of 26/26
Production and Design of Architectural Precast Concrete by C. H. Raths* SYNOPSIS This paper describes typical production methods and design guidelines for architectural precast concrete panels. Recommended design criteria and typical details for precast panels are presented. Design examples are given in the appendix. INTRODUCTION The rapid growth in the use of architectural precast concrete has created many changes and innova- tions in architecture. Today archi- tectural precast concrete is available in complex shapes which serve not only as curtain walls, but combine their attractive appearance with the ability to serve as main structural members. The realization of the pre- cast concrete market potential has resulted in the development of a highly specialized industry. With this market growth, the successful use of architectural precast concrete is be- coming increasingly dependent upon architects' and engineers' under- standing of the precasters' methods of production and of general precast concrete design guidelines. Until the architect, engineer and builder fully comprehend all facets of architectural precast concrete, its maximum potential will be limited. There must be awareness that, when architectural precast concrete con- tracts are awarded on a bid basis, high quality precast producers' bids often reflect costs which are not readily apparent from the plans and °Principal, CHAS. H. RATHS & ASSO- CIATES, Structural Engineers, Hinsdale, Illinois. specifications. When the contract is awarded at some lower bid, the final results can be less than desired or can involve many extra costs. PRODUCTION STAGES OF ARCHITECTURAL PRECAST CONCRETE The general stages in the produc- tion of architectural precast concrete can be broken down into seven broad categories which obviously are overlapping: 1. Engineering and detailing 2. Form construction 3. Production of units 4. Storage 5. Shipping 6. Erection 7. General handling To provide a clear picture of the im- portance of the seven steps and their interaction, each will be examined separately. Engineering and Detailing The first step in preparing the shop 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 that can influence decisions regarding the precast concrete. The goal of this analysis is to produce standardiza- tion of: precast units, modifications required of precast units, connec- 18 PCI Journal

Production and Design of Architectural Precast Concrete

  • View
    0

  • Download
    0

Embed Size (px)

Text of Production and Design of Architectural Precast Concrete

by C. H. Raths*
SYNOPSIS This paper describes typical production methods and design guidelines
for architectural precast concrete panels. Recommended design criteria and typical details for precast panels are presented. Design examples are given in the appendix.
INTRODUCTION
The rapid growth in the use of architectural precast concrete has created many changes and innova- tions in architecture. Today archi- tectural precast concrete is available in complex shapes which serve not only as curtain walls, but combine their attractive appearance with the ability to serve as main structural members. The realization of the pre- cast concrete market potential has resulted in the development of a highly specialized industry. With this market growth, the successful use of architectural precast concrete is be- coming increasingly dependent upon architects' and engineers' under- standing of the precasters' methods of production and of general precast concrete design guidelines.
Until the architect, engineer and builder fully comprehend all facets of architectural precast concrete, its maximum potential will be limited. There must be awareness that, when architectural precast concrete con- tracts are awarded on a bid basis, high quality precast producers' bids often reflect costs which are not readily apparent from the plans and
°Principal, CHAS. H. RATHS & ASSO- CIATES, Structural Engineers, Hinsdale, Illinois.
specifications. When the contract is awarded at some lower bid, the final results can be less than desired or can involve many extra costs.
PRODUCTION STAGES OF ARCHITECTURAL PRECAST CONCRETE
The general stages in the produc- tion of architectural precast concrete can be broken down into seven broad categories which obviously are overlapping:
1. Engineering and detailing 2. Form construction 3. Production of units 4. Storage 5. Shipping 6. Erection 7. General handling
To provide a clear picture of the im- portance of the seven steps and their interaction, each will be examined separately.
Engineering and Detailing
The first step in preparing the shop 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 that can influence decisions regarding the precast concrete. The goal of this analysis is to produce standardiza- tion of: precast units, modifications required of precast units, connec-
18 PCI Journal
tions, shop production techniques, handling methods, and erection. Aside from the general architectural shape requirements, the main factor in establishing standardization of the precast units is the building frame and its relationship to the architec- tural units (i.e. connection locations, clearances, etc.).
The preparation of the precast concrete shop drawings for archi- tectural approval is best divided in- to two separate submissions. Until the architect is satisfied that the pre- caster understands his concepts rela- tive to the panel size, shape, etc., it is fruitless to prepare final shop drawings detailing connections and reinforcement. Further, since the form construction requires the great- est amount of production lead time, the common goal of both the archi- tect and the precast producer at this initial stage is to determine all the details regarding the size and shape of the precast panels for the most economical and efficient production sequence.
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, are usually incorporated.
After receiving approval for shape only, the next step is the design or checking of the panel's reinforce- ment and connections. With the ex- act shape of the panels known, final reinforcement is based upon the re- quirements of stripping from the form, in-plant handling, yarding, shipping, erecting, and wind or other in-place loads. The basic connection design is determined by building frame 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 the connections and their locations, rein- forcement, and required notation is incorporated in the previous shop drawings for final approval prior to production. These shop drawings now explain in complete detail how the precast producer intends to per- form his part of the project.
Form Construction
Form or mold construction usually requires the greatest manufacturing lead time. Thus, the form construc- tion phase is usually initiated as soon as shape approval is received.
The type of form used to produce architectural panels for a given proj- ect depends upon many factors. Among the major factors to be con- sidered are: (1) architectural finish on the panel; (2) the number of casts to be made in the form; (3) the type and sequence of form modifica- tions for greatest production econo- my; (4) dimensional tolerances of precast member; (5) the details of the precast member being produced; (6) the number of similar molds; and (7) the amount of lead time available.
The types of forms or molds used to produce the precast concrete pan- els are mostly fiber-glass-reinforced plastic, wood and steel. Typically, fiber glass molds are used for the manufacture of smooth concrete while wood and steel forms find their greatest applications in making exposed aggregate panels or other types of finishes. The specific mold material selection is based upon the seven points listed above, -as well as on 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 be readily repaired and modified so that the effects of repairing and modify- ing are not reflected in the finished units. Well-constructed wooden molds can be expected to produce 50 casts. However, wood molds greatly show their usage towards the end of their life. As with fiber glass molds, wood forms allow for easy modification and dimensional con- trol. Steel molds, while having a long life, provide the greatest problems in modification and dimensional con- trol. The usual welded construction of a steel mold can induce perma- nent distortions. Other mold types used occasionally to produce archi- tectural units are concrete and sand.
Form or mold tolerances vary with individual precasters and the re- quirements of the projects. High quality mold construction, with the possible exception of steel, should produce units within ±1/s in. of the planned dimensions. Further discus- sion of mold details is presented la- ter in the Design Guidelines section.
Production of Units Production culminates the efforts
of the precaster's engineering and mold fabrication. The reinforcement cage for the architectural panel is usually made in a jig unless the pre- cast panel is a simple flat. In addition to satisfying the structural require- ments of the panel, the reinforcing cage must be designed so that it can be handled within the plant (three- dimensional stability) and have suf- ficient clearance when placed in the form. Usually the cages are pro- duced so that a 1-in, minimum cover of concrete exists over all reinforcing steel. It is common to tack-weld the cages since tie wires can come loose and may appear on the surface of the panel. It is also common to hang
reinforcing cages from the forms rather than to use supports that may show 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 of the form the side rails, end rails, blockouts, concrete inserts and any other materials cast in the panel.
Concrete matching the approved sample is placed in the form and consolidated in a variety of ways. Some precast manufacturers trans- port the form to a vertically vibrat- ing table. External vibrators are employed by other producers or in- ternal vibration may be used. Again, the manner of consolidation is closely related to the experience of the in- dividual precast manufacturer.
After a curing period (generally one day to accommodate a mass- production cycle), the panels are stripped and inspected. This phase in production is probably the most critical because the concrete is green and has a low strength of 1500 to 3000 psi. Frequently cracking results just after stripping due to thermal shock* or through minor mishan- dling. If the panel is to have exposed aggregate, the brushing or washing away of the retarder is done as soon as possible after stripping to accom- modate the production cycle. The same applies if the panel is to be sand-blasted or acid-etched.
An extremely important aspect of production is the inspection and quality control employed by the pre- cast manufacturer. The inspection starts with examination of the form for correct dimensions and must in- clude examination of the reinforcing cage and, most importantly, concrete insert locations. Good quality control
*Thermal shock results from some sections of the precast panel cooling more rapidly than others where they are adjacent.
20 PCI Journal
of the concrete insures that the stripping and handling strength will be achieved. Inspection and quality control 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 role in the production of high quality precast concrete. At initial storage, the concrete strength is still low and the 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 across three or more points, precautions must be taken so that the panel will not bridge over one of the supports and result in bowing and cracking. The problems of warpage cannot be completely eliminated, even with two-point support, when the panel is stored in either a horizontal or verti- cal position, although it can be min- imized by proper blocking of the panel in a given plane.
Often the manner of storing de- pends on how the panel is to be shipped and what limitations the panel's cross-sections impose on han- dling. For all practical purposes, the panel should be stored in the same manner in which it will be shipped. Proper storage must also give con- sideration to the potentially harmful effects of alternating sun and shade on the precast units.
The rule of two-point support also applies to shipping the panel from the precast plant to the job-site. Most precast concrete firms use ei- ther flatbed or low-boy trailers, and these units suffer excessive distor- tions during hauling. Thus, support at more than two points on a trailer
*Relative to blocking, this is an industry term denoting a line of support.
June 1967
unit can be achieved only after con- siderable modification of the unit.
Consideration must also be given to the size and weight of the archi- tectural panel being shipped. If the panel is shipped vertically, the equipment available for shipping and bridge clearances must be care- fully considered. It should be re- called that very often the decision to ship a precast panel flat or vertically is dictated entirely by the structural behavior of the panel when in these positions.
Significant economies can be achieved by having tractor-trailer units carry their maximum capaci- ties. These economies can be accom- plished only if the shape, strength and weight of the precast panels have been taken into account. Erection
Prior to the erection of precast units, the job-site is checked for truck and crane access. If erection is to take place in a congested or constricted area, scheduling and co- ordination with other trades must be worked out. The locations of all con- nections integral with the building frame or foundation (for load-bear- ing panels) are also verified as to position and elevation before any precast units are erected.
The importance of precast panel joint layouts cannot be overempha- sized. This assures even appearance of panel joints as well as identifying problems caused by building frame columns or beams being out of di- mension or alignment. Joint layout,. in short, guarantees that the panels will fit. For multi-story buildings, this joint layout check should be made every third or fourth floor.
There are a variety of ways by which an architectural panel is hoisted into position. The type of erecting equipment is determined by
21
the weight of the panel and distance of reach to set the panel. Whether the panel is simply picked off the truck or "spun" in the air is deter- mined by the limitations the panel poses to handling. The problem of how the panel is handled is the same as that discussed previously in pro- duction and storing.
Significant economies can be achieved if the panel is sized to min- imize the number of units which must be erected. A good rule is to make the panels as large as possible relative to general handling, erec- tion equipment or methods avail- able, and to the structural building frame. In addition to providing sav- ings on erection costs, larger sized panels provide secondary benefits of reduced amounts of caulking, better dimensional controls and fewer con- nections.
Connections play a key part in the erection procedure. Properly de- signed connections allow the panel to be secured in place while allowing for final alignment later. Connec- tions of this type are achieved gen-
erally by the use of bolting and economies are gained by using con- nections that are standard through- out the job. While material costs may be greater for standardized con- nections, the economy produced by efficient crane operation far out- weighs the increased material costs.
The final part of the erection phase is the cleaning and patching of the precast panels, if required.
General Handling
A knowledge of the various possi- ble modes of handling is pertinent to the proper understanding of an architectural precast panel.
Most precast concrete panels are cast 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, the panel may be stripped with or with- out an auxiliary spreader beam by means of overhead cranes or lift trucks. Usually the second step is to temporarily store the panel in a flat position to free the crane or lift truck for other operations. Next
__._. crane lineF spreader beam
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 into a vertical position for further work on the panel face (viz, patching, sandblasting, removing retarder, etc.) and then it is often supported at three or four points on a leveled area within the plant. The panel is either rotated back down on its face or left vertical to be transferred to storage. From storage, the panel is transferred to a tractor-trailer for shipping. It is well to repeat that the panels are stored and shipped on two points. After the panel arrives at the job site, it is either stored again or immediately erected. Depending upon how the panel is shipped, it may be either tipped up from the trailer bed, lifted off and installed or the panel may be rotated 90° while suspended from a crane after being lifted off the trailer.
Figs. 1 and 2 illustrate schemati- cally the various possible handling operations of an architectural pre- cast concrete panel.
DESIGN GUIDELINES OF ARCHITECTURAL PRECAST CONCRETE
The breakdown of design guide- lines into separate categories can be endless due to the variety of custom designs employed. However, for conditions usually encountered in architectural precast design, guide- lines can be set out for: (1) shape details, (2) inserts, (3) engineering properties of architectural precast concrete, (4) in-place loadings, (5) connections and (6) tolerances.
Shape Details
The shape details play a large role in determining the cost of a precast panel. 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
crane line
crane line
Fig. 2—Erection Handling
el is one where the form has no removable parts. This creates a min- imum set-up time and assures excel- lent dimensional control. An impor- tant aspect of shape relative to the form is the amount of draft. Without draft (generally a--minimum of 1-in. horizontal to 12 in. vertically), strip- ping cannot be accomplished with- out removing parts of the form. In addition to being a factor in strip- ping, draft also affects the surface appearance of the precast unit. Too steep a draft (greater than 1 to 8) can produce some entrapped air on smooth concrete. A good rule of thumb for smooth concrete is a draft
23
3 block
of l to 5. For exposed aggregate con- crete, good finish results are ob- tained with drafts varying between 1 to 5 and 1 to 12. While draft in- volves both production and surface appearance, it must be balanced with the architectural concept. Fig. 3 illustrates the draft concept.
Another aspect of shape involves a minimum radius of '/s in. on all edges to prevent chipping. This is more important for smooth, etched and sand-blasted panels than for exposed aggregate panels .Also, rela- tive mainly to smooth and sand- blasted panels, thought must be giv- en to preventing leakage especially where removable side or end rails attach to forms. This point of leak- age, which can mar the finish, is shown by Fig. 4a. A return as indi- cated in Fig. 4b will cause the leak-
A 1I hV
A.J Sec. AA
minimum for easystripping. concetesmooth
Fig. 3—Draft Concept
age to take place where it will not be seen.
Occasionally two types of finishes are adjacent as shown by Fig. 5. Un- less a distinct separation is made with a separator groove, the two fin- ishes can intermingle creating an undesirable effect. The dimensions given for the separator groove are
X to^4„ point of
—leakage mottled surface appearance
anel removeable
Fig. 4—Form Leakage
considered a minimum to insure the desired finished appearance.
Careful attention must be given to window details. The primary cause of problems is dimensional control. Fig. 6 shows several common details used to secure windows to the archi- tectural precast concrete. Here again, the best dimensional control is achieved by having the window blockout as a permanent part of the form, or, if a removable blockout is
used, the blockout should be of one piece.
Whenever possible, it is advisable to have the back of the precast panel flat. This relates to the previous dis- cussion on having the shape fixed as much as possible by non-removable parts of the form. By having the back of the panel flat, its back sur- face appearance is uniform and lev- el, leakage where two parts of a form join are eliminated, and good dimen- sional control is achieved.
A final consideration on shape is to provide a sufficient cross-section for embedment of inserts and reinforce- ment and, most importantly, strength for handling. Also, as was discussed earlier, shape determines how the panel will be handled. Fig. 7 illustrates the importance of shape as it concerns inserts, reinforcement and handling.
Inserts in Concrete
Inserts are a fundamental part of architectural precast concrete. With- out them, it would be virtually fin-
Structural window gasket Structural window gasket Blackout to receive on concrete lug into concrete groove metal window frame
Fig. 6—Typical Window Attachment Details
June 1967 25
not sufficient clearance,
clearance for Insert
vjJ dimemsionally unstable cage, uniform cover hard to maintain and unsym- metrical reinf. avoid if possible
proper cages-dimension- ally stable, easy to handle
Shape and reinforcement
V
LH
V
^H
Shape and handlir-
improper
imi?roper, vertical load
will crack -ng
Shape and inserts
possible to strip, handle and erect panels. Concrete inserts can be di- vided into two types: one type con- sists of bolted inserts and the other a wire cable type of insert.
The bolted insert is most widely used. They are available in many va- rieties and Fig. 8 illustrates some of the common types. These inserts are made with either a coarse coil bolt thread 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 their greatest use in connecting the pre- cast unit to the building frame. Fig. 8 lists the capacity of the inserts when tested in pull-out from plain concrete* blocks ranging from 2000 to 3000 psi in strength.
The looped cable type of insert finds application in stripping small flat 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 cable inserts is generally limited to two- point pick-up since their projection from the concrete is subject to varia- tion. Fig. 9 shows typical use of ca- ble type inserts and gives their usual load capacities.
Previous discussion has been lim- ited to the capacity of bolted con- crete inserts when its full shear cone is developed at failure. However, in- serts are frequently located close to the edges of precast concrete or in narrow sections so that a normal full shear cone cannot develop for a ten- sion failure (see Fig. 10) . Test data indicate 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 the insert which approxi- mates the shear cone surface area, sq. in.
k = a constant, which is a function of mix design, that varies from 1.9 for low-strength concrete to 6.0 for high-strength concrete.
Fig. 10 illustrates the actual shear cone and the assumed approximate cylindrical cone. If the full shear cone can develop, inserts with loops or struts 6 in. or greater generally fail by fracture of their steel at the junction of the loop and the threaded part after a small initial shear cone develops. For cases where it is obvious that the full shear cone can not develop because of the location of the insert, careful judgment must be used as to the safe load that the insert can carry. The capacity is reduced in proportion to the shear cone that can develop, or
reinforcement can be added that will insure development of the insert's safe working load. Fortunately, in- serts are rarely required to develop their full working load value.
Test data indicate that an insert loaded in direct tension has a smaller capacity than when loaded in pure shear. Therefore, shear values for in- serts may be assumed to be equal to their tensile values except when the insert is near a free edge.
A swivel plate, shown in Fig. 11, should always be used with a bolted insert. This assures that an angular pull will place the concrete insert primarily in tension. Also, inserts should provide a minimum safety factor of 21/2.
Engineering Properties of Architec- tural Precast Concrete
One of the basic criteria from an engineering viewpoint is to design the stripping, storing, handling, ship- ping and erection of an architectural precast panel so that it will be crack
coil coil thread thread wire
loop
3/q s.w.l. 4500`
3/q or 1 s.w.l. 9000* Note! threads can also be machine threaded weldments
coi le oil thread
(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) allowable concrete stresses; and (2) reinforce- ment design criterion.
With regard to stripping, han- dling, shipping and erection, this writer's experience has led to the development of the following cri- teria:
1. Initial concrete flexural crack- ing which can be observed, but not measured, occurs at a con- crete strain of 150 millionths.
2. Form suction and handling im- pact can be neglected if the maximum flexural strain is lim- ited to 75 millionths.
3. The modulus of elasticity of the concrete for tension and compression is as defined by ACI 318-63 and equal to E0 _ w1.5 331 fc•
4. Reinforcing design is based up- on the conventional cracked section used in the working stress method, at a stress level to keep any potential cracks hairline in width.
mesh
usual working loads for aircraft cable
diameter in. capacity lbs. 3/ g 1250 1/4 2000 5/ 3000
5000 1/2 10000
5 assumed point of wireshear cone failure usual failure
45°± shear cone
r\ total possible
cRactual shear cone
28 PCI Journal
insert placed — mostly in tension
Fig. 11—Swivel Plate Assembly
The major problem in design of architectural precast concrete is cracking and thus flexural tension becomes the controlling factor. Since it is awkward to work with a strain criterion as stated in the assumptions, Table I represents allowable stresses, fb = E0e, for different concrete strengths where w = 150 pcf.
TABLE 1—Recommended Maximum Concrete Flexural Stresses
Concrete Strength, psi
Maximum Flexural Stress, psi
2000 200 2500 225 3000 250 3500 270 4000 285 4500 305 5000 325 5500 335 6000 350
The bending or flexural stress, f b = M/S, is determined from the bend- ing moments the section must resist due to the dead weight of the panel and the section modulus of the gross cross-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 cracked section. If the strains never exceeded 75 millionths, only temperature rein- forcement would be necessary. How- ever, if cracking does develop, it should be limited by the reinforce- ment to minor hairline cracking hardly visible to the eye and not measurable by ordinary means. As- suming that initial flexural cracking which can be observed occurs at a strain of 150 millionths, then to keep the cracks hairline in width, the steel stress should be
fs=E'8e using e$ = 150 X 10 -6 and
E8 = 29 x 106 psi f8 = 29 x150 = 4500 psi
The limitation of f , to 4500 psi is extremely severe and, since the full cracked section will not be permit- ted to develop, this writer has found that an f $ of 12,000 to 14,000 psi is a practical value for design use. Rather than determine the location of the neutral axis based on the cracked section properties, it is con- venient to calculate the A8 required from the general approximate rela- tionship
A=_M 8
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 psi d = depth from extreme
compression fiber to cen- ter of reinforcement.
After complete reinforcing steel area is selected, care should be taken
June 1967 29
in arranging the cage configuration. As previously explained, the cage must be handled within the produc- tion shop and therefore requires handling rigidity. Also, thought must be given to placing the reinforce- ment as symmetrically as possible about the panel's cross-sectional cen- troid. This is particularly true for flat panels. If the reinforcement is not placed symmetrically, shrinkage-in- duced bending will cause bowing of the panel.
An area of controversy regarding precast concrete reinforcement is whether it should be galvanized. This writer's experience leads to the recommendation that welded wire fabric be galvanized only in thin panels. Reinforcing bars should not require galvanizing if proper clear distances are maintained (1 in. mini- mum) and quality concrete is em- ployed having strengths of 5000 to 6000 psi at 28 days.
In-Place Loadings Architectural precast panels can
be subjected to three types of load- ings—gravity, wind or earthquake, and, for load-bearing panels, floor loads.
Gravity loading design usually presents no problems unless the main support for the panel is at the top, thereby placing the panel cross- section in tension. Good design pro- cedure dictates that the entire panel should be supported entirely at one level (only two points) so that the panel weight keeps the entire cross- section in compression. This means locating the main connections near the bottom of the panel.
The usual lateral force -considera- tion is wind. In addition to the wind pressure normal to the plane of the panel, wind suction should also be considered. Experience indicates that the suction loading applies more
to the connections than it does to the panel. 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 details must be developed to completely eliminate them, are loads created by restraint of panel movement caused by temperature changes or by floor live loads applied to the panel through kicker or tie-back connec- tions. Vertical slots, as discussed in the section on connections, provide a method for preventing temperature and floor live load forces from being induced into the precast panel.
Connections
Enough emphasis cannot be given to connections. They are the key to proper structural behavior and econ- omy. Basically there are four con- nection types: shear, combined mo- ment-shear, tension or compression, and partial moment-shear. Design factors include allowable tensile and compressive 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 panel supporting the panel's gravity load. With a shear connection, provision must be made for resisting torsional stresses induced into the supporting member. In both cases, steel and concrete, the structural building frame must develop adequate rota- tional resistance. Depending upon the cantilever projection beyond the structural frame, the connection, if an angle, may require a gusset. The connection should be completed by bolting to the precast unit, and, whenever possible, by bolting the connection to the building frame.
30 PCI Journal
gussets, if required
(a) Shear connection to steel beam
cast in plate, anchored to resist all forces
note, boltinq also used where weld indicated
—c-i. p. concrete beam
Fig. 12—Panel Shear Connections
If the building frame spandrel cannot provide rotational resistance (this applies more to a structural steel frame than to concrete), the connection at the precast panel must develop moment capacity. Fig. 13 shows three types of moment con- nections—two for heavy bending and the other for light bending. An economical solution for situations re- quiring heavy or large bending to be resisted by precast panels is to cast a wide flange or similar section direct- ly into the panel as shown in Fig. 13a. Fig, 13b indicates an alternate bolted system where that part of the connection within the panel must in- clude a steel plate to insure proper contact between the precast panel and the connection for heavy bend- ing. This type of connection requires inserts to resist tension and shear and 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 the plate cast into the precast unit and letting the connection device bear directly against the precast panel, Fig. 13c.
Tension or compression connec- tions keep the precast panel in a plumb position and are sometimes referred to as "kicker" connections (see Fig. 14a) . They also serve to re- sist wind loadings. Usually these connections are made with angles and gussetted if required by the magnitude of horizontal forces. A variation of this type of connection is shown with a typical tension-com- pression connection in Fig. 14b. The
V^ cast in panel
into steel beam Yz clear
(a) Heavy bending, connection cost into panel
FC cost in double concrete-_ panel inserts, welded togusset plate _ I welded studs or hooks as required
shim over minimum
web only dimension
other details same as in (b)
(c) Light bending, bolted connection
Fig. 13—Panel Moment Connections
31
Mi Note! kicker angle can induce torsion into VF beam
resulting from Overturning and wind suction
(a) Plumbing tension-compression connection
or s
double nut and double square washer
threaded rod
ered if requiredrui
Fig. 14—Panel Plumbing Connections
advantage of this variation is that it allows for plumbing after the precast panel is in position on the structural building 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 general method of preventing movement re- straint is to provide slotted holes vertically. An additional means of eliminating restraint is to select a connection that does not develop flexural rigidity (i.e. use a minimum thickness of clip angle, etc.) . How- ever, care must be taken to insure that the kicker connection can satis- factorily resist panel overturning forces as well as wind suction forces without excessive deflection or stress.
The partial moment-shear connec- tion is a type frequently used for load-bearing panels. The connection is accomplished through the use of neoprene pads and coil rods as shown in Fig. 15. This connection permits development of a horizontal force necessary for panel equilibri- um, but it is necessary to tie the floor slab back to another part of the structure to develop full lateral force resistance. An advantage of this con- nection type is the speed with which it can be accomplished. Slight foun- dation settlements or floor live loads will not create a significant bending action in the floor system or the pre- cast panel. The basic principle of this partial moment-shear connec- tion is the ability of the properly de- signed neoprene pad to deform in shear and thus prevent the formation of a force couple. If the shear force becomes greater than that causing deformation, the neoprene pad will simply slip and destroy the moment couple.
A simple, yet effective, base con- nection for precast load-bearing pan- els is that illustrated by Fig. 15b. Leveling blocks produced by the precast manufacturer in lengths from 8 to 24 in. are shipped to the job site prior to panel erection. These leveling blocks are set in grout in the previously formed grooves in the foundation wall and adjusted to the proper elevation and alignment. The panel, when simply set on the grouted-in-place leveling block, is automatically set properly. Follow- ing the panel's erection, a dry pack, non-shrink grout is placed between the panel and foundation wall. Gen- erally the grout is placed only in the vicinity of the leveling block to in- sure that loads are transferred at known points (usually under the panel mullions) .
32 PCI Journal
neoprene pad
panel haunch
(a)
precast leveling block set in grout bed ,. ,dry pack as required
groove cast in foundatio wall (b,
interference with completion of the connection.
In summary, all connections should provide for temporary and final securement of the precast panel to the building frame; allow for free movement due to temperature ex- pansion and contraction; different types should be kept to a minimum; and the connections should be struc- turally adequate.
Tolerances
The selection of proper tolerances is a difficult task. Two types of toler- ances must be considered, those af- fecting the dimensions of the precast units and those required for connec- tion of the precast elements to the building frames.
Fig. 15—Load-Bearing Panel Connection Details
A problem all too frequently en- countered with architectural precast panels is the lack of room to make the connection. This results in in- creased connection costs both in ma- terials and in requiring different kinds. Moreover, using a different connection type may require altering the position of the insert within the precast panel. In short, required con- nection clearances are a critical part of the preliminary architectural planning.
Some mention should be made about 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 the column cover and the column, and adjustability of the connections in the field. The very minimum clear- ance between column covers and column is 1 in.; 1'/z to 2 in. is recom- mended because of columns being out of plumb or dimension causing
June 1967
mn. - ^ horizontal
inslot angle
caulK % min.
cover I
1 concrete insert
concrete insertweld shelf angle to ga steel col. flange or angle support
Section A
anangle for steel column secure to adjacent
support covers or connect between II to angle support flanges L—. ^__g se
Column cover plan section
33
Fig. 17—Typical Recommended Tolerances For Architectural Precast Panels
A reasonable tolerance for the pre- cast unit's dimensions is -1- 1/s in. For either steel or concrete framed struc- tures supporting precast concrete units an overall vertical and horizon- tal connection tolerance of ½ in. seems to be the most practical, as shown by Fig. 17. Fig. 17 also shows typical tolerances between the pre- cast panel and the building frame. Occasionally the case for greater tol- erances can be stated. However, it is better to make adjustments in the field since the percentage of connec- tions requiring a tolerance greater than 1/z in. represents a minor pro- portion of the total. Another guide to the selection of connection toler- ances are the standard tolerances set
for construction in steel and concrete by the AISC and ACI building speci- fications and recommendations.
SUMMARY
Obviously, the wide range of top- ics discussed in this paper can only consider the major or general items. Subjects in architectural precast panels which were not discussed in- clude, but are not limited to, pre- stressing of panels, thermal and moisture gradients within the panels, concrete backups (lightweight and normal weight concrete), expen- sive facing concretes, sculptured concrete panels, and insulated pan- els.
The successful and proper use of architectural precast concrete is de- pendent upon panel size or dimen- sion for best production, transporta- tion and erection; cross-sectional shape 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 available by Superior Concrete Accessories, Franklin Park, I11. All figures were prepared by R. W. Johnson.
34 PCI Journal
DESIGN EXAMPLE I
Design the simple flat panel shown in Fig. Al for all conditions. Assume concrete strength at stripping is 3000 psi.
Panel Weight
Stripping Design—Longitudinal
50 (2.54) 2 __ —161 ft.-lb.Mrna. = 2
Check for cracking:
Reinforcement required:
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
Total load carried per tranverse strip = 5080 = 318 plf 2(8) —
318(1.6) 2 = -407 ft.-lb.Mmax - 2
Check for cracking:
S- 2464 2 =64in3
f b _ 4 (12) - ±76 psi (o.k. for tension) 64
Reinforcement required:
Section A (a) Elevation and section
2.54 25
typical 1 , coil I 1.6' wing nut (4 req'd.) 4
1.6'
t temporary blocking
(d) Panel reinforcement 8<6'x 1 , 6'lg. with 116 x 2 ' horiz. slo
1,Z--1 typ.
30501b F forces= 1020 lb. (e) Base connection
6'x4? 5"Ig. with 2 hole r4- 1' threaded and sq. washer, I wing nut
(21 x21x) 560 lb. _IL
l ' shim as req'd.
5
(0 Top connection n
_ 0.407 = 0.25 in.2 A8 0.83(2) —
4 x 4-4/4 WWF present provides 0.24 in.2 add 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:
bo.2 = 0.4(8)12 - 38 in.
Reinforcement required @ 0.2 width:
4 x 4-4/4 WWF present = 381 12) = 0.38 in.2 2
provide additional 2—No. 4 above and below WWF.
Reinforcement required @ ctr.: f, = 12,000 psi
(A8)ctr - 2.54 1.22 in.=
Wind Design
Assume 25 psf wind pressure. Place connections at length/5. Cracking and reinforcement o.k. by inspection since panel weight of 50 psf exceeds wind 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:
S = 4570 = 0.25 in. 3 for f3 =18 ksi
18,000
Check shear:
6(0.5)
by inspection, c,-in. fillet, 3 1/z-in. long
Note: WF supporting beam should be checked for torsion.
Top Connection Design
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. inward overturning force — 80 lb. outward total = 560 lb. inward
Select angle size:'
5= 560(3.5) =0.089in.3 for f8 = 18 ksi 18,000
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, with 2" x 2" x s " square washer.
Handling Insert Selection
Turning and erection:
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 Weight Areas of sections A, B, C and D as well as others not shown have been determined 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 plf P1 = [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:
June 1967 39
1 — ___
(a) Panel dimension
P1 P2 41 E P P4 7W 5^ top 120 pIf71
3.5'* 6A'. a2'± B Sec,E
A 13.1± (approx.)
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]
21.2 111.6
10.9 4.4
231 491
5.5 1.0
640 112
32 494
40 PCI Journal
r 2A'^
A B
5• 1
Sec. F (approx.)
08^ C I L ^0.8 —.f ty
^^^--- 9:0= typ A B
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
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 occurs RA = 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 shown by Sect. F):
June 1967 41
[1] [2]
24.4 147.2
6.9 2.4
168 353
3.9 0.6
371 53
37 277
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 before P5 = 1600 lb., as before Ps =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
_ —218 psi (less than 250 psi, therefore o.k.) 126 — 247
Wind Design Use 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 for stripping)
No further wind design required since handling controls the design for cracking and reinforcement.
Reinforcement Design (See Fig. Ate)
Sect. A:
42 PCI Journal
select 1—No. 5 for tension reinforcement. Remaining reinforcement as well 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 by inspection 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 temporary blocking to be directly under stripping insert locations. To erect, lift panel off trailer and rotate 90° into final erection position. Erection requires two side and top inserts.
Handling Insert Selection
Turning and erection:
select 3/4-in, coil with two 12-in, coil legs.
Discussion of this paper is invited. Please forward your Discussion to PCI Headquarters before September 1 to permit publication in the December issue of the PCI JOURNAL.
June 1967 43