Production and Design ofArchitectural Precast Concrete

by C. H. Raths*

SYNOPSISThis paper describes typical production methods and design guidelines

for architectural precast concrete panels. Recommended design criteria andtypical details for precast panels are presented. Design examples are givenin the appendix.

INTRODUCTION

The rapid growth in the use ofarchitectural precast concrete hascreated many changes and innova-tions in architecture. Today archi-tectural precast concrete is availablein complex shapes which serve notonly as curtain walls, but combinetheir attractive appearance with theability to serve as main structuralmembers. The realization of the pre-cast concrete market potential hasresulted in the development of ahighly specialized industry. With thismarket growth, the successful use ofarchitectural precast concrete is be-coming increasingly dependent uponarchitects' and engineers' under-standing of the precasters' methodsof production and of general precastconcrete design guidelines.

Until the architect, engineer andbuilder fully comprehend all facetsof architectural precast concrete, itsmaximum potential will be limited.There must be awareness that, whenarchitectural precast concrete con-tracts are awarded on a bid basis,high quality precast producers' bidsoften reflect costs which are notreadily apparent from the plans and

°Principal, CHAS. H. RATHS & ASSO-CIATES, Structural Engineers, Hinsdale,Illinois.

specifications. When the contract isawarded at some lower bid, the finalresults can be less than desired orcan involve many extra costs.

PRODUCTION STAGES OFARCHITECTURAL PRECAST CONCRETE

The general stages in the produc-tion of architectural precast concretecan be broken down into sevenbroad categories which obviouslyare overlapping:

1. Engineering and detailing2. Form construction3. Production of units4. Storage5. Shipping6. Erection7. General handling

To provide a clear picture of the im-portance of the seven steps and theirinteraction, each will be examinedseparately.

Engineering and Detailing

The first step in preparing theshop drawings necessary for a proj-ect, is a thorough review and re-study of the job plans and specifica-tions to determine all the factors thatcan influence decisions regarding theprecast concrete. The goal of thisanalysis is to produce standardiza-tion of: precast units, modificationsrequired of precast units, connec-

18 PCI Journal

tions, shop production techniques,handling methods, and erection.Aside from the general architecturalshape requirements, the main factorin establishing standardization of theprecast units is the building frameand its relationship to the architec-tural units (i.e. connection locations,clearances, etc.).

The preparation of the precastconcrete shop drawings for archi-tectural approval is best divided in-to two separate submissions. Untilthe architect is satisfied that the pre-caster understands his concepts rela-tive to the panel size, shape, etc., itis fruitless to prepare final shopdrawings detailing connections andreinforcement. Further, since theform construction requires the great-est amount of production lead time,the common goal of both the archi-tect and the precast producer at thisinitial stage is to determine all thedetails regarding the size and shapeof the precast panels for the mosteconomical and efficient productionsequence.

During the preparation of the ini-tial sections and elevations for shape,minor changes and other recommen-dations, made for many varying rea-sons as will be discussed later, areusually incorporated.

After receiving approval for shapeonly, the next step is the design orchecking of the panel's reinforce-ment and connections. With the ex-act shape of the panels known, finalreinforcement is based upon the re-quirements of stripping from theform, in-plant handling, yarding,shipping, erecting, and wind or otherin-place loads. The basic connectiondesign is determined by buildingframe conditions and by forces in-duced into the connection support-ing the architectural precast panels.However, final selection of connec-tions is made only after considering

standardization and ease of complet-ing connections during erection.

The information regarding theconnections and their locations, rein-forcement, and required notation isincorporated in the previous shopdrawings for final approval prior toproduction. These shop drawingsnow explain in complete detail howthe precast producer intends to per-form his part of the project.

Form Construction

Form or mold construction usuallyrequires the greatest manufacturinglead time. Thus, the form construc-tion phase is usually initiated as soonas shape approval is received.

The type of form used to producearchitectural panels for a given proj-ect depends upon many factors.Among the major factors to be con-sidered are: (1) architectural finishon the panel; (2) the number ofcasts to be made in the form; (3) thetype and sequence of form modifica-tions for greatest production econo-my; (4) dimensional tolerances ofprecast member; (5) the details ofthe precast member being produced;(6) the number of similar molds;and (7) the amount of lead timeavailable.

The types of forms or molds usedto produce the precast concrete pan-els are mostly fiber-glass-reinforcedplastic, wood and steel. Typically,fiber glass molds are used for themanufacture of smooth concretewhile wood and steel forms findtheir greatest applications in makingexposed aggregate panels or othertypes of finishes. The specific moldmaterial selection is based upon theseven points listed above, -as well ason the experience of the precast con-crete producer.

Fiber glass molds probably pro-vide the best overall performance.They have a relatively long life in

June 1967 19

excess of 75 casts. Further, fiber-glass-reinforced plastic molds can bereadily repaired and modified so thatthe effects of repairing and modify-ing are not reflected in the finishedunits. Well-constructed woodenmolds can be expected to produce50 casts. However, wood moldsgreatly show their usage towards theend of their life. As with fiber glassmolds, wood forms allow for easymodification and dimensional con-trol. Steel molds, while having a longlife, provide the greatest problemsin modification and dimensional con-trol. The usual welded constructionof a steel mold can induce perma-nent distortions. Other mold typesused occasionally to produce archi-tectural units are concrete and sand.

Form or mold tolerances vary withindividual precasters and the re-quirements of the projects. Highquality mold construction, with thepossible exception of steel, shouldproduce units within ±1/s in. of theplanned dimensions. Further discus-sion of mold details is presented la-ter in the Design Guidelines section.

Production of UnitsProduction culminates the efforts

of the precaster's engineering andmold fabrication. The reinforcementcage for the architectural panel isusually made in a jig unless the pre-cast panel is a simple flat. In additionto satisfying the structural require-ments of the panel, the reinforcingcage must be designed so that it canbe handled within the plant (three-dimensional stability) and have suf-ficient clearance when placed in theform. Usually the cages are pro-duced so that a 1-in, minimum coverof concrete exists over all reinforcingsteel. It is common to tack-weld thecages since tie wires can come looseand may appear on the surface ofthe panel. It is also common to hang

reinforcing cages from the formsrather than to use supports that mayshow on panel surfaces.

Following placement of the rein-forcing cage in the form, the remain-ing set-up is completed. This in-cludes securing to the main part ofthe form the side rails, end rails,blockouts, concrete inserts and anyother materials cast in the panel.

Concrete matching the approvedsample is placed in the form andconsolidated in a variety of ways.Some precast manufacturers trans-port the form to a vertically vibrat-ing table. External vibrators areemployed by other producers or in-ternal vibration may be used. Again,the manner of consolidation is closelyrelated to the experience of the in-dividual precast manufacturer.

After a curing period (generallyone day to accommodate a mass-production cycle), the panels arestripped and inspected. This phasein production is probably the mostcritical because the concrete is greenand has a low strength of 1500 to3000 psi. Frequently cracking resultsjust after stripping due to thermalshock* or through minor mishan-dling. If the panel is to have exposedaggregate, the brushing or washingaway of the retarder is done as soonas possible after stripping to accom-modate the production cycle. Thesame applies if the panel is to besand-blasted or acid-etched.

An extremely important aspect ofproduction is the inspection andquality control employed by the pre-cast manufacturer. The inspectionstarts with examination of the formfor correct dimensions and must in-clude examination of the reinforcingcage and, most importantly, concreteinsert locations. Good quality control

*Thermal shock results from some sectionsof the precast panel cooling more rapidlythan others where they are adjacent.

20 PCI Journal

of the concrete insures that thestripping and handling strength willbe achieved. Inspection and qualitycontrol also has to cover the block-ing and handling of the precast pan-el to insure high quality units.

Storage and Shipping

Storage plays an important rolein the production of high qualityprecast concrete. At initial storage,the concrete strength is still low andthe panel can be subject to warpage,bowing and/or cracking.

A basic axiom to storage is to sup-port the precast unit at two points*only. If support is continuous acrossthree or more points, precautionsmust be taken so that the panel willnot bridge over one of the supportsand result in bowing and cracking.The problems of warpage cannot becompletely eliminated, even withtwo-point support, when the panel isstored in either a horizontal or verti-cal position, although it can be min-imized by proper blocking of thepanel in a given plane.

Often the manner of storing de-pends on how the panel is to beshipped and what limitations thepanel's cross-sections impose on han-dling. For all practical purposes, thepanel should be stored in the samemanner in which it will be shipped.Proper storage must also give con-sideration to the potentially harmfuleffects of alternating sun and shadeon the precast units.

The rule of two-point support alsoapplies to shipping the panel fromthe precast plant to the job-site.Most precast concrete firms use ei-ther flatbed or low-boy trailers, andthese units suffer excessive distor-tions during hauling. Thus, supportat more than two points on a trailer

*Relative to blocking, this is an industryterm denoting a line of support.

June 1967

unit can be achieved only after con-siderable modification of the unit.

Consideration must also be givento the size and weight of the archi-tectural panel being shipped. If thepanel is shipped vertically, theequipment available for shippingand bridge clearances must be care-fully considered. It should be re-called that very often the decision toship a precast panel flat or verticallyis dictated entirely by the structuralbehavior of the panel when in thesepositions.

Significant economies can beachieved by having tractor-trailerunits carry their maximum capaci-ties. These economies can be accom-plished only if the shape, strengthand weight of the precast panelshave been taken into account.Erection

Prior to the erection of precastunits, the job-site is checked fortruck and crane access. If erectionis to take place in a congested orconstricted area, scheduling and co-ordination with other trades must beworked out. The locations of all con-nections integral with the buildingframe or foundation (for load-bear-ing panels) are also verified as toposition and elevation before anyprecast units are erected.

The importance of precast paneljoint layouts cannot be overempha-sized. This assures even appearanceof panel joints as well as identifyingproblems caused by building framecolumns or beams being out of di-mension or alignment. Joint layout,.in short, guarantees that the panelswill fit. For multi-story buildings,this joint layout check should bemade every third or fourth floor.

There are a variety of ways bywhich an architectural panel ishoisted into position. The type oferecting equipment is determined by

21

the weight of the panel and distanceof reach to set the panel. Whetherthe panel is simply picked off thetruck or "spun" in the air is deter-mined by the limitations the panelposes to handling. The problem ofhow the panel is handled is the sameas that discussed previously in pro-duction and storing.

Significant economies can beachieved if the panel is sized to min-imize the number of units whichmust be erected. A good rule is tomake the panels as large as possiblerelative to general handling, erec-tion equipment or methods avail-able, and to the structural buildingframe. In addition to providing sav-ings on erection costs, larger sizedpanels provide secondary benefits ofreduced amounts of caulking, betterdimensional controls and fewer con-nections.

Connections play a key part in theerection procedure. Properly de-signed connections allow the panelto be secured in place while allowingfor final alignment later. Connec-tions of this type are achieved gen-

erally by the use of bolting andeconomies are gained by using con-nections that are standard through-out the job. While material costsmay be greater for standardized con-nections, the economy produced byefficient crane operation far out-weighs the increased material costs.

The final part of the erectionphase is the cleaning and patching ofthe precast panels, if required.

General Handling

A knowledge of the various possi-ble modes of handling is pertinentto the proper understanding of anarchitectural precast panel.

Most precast concrete panels arecast in a flat position with the ex-terior face of the panel down. De-pending upon the weight of the pan-el and the location of the inserts, thepanel may be stripped with or with-out an auxiliary spreader beam bymeans of overhead cranes or lifttrucks. Usually the second step is totemporarily store the panel in a flatposition to free the crane orlift truck for other operations. Next

__._. crane lineF spreader beam

panel

formIFF-

Precast panel - plan view Stripping from form

crane iline e b tee shore

padding

Tipping panel 90' for two point handling Storage of panel

Fig. 1—In-Plant Handling

22 PCI Journal

the panel might be rotated 90 0 intoa vertical position for further workon the panel face (viz, patching,sandblasting, removing retarder,etc.) and then it is often supportedat three or four points on a leveledarea within the plant. The panel iseither rotated back down on its faceor left vertical to be transferred tostorage. From storage, the panel istransferred to a tractor-trailer forshipping. It is well to repeat that thepanels are stored and shipped ontwo points. After the panel arrives atthe job site, it is either stored againor immediately erected. Dependingupon how the panel is shipped, itmay be either tipped up from thetrailer bed, lifted off and installed orthe panel may be rotated 90° whilesuspended from a crane after beinglifted off the trailer.

Figs. 1 and 2 illustrate schemati-cally the various possible handlingoperations of an architectural pre-cast concrete panel.

DESIGN GUIDELINES OFARCHITECTURAL PRECAST CONCRETE

The breakdown of design guide-lines into separate categories can beendless due to the variety of customdesigns employed. However, forconditions usually encountered inarchitectural precast design, guide-lines can be set out for: (1) shapedetails, (2) inserts, (3) engineeringproperties of architectural precastconcrete, (4) in-place loadings, (5)connections and (6) tolerances.

Shape Details

The shape details play a large rolein determining the cost of a precastpanel. They influence the form costs,the production set-up, labor, appear-ance of the precast unit and dimen-sional tolerances.

The ideal shape for a precast pan-

June 1967

craneline

trailer bunit

O

Erected in same position as shipped

craneline

I

trailer paddingunit

pp

Tipping panel 90° into erection position

Panel rotated 90° into erection position;

Fig. 2—Erection Handling

el is one where the form has noremovable parts. This creates a min-imum set-up time and assures excel-lent dimensional control. An impor-tant aspect of shape relative to theform is the amount of draft. Withoutdraft (generally a--minimum of 1-in.horizontal to 12 in. vertically), strip-ping cannot be accomplished with-out removing parts of the form. Inaddition to being a factor in strip-ping, draft also affects the surfaceappearance of the precast unit. Toosteep a draft (greater than 1 to 8)can produce some entrapped air onsmooth concrete. A good rule ofthumb for smooth concrete is a draft

23

crjib

rolling bl

^Im

3 block

of l to 5. For exposed aggregate con-crete, good finish results are ob-tained with drafts varying between1 to 5 and 1 to 12. While draft in-volves both production and surfaceappearance, it must be balancedwith the architectural concept. Fig.3 illustrates the draft concept.

Another aspect of shape involvesa minimum radius of '/s in. on alledges to prevent chipping. This ismore important for smooth, etchedand sand-blasted panels than forexposed aggregate panels .Also, rela-tive mainly to smooth and sand-blasted panels, thought must be giv-en to preventing leakage especiallywhere removable side or end railsattach to forms. This point of leak-age, which can mar the finish, isshown by Fig. 4a. A return as indi-cated in Fig. 4b will cause the leak-

A 1I hV

A.JSec. AA

h v where required712

minimum for easystripping.concetesmooth

Fig. 3—Draft Concept

age to take place where it will not beseen.

Occasionally two types of finishesare adjacent as shown by Fig. 5. Un-less a distinct separation is madewith a separator groove, the two fin-ishes can intermingle creating anundesirable effect. The dimensionsgiven for the separator groove are

X to^4„point of

—leakagemottledsurfaceappearance

(a) Leakage on exposed surface

anelremoveable

panel— —[side rail

point ofleakage

form4

mottledsurfacehidden byjoint

(b) Leakage on hidden surface

Fig. 4—Form Leakage

24 I'd journal

groove toseparatefifinishes

windowdetail

1 34

paneldifferent mullionsurfacetreat me nts

Fig. 5—Separation Of Different Surface Finishes

considered a minimum to insure thedesired finished appearance.

Careful attention must be given towindow details. The primary causeof problems is dimensional control.Fig. 6 shows several common detailsused to secure windows to the archi-tectural precast concrete. Hereagain, the best dimensional controlis achieved by having the windowblockout as a permanent part of theform, or, if a removable blockout is

used, the blockout should be of onepiece.

Whenever possible, it is advisableto have the back of the precast panelflat. This relates to the previous dis-cussion on having the shape fixed asmuch as possible by non-removableparts of the form. By having theback of the panel flat, its back sur-face appearance is uniform and lev-el, leakage where two parts of a formjoin are eliminated, and good dimen-sional control is achieved.

A final consideration on shape is toprovide a sufficient cross-section forembedment of inserts and reinforce-ment and, most importantly,strength for handling. Also, as wasdiscussed earlier, shape determineshow the panel will be handled. Fig.7 illustrates the importance of shapeas it concerns inserts, reinforcementand handling.

Inserts in Concrete

Inserts are a fundamental part ofarchitectural precast concrete. With-out them, it would be virtually fin-

Structural window gasket Structural window gasket Blackout to receiveon concrete lug into concrete groove metal window frame

Fig. 6—Typical Window Attachment Details

June 1967 25

not sufficientclearance,

msert___J improper

1mi n.insert I proper

clearancefor Insert

vjJdimemsionally unstablecage, uniform cover hardto maintain and unsym-metrical reinf. avoid ifpossible

proper cages-dimension-ally stable, easy to handle

Shape and reinforcement

Fig. 7—Shape Considerations

V

Ugood for handling invertic of position

V

LH

improper for handlingin horizontal position

V

^H

good for handlingin both verticaland horizontalpositions

Shape and handlir-

insert

P ',not sufficin clearance,

improper

imi?roper,vertical load

will crack-ng

Shape and inserts

possible to strip, handle and erectpanels. Concrete inserts can be di-vided into two types: one type con-sists of bolted inserts and the other awire cable type of insert.

The bolted insert is most widelyused. They are available in many va-rieties and Fig. 8 illustrates some ofthe common types. These inserts aremade with either a coarse coil boltthread or a common machine thread-ing. For handling precast concrete,the coil type bolt is used in conjunc-tion with a swivel plate. The ma-chine threaded inserts find theirgreatest use in connecting the pre-cast unit to the building frame. Fig.8 lists the capacity of the insertswhen tested in pull-out from plainconcrete* blocks ranging from 2000to 3000 psi in strength.

The looped cable type of insertfinds application in stripping smallflat panels or for edge lifting of flat

*150 pcf concrete with aggregates not ex-ceeding 1'9'i" in size.

panels. The use of the looped cableinserts is generally limited to two-point pick-up since their projectionfrom the concrete is subject to varia-tion. Fig. 9 shows typical use of ca-ble type inserts and gives their usualload capacities.

Previous discussion has been lim-ited to the capacity of bolted con-crete inserts when its full shear coneis developed at failure. However, in-serts are frequently located close tothe edges of precast concrete or innarrow sections so that a normal fullshear cone cannot develop for a ten-sion failure (see Fig. 10) . Test dataindicate that the capacity of an in-sert in direct tension can be esti-mated by

Pu —k fcAc

where P. = ultimate load in lb.f l = concrete compressive

strength, psi

26 PCI Journal

A, = surface area of a concen-tric cylinder about theinsert which approxi-mates the shear conesurface area, sq. in.

k = a constant, which is afunction of mix design,that varies from 1.9 forlow-strength concrete to6.0 for high-strengthconcrete.

Fig. 10 illustrates the actual shearcone and the assumed approximatecylindrical cone. If the full shearcone can develop, inserts with loopsor struts 6 in. or greater generallyfail by fracture of their steel at thejunction of the loop and thethreaded part after a small initialshear cone develops. For caseswhere it is obvious that the fullshear cone can not develop becauseof the location of the insert, carefuljudgment must be used as to the safeload that the insert can carry. Thecapacity is reduced in proportion tothe shear cone that can develop, or

reinforcement can be added that willinsure development of the insert'ssafe working load. Fortunately, in-serts are rarely required to developtheir full working load value.

Test data indicate that an insertloaded in direct tension has a smallercapacity than when loaded in pureshear. Therefore, shear values for in-serts may be assumed to be equal totheir tensile values except when theinsert is near a free edge.

A swivel plate, shown in Fig. 11,should always be used with a boltedinsert. This assures that an angularpull will place the concrete insertprimarily in tension. Also, insertsshould provide a minimum safetyfactor of 21/2.

Engineering Properties of Architec-tural Precast Concrete

One of the basic criteria from anengineering viewpoint is to designthe stripping, storing, handling, ship-ping and erection of an architecturalprecast panel so that it will be crack

coil coilthread thread wire

loop

3/q s.w.l. 4500`

3/q or 1 s.w.l. 9000*Note! threads canalso be machinethreaded weldments

coi leoil thread

coil

lgs

thread

2 ^2

3i4 or 1" s.w.l. 9000* 3iq s.w.l. 2500k

(s w.l.=safe working load when full shear cone develops)

Fig. 8—Representative Concrete Inserts

June 1967 27

free. Engineering design of an ar-chitectural precast panel can be di-vided into two areas: (1) allowableconcrete stresses; and (2) reinforce-ment design criterion.

With regard to stripping, han-dling, shipping and erection, thiswriter's experience has led to thedevelopment of the following cri-teria:

1. Initial concrete flexural crack-ing which can be observed, butnot measured, occurs at a con-crete strain of 150 millionths.

2. Form suction and handling im-pact can be neglected if themaximum flexural strain is lim-ited to 75 millionths.

3. The modulus of elasticity ofthe concrete for tension andcompression is as defined byACI 318-63 and equal to E0 _w1.5 331 fc•

4. Reinforcing design is based up-on the conventional crackedsection used in the workingstress method, at a stress levelto keep any potential crackshairline in width.

mesh

i.Jaircraftcable

usual working loadsfor aircraft cable

diameter in. capacity lbs.3/ g 12501/4 20005/ 3000

50001/2 10000

Fig. 9—Aircraft Cable Inserts

5 assumed point of wireshear cone failureusual failure

45°±shear cone

r\ totalpossible

assumed shearshear cone cone

cRactualshearcone

Fig. 10—Insert Failure In Concrete

28 PCI Journal

ngular pull

plate

horizontal componentresisted by friction inthis area

insert placed —mostly in tension

Fig. 11—Swivel Plate Assembly

The major problem in design ofarchitectural precast concrete iscracking and thus flexural tensionbecomes the controlling factor. Sinceit is awkward to work with a straincriterion as stated in the assumptions,Table I represents allowable stresses,fb = E0e, for different concretestrengths where w = 150 pcf.

TABLE 1—Recommended Maximum ConcreteFlexural Stresses

ConcreteStrength, psi

Maximum FlexuralStress, psi

2000 2002500 2253000 2503500 2704000 2854500 3055000 3255500 3356000 350

The bending or flexural stress, f b =M/S, is determined from the bend-ing moments the section must resistdue to the dead weight of the paneland the section modulus of the grosscross-section under examination.Quite frequently, complex architec-tural units require the use of inde-terminate structural analysis to de-

termine the bending moments.Handling stresses will control the de-sign in the majority of cases.

Criteria 4 states that the reinforc-ing design is based upon a crackedsection. If the strains never exceeded75 millionths, only temperature rein-forcement would be necessary. How-ever, if cracking does develop, itshould be limited by the reinforce-ment to minor hairline crackinghardly visible to the eye and notmeasurable by ordinary means. As-suming that initial flexural crackingwhich can be observed occurs at astrain of 150 millionths, then to keepthe cracks hairline in width, the steelstress should be

fs=E'8eusing e$ = 150 X 10 -6 and

E8 = 29 x 106 psif8 = 29 x150 = 4500 psi

The limitation of f , to 4500 psi isextremely severe and, since the fullcracked section will not be permit-ted to develop, this writer has foundthat an f $ of 12,000 to 14,000 psi is apractical value for design use.Rather than determine the locationof the neutral axis based on thecracked section properties, it is con-venient to calculate the A8 requiredfrom the general approximate rela-tionship

A=_M8

where A$ = area of reinforcement,sq. in.

M = bending moment, kip-ft.a = 0.83 for fs =12,000 psi or

0.98 for f$ =14,000 psid = depth from extreme

compression fiber to cen-ter of reinforcement.

After complete reinforcing steelarea is selected, care should be taken

June 1967 29

in arranging the cage configuration.As previously explained, the cagemust be handled within the produc-tion shop and therefore requireshandling rigidity. Also, thought mustbe given to placing the reinforce-ment as symmetrically as possibleabout the panel's cross-sectional cen-troid. This is particularly true for flatpanels. If the reinforcement is notplaced symmetrically, shrinkage-in-duced bending will cause bowing ofthe panel.

An area of controversy regardingprecast concrete reinforcement iswhether it should be galvanized.This writer's experience leads to therecommendation that welded wirefabric be galvanized only in thinpanels. Reinforcing bars should notrequire galvanizing if proper cleardistances are maintained (1 in. mini-mum) and quality concrete is em-ployed having strengths of 5000 to6000 psi at 28 days.

In-Place LoadingsArchitectural precast panels can

be subjected to three types of load-ings—gravity, wind or earthquake,and, for load-bearing panels, floorloads.

Gravity loading design usuallypresents no problems unless themain support for the panel is at thetop, thereby placing the panel cross-section in tension. Good design pro-cedure dictates that the entire panelshould be supported entirely at onelevel (only two points) so that thepanel weight keeps the entire cross-section in compression. This meanslocating the main connections nearthe bottom of the panel.

The usual lateral force -considera-tion is wind. In addition to the windpressure normal to the plane of thepanel, wind suction should also beconsidered. Experience indicatesthat the suction loading applies more

to the connections than it does to thepanel. For normal wind loads, a suc-tion loading of half the wind pres-sure should prove sufficient for de-sign.

Additional loadings which defi-nitely must be considered, or detailsmust be developed to completelyeliminate them, are loads created byrestraint of panel movement causedby temperature changes or by floorlive loads applied to the panelthrough kicker or tie-back connec-tions. Vertical slots, as discussed inthe section on connections, provide amethod for preventing temperatureand floor live load forces from beinginduced into the precast panel.

Connections

Enough emphasis cannot be givento connections. They are the key toproper structural behavior and econ-omy. Basically there are four con-nection types: shear, combined mo-ment-shear, tension or compression,and partial moment-shear. Designfactors include allowable tensile andcompressive stresses, shear stresses,bearing of steel on concrete, preven-tion of temperature-induced forces,and standardization.

Fig. 12 illustrates a shear connec-tion near the bottom of the panelsupporting the panel's gravity load.With a shear connection, provisionmust be made for resisting torsionalstresses induced into the supportingmember. In both cases, steel andconcrete, the structural buildingframe must develop adequate rota-tional resistance. Depending uponthe cantilever projection beyond thestructural frame, the connection, ifan angle, may require a gusset. Theconnection should be completed bybolting to the precast unit, and,whenever possible, by bolting theconnection to the building frame.

30 PCI Journal

gussets, if required

panelweight

normalI gage

min, clear

~—resisting equilibrium forcescause torsion in beam

(a) Shear connection to steel beam

cast in plate, anchored to resistall forces

note, boltinq also usedwhere weld indicated

—c-i. p. concrete beam

min, clear

(b) Shear connection to concrete beam

Fig. 12—Panel Shear Connections

If the building frame spandrelcannot provide rotational resistance(this applies more to a structuralsteel frame than to concrete), theconnection at the precast panel mustdevelop moment capacity. Fig. 13shows three types of moment con-nections—two for heavy bendingand the other for light bending. Aneconomical solution for situations re-quiring heavy or large bending to beresisted by precast panels is to cast awide flange or similar section direct-ly into the panel as shown in Fig.13a. Fig, 13b indicates an alternatebolted system where that part of theconnection within the panel must in-clude a steel plate to insure propercontact between the precast paneland the connection for heavy bend-ing. This type of connection requiresinserts to resist tension and shearand a sufficient precast concrete vol-ume to encase the cast-in plate as-

June 1967

sembly. Light bending can be satis-factorily resisted by eliminating theplate cast into the precast unit andletting the connection device beardirectly against the precast panel,Fig. 13c.

Tension or compression connec-tions keep the precast panel in aplumb position and are sometimesreferred to as "kicker" connections(see Fig. 14a) . They also serve to re-sist wind loadings. Usually theseconnections are made with anglesand gussetted if required by themagnitude of horizontal forces. Avariation of this type of connectionis shown with a typical tension-com-pression connection in Fig. 14b. The

V^ cast in panel

mover web only,— '— no torsion induced

into steel beamYz clear

(a) Heavy bending, connection cost into panel

FC cost indouble concrete-_ panelinserts, welded togussetplate _ Iwelded studs orhooks as required

shim over minimum

web only dimension

Y dimension

(b)Heavy bending, bolted connection

single no steelsin fCinsert cast in panel

other detailssame as in (b)

(c) Light bending, bolted connection

Fig. 13—Panel Moment Connections

31

MiNote! kicker angle caninduce torsion into VF beam

resulting fromOverturning and windsuction

(a) Plumbing tension-compression connection

steel I ,r– concrete.beam beam

or s

2 1/ clear min.concreteinsert ngle

double nut and doublesquare washer

threaded rod

(b) Variation of tension-compression connectionF-A

ered ifrequiredrui

forcespossible

—• min . h0

to forhole fotolerance

ViewA

(c) Slots for tolerance and for allowing for movement

Fig. 14—Panel Plumbing Connections

advantage of this variation is that itallows for plumbing after the precastpanel is in position on the structuralbuilding frame.

An important aspect of the kicker-type connection, shown by Fig. 14c,is that it does not restrain move-ments of the panel and induce addi-tional loadings. The most generalmethod of preventing movement re-straint is to provide slotted holesvertically. An additional means ofeliminating restraint is to select aconnection that does not developflexural rigidity (i.e. use a minimumthickness of clip angle, etc.) . How-ever, care must be taken to insurethat the kicker connection can satis-factorily resist panel overturningforces as well as wind suction forceswithout excessive deflection orstress.

The partial moment-shear connec-tion is a type frequently used forload-bearing panels. The connectionis accomplished through the use ofneoprene pads and coil rods asshown in Fig. 15. This connectionpermits development of a horizontalforce necessary for panel equilibri-um, but it is necessary to tie thefloor slab back to another part of thestructure to develop full lateral forceresistance. An advantage of this con-nection type is the speed with whichit can be accomplished. Slight foun-dation settlements or floor live loadswill not create a significant bendingaction in the floor system or the pre-cast panel. The basic principle ofthis partial moment-shear connec-tion is the ability of the properly de-signed neoprene pad to deform inshear and thus prevent the formationof a force couple. If the shear forcebecomes greater than that causingdeformation, the neoprene pad willsimply slip and destroy the momentcouple.

A simple, yet effective, base con-nection for precast load-bearing pan-els is that illustrated by Fig. 15b.Leveling blocks produced by theprecast manufacturer in lengthsfrom 8 to 24 in. are shipped to thejob site prior to panel erection.These leveling blocks are set in groutin the previously formed grooves inthe foundation wall and adjusted tothe proper elevation and alignment.The panel, when simply set on thegrouted-in-place leveling block, isautomatically set properly. Follow-ing the panel's erection, a dry pack,non-shrink grout is placed betweenthe panel and foundation wall. Gen-erally the grout is placed only in thevicinity of the leveling block to in-sure that loads are transferred atknown points (usually under thepanel mullions) .

32 PCI Journal

concrete—insert 2" topping

c i. p. or precast floor

neoprene pad

panel haunch

""- precast panelload bearing

(a)

precast levelingblock set ingrout bed ,. ,dry pack as required

groove cast in foundatiowall (b,

interference with completion of theconnection.

In summary, all connectionsshould provide for temporary andfinal securement of the precast panelto the building frame; allow for freemovement due to temperature ex-pansion and contraction; differenttypes should be kept to a minimum;and the connections should be struc-turally adequate.

Tolerances

The selection of proper tolerancesis a difficult task. Two types of toler-ances must be considered, those af-fecting the dimensions of the precastunits and those required for connec-tion of the precast elements to thebuilding frames.

Fig. 15—Load-Bearing Panel Connection Details

A problem all too frequently en-countered with architectural precastpanels is the lack of room to makethe connection. This results in in-creased connection costs both in ma-terials and in requiring differentkinds. Moreover, using a differentconnection type may require alteringthe position of the insert within theprecast panel. In short, required con-nection clearances are a critical partof the preliminary architecturalplanning.

Some mention should be madeabout column cover connections.Fig. 16 shows some typical connec-tions. The two most important con-siderations in column cover connec-tions are the clearance between thecolumn cover and the column, andadjustability of the connections inthe field. The very minimum clear-ance between column covers andcolumn is 1 in.; 1'/z to 2 in. is recom-mended because of columns beingout of plumb or dimension causing

June 1967

mn.- ^ horizontal

inslot angle

steel weldplatecast in concrete

caulK% min.

column

precastdowel or colt rodcolumn

cover I

1 concrete insert

concrete insertweld shelf angle to gasteel col. flange orangle support

Section A

A

1^^^ II

anangle for steel column secure to adjacent

support covers or connectbetween II to angle supportflanges L—. ^__g se

Column cover plan section

Fig. 16—Column Cover Connection Details

33

precas±I_..

steel beam.

pnastpanel V concrete

Fig. 17—Typical Recommended Tolerances ForArchitectural Precast Panels

A reasonable tolerance for the pre-cast unit's dimensions is -1- 1/s in. Foreither steel or concrete framed struc-tures supporting precast concreteunits an overall vertical and horizon-tal connection tolerance of ½ in.seems to be the most practical, asshown by Fig. 17. Fig. 17 also showstypical tolerances between the pre-cast panel and the building frame.Occasionally the case for greater tol-erances can be stated. However, it isbetter to make adjustments in thefield since the percentage of connec-tions requiring a tolerance greaterthan 1/z in. represents a minor pro-portion of the total. Another guideto the selection of connection toler-ances are the standard tolerances set

for construction in steel and concreteby the AISC and ACI building speci-fications and recommendations.

SUMMARY

Obviously, the wide range of top-ics discussed in this paper can onlyconsider the major or general items.Subjects in architectural precastpanels which were not discussed in-clude, but are not limited to, pre-stressing of panels, thermal andmoisture gradients within the panels,concrete backups (lightweight andnormal weight concrete), expen-sive facing concretes, sculpturedconcrete panels, and insulated pan-els.

The successful and proper use ofarchitectural precast concrete is de-pendent upon panel size or dimen-sion for best production, transporta-tion and erection; cross-sectionalshape for handling in all planes; sur-face finish requirements; details re-lated to production, forms and han-dling; total erection requirements;and, most importantly—repetition,repetition, repetition....

ACKNOWLEDGMENTS

Test data and other concrete in-sert information was made availableby Superior Concrete Accessories,Franklin Park, I11. All figures wereprepared by R. W. Johnson.

34 PCI Journal

APPENDIX

DESIGN EXAMPLE I

Design the simple flat panel shown in Fig. Al for all conditions. Assumeconcrete strength at stripping is 3000 psi.

Panel Weight

W = 150 (0.33) 8 (12.7) = 5080 lb.w=50psf

Stripping Design—Longitudinal

By inspection, strip with inserts @ length/5 = 2.54 ft.

50 (2.54) 2 __ —161 ft.-lb.Mrna. = 2

Check for cracking:

S = bs2= 12(64)-=32in3/ft.

__M 161(12)_fb S

32 — ±60 psi (o.k. for tension)

Reinforcement required:

use f, =12,000 psi

Ag = M _ 0.161 0.097 in. /ft.

ad 0.83(2)

select 4 x 4-4/4 WWF (A8 = 0.12 in.-/ft.) add 1—No. 3 across stripping in-serts above and below centered WWF.

Stripping Design—Transverse

By inspection, strip with inserts @ width/5 = 1.6 ft.

Total load carried per tranverse strip = 5080 = 318 plf2(8) —

318(1.6) 2 = -407 ft.-lb.Mmax - 2

Check for cracking:

estimate transverse beam to be 24 in. wide.

S- 2464 2 =64in3

f b _ 4 (12) - ±76 psi (o.k. for tension)64

Reinforcement required:

use 18 =12,000 psi

June 1967 35

typi 2S4 -..254

1.6_ 2(–r–r)8A

i6^' B 2'3(– "-) Section B

Section A(a) Elevation and section

2.54 25

^-- -^ 3,4 coil, 12 ' coil rodlegs (2 req'd.)

typical 1 , coil I 1.6'wing nut (4 req'd.) 4

1.6'

(b) Handling insert location

2.54' 2.54'

assumed 45' ^transversebeam

t temporary blocking

(d) Panel reinforcement8<6'x 1 , 6'lg. with 116 x 2 ' horiz. slo

1,Z--1 typ.

1' planned, use11 for design

30501b F forces= 1020 lb.(e) Base connection

6'x4? 5"Ig.with 2 hole r4- 1' threadedand sq. washer, I wing nut

(21 x21x) 560 lb. _IL

l ' shim as req'd.

4_ 1' threawing nut

5

shim as req'd.

Od (c) Transverse beam for edge turning

(0 Top connectionn

0

Fig. AI–Details for Design Example 1

_ 0.407 = 0.25 in.2A8 0.83(2) —

4 x 4-4/4 WWF present provides 0.24 in.2add additional 1—No. 3 across stripping inserts above and below WWF.

Turning Panel 90 0 Into Vertical Design

Turn panel with side inserts @ length/5 = 2.54 ft.Loads per tranverse beam = 318 plf

318(8)2 = 2540 ft.-lb.M^rr = 8

Critical point for cracking @ 0.2 width (see Fig. Ale)Mo.Q = 0.64 Met,. = 0.64 (2540) = 1630 ft.-lb.

Check cracking @ 0.2 width:

_ bh2 _ 38(4) 2 _ 101 in.3S°'^ 6 6

bo.2 = 0.4(8)12 - 38 in.

1630(12)=fb 101

±194 psi (o.k. for tension)

Reinforcement required @ 0.2 width:

f$ =12,000 psi

(A8)0.2 = 1.63 = 0.79 in.20.83(2.5)

4 x 4-4/4 WWF present = 381 12) = 0.38 in.22

provide additional 2—No. 4 above and below WWF.

Reinforcement required @ ctr.:f, = 12,000 psi

(A8)ctr - 2.54 1.22 in.=

0.83(2.5) _

b=12(2.54+4.0)=78in.

provide additional 2—No. 4 above and below WWF.

Wind Design

Assume 25 psf wind pressure. Place connections at length/5. Cracking andreinforcement o.k. by inspection since panel weight of 50 psf exceedswind load of 25 psf.

Storing, Shipping and Erection

All blocking to be at length/5 when panel in either flat or vertical position.Ship panel with 8-ft. dimension vertical (see Fig Aid).

June 1967 37

Base Connection Design

Assume each shear connection to resist 0.6 weight (see Fig. Ale)0.6 (5080) = 3050 lb.Select 1 in. 4) threaded wing nut insert

Select angle size:

Try 8" x 6" x'/2" L —insure that connection insert is 5d from free edge.Use 5-in, gage on vertical leg (2 required)

Check bending:

M = 1.5 (3050) = 4570 in.-lb. to angle

S = 4570 = 0.25 in. 3 for f3 =18 ksi

18,000

length b = 6S — 6(0.25) = 6 in.h2 — (05):

Check shear:

_ 3050v = 1000 psi (o.k. for shear)

6(0.5)

Determine connection weld:

F 4570 5 1020 1b.

by inspection, c,-in. fillet, 3 1/z-in. long

Note: WF supporting beam should be checked for torsion.

Top Connection Design

Wind suction and overturning force:

wind suction - 25 (12.7)8 ('/4) = 320 lb. per insert

overturning force -= 6 . 5 1 2) = 801b. per insert

6.5(12)

total = 400 lb. outward force

Wind pressure and overturning force:

wind pressure = 2 (320) = 640 lb. inwardoverturning force — 80 lb. outwardtotal = 560 lb. inward

Select angle size:'

try 6" x 4" x 3/s" L

5= 560(3.5) =0.089in.3 for f8 = 18 ksi18,000

b = 6(0.089) _ 3.8 in. long, use b = 5 in.(0.375)'1

38 PCI Journal

To allow for thermal volume changes provide oversized holes, 2 in. (A, with2" x 2" x s " square washer.

Handling Insert Selection

Stripping:

load = 50 =1270 lb.

select 3/4-in, coil, wing nut type

Turning and erection:

load = 5 280 = 2540 lb.max

select 3/4-in, coil with two 12-in, coil legs

DESIGN EXAMPLE 2

Design the window panel shown in Fig. A2 for all handling conditions.Assume concrete strength at stripping is 3000 psi.

Panel WeightAreas of sections A, B, C and D as well as others not shown have beendetermined by a planimeter, and these areas have been converted to lb.per ft. (plf) as indicated by parts [ 1] to [7] .

Part Weight (pif) Part Panel Weight (lb.)

[1] 26[2] 119 [2], Sect. A 2(13.14)119=3130[3] 244 [3], Sect. B (13.14)244 = 3200[4] 205 [4], Sect. D 2(2.37)205 = 970[5] 244 [5], Sect. C 2(2.37)244 = 1160[6] —19 [6] 4(1.15)(-19) _ —90[7] —19 [7] 4(1.17)(-19) _ —90

Total Wgt. = 8280

Exterior Mullion Stripping Design (Sect. A)

Study of the panel sections and dimensions reveals the only practical in-sert location is that shown in Fig. Ala.Exterior mullion loading (approx.) is given by Fig. Alb:

w= [2] - 120 plfP1 = [5] /4 = 1160/4 = 290 lb.P2 [3]/4+ [5]/4=290+3200/4=1090 lb.P3=[3]/4 +[4]/4 - 800 + 970/4=10401b.P4 — [4]/4=2401b.

Find bending moments:

MA = 290(1.7) + 120(3.5) 2 = —1230 ft.-lb.

June 1967 39

1.17'

8.97'0 r t29^

13,14

A

stripping I®inserts D

Back view

7061 5,

1=1 ^^8i' A

jI t Sec.

38a.

4q

C 53

2-112x5^^- M -15

6 1 1,a

D 3=5^20 434

1 — ___

(a) Panel dimension

P1 P2 41 E P P47W 5^top 120 pIf71

3.5'* 6A'. a2'±B Sec,E

A 13.1± (approx.)

(b) Section A stripping design

Fig. A2—Details for Design Example 2

MR = 240(1.6) + 120(3.2)2 _ —1000 ft-lb.

Mctr _ _ (1230+1000) 2 + 120(6.4) 2 = _500 ft.-lb.

Check cracking (approximate section properties, refer to Sect. E) :

Part A =y.A yo Ay 2 to

[1][2]

21.2111.6

10.94.4

231491

5.51.0

640112

32494

132.8 in.2 722 in .3 752 in.4 526 in.4

40 PCI Journal

P5r _ w=160 pit

r2A'^

A B

5•1

1_ 2L—! 414^43}L2'

Sec. F(approx.)

(c) Section C stripping design

P6FF P5 P6 AGO plf

08^ C I L ^0.8—.f ty

^^^--- 9:0= typA B

(d) Section C turning design

02 at 18 «2 '4'3(_ ^2at12 `4(n) "3(`-)- x444Wwf

ii

^5( 4x44wwf '5(—) 4(^) p 5(—) ^5(^) 4(`^) 4 (—) © 0 at 1S`

(e) Panel reinforcement

Fig. A2—Details for Design Example 2 (Cont.)

722 = 5.4 in.7.6 in.yb 132.8 ^`

T1280in.4; S t = 176 =168in."; Sb= 1580 =237in."

M, —1230(12) = —88 psi (o.k. for tension)(fv)^^,a^ = St 168

Transverse Stripping Design (Sect. C)

Transverse loading (approx.) is given by Fig. A2c:w = 0.4(2.6)155 = 160 plf (see Sect. F)P5 = [3] /2 = 3200/2 = 1600 lb.

Find critical bending moment:

by inspection, critical cracking at C where change in cross-section occursRA = RB = 800 + 160(2.4) = 1180 lb.

MC =1180(2.4) - 16022.4) 2 = 2370 ft.-lb.

Check cracking @ critical point C (assume approximate cross-section shownby Sect. F):

June 1967 41

Part A yo Ayo yo Ayo2 to

[1][2]

24.4147.2

6.92.4

168353

3.90.6

37153

37277

171.6 in. 2 521 in. 3 424 in. 4 314 in.4

521 —yb

_ 171.6 ns3.Oin.; yt=6.0in.

I = 740 in. 4; St= 740 =123 in. 3; S b = 340 = 247 in 3

(f) _ 2370(12) _ _115 psi (o.k. for tension)

b maa, — 247

Transverse In-Plant Turning Design (Sect. C)

Panel to be rotated 90° from flat position with side inserts to 9.0 ft. (±)dimension vertical for storing and shipping. Dimensions and loadings(approx.) are given by Fig. A2d.

w -160 plf, as beforeP5 = 1600 lb., as beforePs =P2 =1090 lb.

Find critical bending moment at point C:

RA = RB = 1090 + 800 + 3.4(160) = 2430 lb.

Mc = 2430(3.4) — 16i^(3.4)2

_1090(2.6) = 4500 ft.-lb.

Check cracking at point C:(f) 4500(12)

_ —218 psi (less than 250 psi, therefore o.k.)126 — 247

Wind DesignUse wind = 25 psf

Information not given concerning connection locations; assume the con-nection locations are similar to stripping locations.

wwind to exterior mullion = 2.2(25) = 55 plf (less than the 120 plf used forstripping)

No further wind design required since handling controls the design forcracking and reinforcement.

Reinforcement Design (See Fig. Ate)

Sect. A:

use f8 = 14,000 psi

1.23 = 0.15 in.2A. = 0.98(8.5)

42 PCI Journal

select 1—No. 5 for tension reinforcement. Remaining reinforcement aswell as the 1—No. 5 selected are for cage handling stability.

Sect. B:use same as for Sect. A (double mullion)

Sect. C:use f8 = 14,000 psi

_ 4.5 _0.61in.2`4$ 0.98(7.5) —

select 2—No. 5 for tension reinforcement. Remaining reinforcement byinspection and for cage handling stability.

Sect. D:provide same basic reinforcement as used for Sect. C.

Storing, Shipping and Erection

Store and ship panel with 9.0 ft. (-t) dimension vertical. All temporaryblocking to be directly under stripping insert locations. To erect, lift paneloff trailer and rotate 90° into final erection position. Erection requires twoside and top inserts.

Handling Insert Selection

Stripping:

load = 8 280

= 2070 lb.

select 3/4-in, coil with 9-in. loop.

Turning and erection:

ldmax = 8280 = 4140 lb.oa 2

select 3/4-in, coil with two 12-in, coil legs.

Discussion of this paper is invited. Please forward your Discussion to PCI Headquartersbefore September 1 to permit publication in the December issue of the PCI JOURNAL.

June 1967 43