchitectural philosophy specifi-cally, a system in which the wallsare actually doing the structuralwork they appear to be doing.

❑ Architectural loadbearing wall pan-els can be used effectively to reno-vate and rehabilitate old deterioratedstructures.

❑ Architectural loadbearing wall pan-els can be used not only in all-pre-cast structures but also in structuralsteel framed structures and cast-in-place concrete structures.

❑Architectural precast concrete usedinnovatively for loadbearing wallsmakes possible a nearly unlimitedrange of aesthetic expression, newdesign concepts and more efficientand less costly construction.

ACKNOWLEDGEMENTThe author wishes to express his ap-

preciation to the reviewers of this arti-cle for their technical comments andhelpful suggestions: Alex Aswad,Kenneth C. Baur, Paul Carr, Ned M.Cleland, John Garlich, J im King,Charles LeMaster, George D. Nasser,Dennis L. Nemenz, H. W. Reinking,Sami H. Rizkalla , and Stanley J.Ruden.


1. PCI Industry Handbook Committee,PCI Design Handbook, Fifth Edition,Precast/Prestressed Concrete Institute,Chicago, IL, 1999.

2 . PCI Committee on Connections, D e s i g nand Typical Details of Connections forPrecast and Prestressed Concrete,MNL 123-88, Precast/Prestressed Con-crete Institute, Chicago, IL, 1988, 270pp., and PCI Committee on ConnectionDetails, Addendum to Design andTypical Details of Connections for Pre-cast and Prestressed Concrete, PCIJOURNAL, V. 40, No. 5, September-October 1995, pp. 46-57.

3. Du Bois, Cornelius R. (Kin), Kipp,Brian R., Muir, R. Wayne and Wolf,Bruce R., Design-Construction ofUnited Bank Tower, PCI JOURNAL,V. 35, No. 6, November-December1990, pp. 26-33.

4. PCI Committee on Precast SandwichWall Panels, Sta te-of-the-Art of Pre-cast/Prestressed Sandwich Wall Pan-els, PCI JOURNAL, V. 42, No. 2,and 3, March-April 1997 and May-June 1997, pp. 92-134 and pp. 32-49.

5 . ACI Committee 318, Building CodeRequirements for Structural Con-crete (ACI 318-99), American Con-crete Institute, Farmington Hills, MI,1 9 9 9 .

6. Klein, Gary J., Design of SpandrelBeams, Specially Funded R&D Pro-gram, Research Project No. 5, Pre-cast/Prestressed Concrete Institute,Chicago, IL, 1986.

7. Cleland, N. M., Design for LateralForce Resistance with Precast Con-crete Shear Walls, PCI JOURNAL,V. 42, No. 5, September-October1997, pp. 44-63.

8. Aswad, G. Gus, Djazmati, Basel andAswad, Alex, Comparison of ShearWall Deformations and Forces UsingTwo Approaches, PCI JOURNAL, V.44, No. 1, January-February 1999, pp.34-46.

9 . Mackertich, Seroj, and Aswad, Alex,Lateral Deformations of Perforated

Shear Walls for Low and Mid-RiseBuildings, PCI JOURNAL, V. 42,No. 1, January-February 1997, pp.3 0 - 4 1 .

10. Kulka, Felix, Lin, T. Y., and Yang, Y.C., Prestressed Concrete BuildingConstruction Using Precast Wall Pan-els, PCI JOURNAL, V. 20, No. 1,January-February 1975, pp. 62-72.

11. Yee, Alfred A., One Hundred Wash -ington Square Structural Designand Construction, PCI JOURNAL, V.29, No. 1, January-February 1984, pp.24-48.

12. Pickersgill, David, One HundredWashington Square The Precaster sStory, PCI JOURNAL, V. 29, No. 1,January-February 1984, pp. 49-63.

13. A rchitectural Detailing andP r e c a s t /Prestressed Concrete Play KeyRole in Success of the Boston CollegeParking Structure, PCI JOURNAL,V. 41, No. 2, March-April 1996, pp.12-20.

14. Matthews, Lisa, Himes, Herman C.,and Cronin, John L., Design-Con-struction of the Oklahoma City IRSBuilding, PCI JOURNAL, V. 39, No.3, May-June 1994, pp. 12-25.

REFERENCES

Documents

Loadbearing Architectural Precast