LOAD-BEARING WALL PANELSDESIGN AND APPLICATION

Ivan Varkay

Schokbeton Quebec, Inc. St. Eustache, Quebec, Canada

Looking back a few decades in the history of architectural precast concrete, we can detect a definite trend in this man-made product. In the earlier days, it was a simple substitute for other natural stone products such as window sills, copings, paving stones, and the like. Later, partial or complete cladding of buildings was done with concrete units as it has been done for centuries in marble and other stone. With the development of curtain walls, the precast concrete industry followed suit and, by improved manufacturing and erection techniques, soon produced large elements such as whole window units or wall components. With the continuous drive to build buildings more efficiently and economically, eve started to use curtain wall elements not only as architectural building products but as structural components as well by having them substitute for normally used columns and beams or block and brick walls. In this presentation, I would like to analyze load-bearing wall panels, their application and design, their manufacturing and erection requirements, and attempt to evaluate them from an economic point of view as well. 34ҟ

APPLICATION OF LOAD-BEARING WALL PANELS

Load-bearing wall panels can be divided into two categories: 1) single-story elements for industrial type buildings; 2) single-story or multistory elements for institutional or residential buildings. Single-story industrial buildings can be constructed economically with load-bearing wall panels, where the roof may consist of precast, prestressed elements such as double tees, hollow-core slabs or other conventionally used structural elements. The wall components are most commonly the less expensive type of standard units, such as double tees or single tees, which can be mass produced economically in a prestressing plant. Depending on the climatic condition and the use of the building, they can be insulated or non-insulated. The insulation can be applied either in the field after erection of the wall components or produced with the unit in the plant and covered with a protective layer of concrete. From a structural point of view, the wall panels provide support for the roof structure and serve as shear walls for lateral forces. From an arPCI Journal

Design of load-bearing wall panels requires consideration of manufacturing, transportation and erection requirements in addition to design loads of the completed building. Full recognition of all factors results in efficient and economical wall systems.

chitectural point of view, the aesthetic requirements are less demanding, therefore the finish can be ordinary gray concrete using off-theform finish or various degrees of sandblasting to give a more uniform appearance. For the institutional or residential buildings, the load-bearing wall panels can be made several stories in height up to the maximum transportable length, or one story high and jointed at every floor level. The architectural requirements generally govern, therefore we can have the variety of shape and surface finish commonly associated with curtain walls, provided the structural and other technical requirements can be satisfied at the same time. From a structural point of view, they are also an integral part of the structure, taking the vertical and horizontal loads imposed on the building, or transferring loads to other structurally used building parts such as castin-place shear walls or elevator cores. DESIGN OF LOAD-BEARING WALL PANELS

When designing a load-bearing wall panel the following requirements must be taken into account: July-August 1971ҟ

1. Create good architectural effect 2. Withstand the load imposed on the elements 3. Take movements due to temperature variations, creep and shrinkage 4. Provide the required thermal insulation 5. Comply with fire rating requirements 6. Ensure minimum maintenance cost. To create the desired architectural effect, the architect has a full range of standard or custom-made shapes, sizes, textures, and colour attainable through the use of precast concrete. Through this infinite variation, the final result is limited only by the designer's imagination—something no other material can offer in such a variety. In addition, the wall elements can incorporate insulation, openings, windows, doors and ultimately mechanical components as well. Nevertheless, the architect must fully recognize the limitation of the elements from a structural point of view. Therefore, in order to achieve the best results, he must work hand in hand with the structural designer and the manufacturer. The structural design of the load35

II L

1 _ 1- 7 .

—

Fig. 1. Single-story industrial buildings

bearing wall elements can be divided into three steps: 1. Requirements during the manufacturing process 2. Hauling and erection loads and forces 3. In-service design loads In the manufacturing stage, the elements must withstand, without any ill effect, the stresses imposed on them during stripping, handling and storage. When analyzing the concrete stresses due to stripping, depending on the shape of the elements, the type of mould, and re36ҟ

lease agent used, an impact factor varying between 50 and 100 percent must be taken into account which can only be assumed based on past experience. In addition, the stresses in the reinforcing steel, if the product is not prestressed, should be kept preferably in the 12,000 to 14,000 psi range which will allow production of practically crack-free elements. In case of prestressed products, based on my past experience, the tensile stresses should be limited to not more than 200 to 300 psi in order to insure a high quality product. If these conditions cannot be satisfied, it is preferable to use auxiliary bracing, such as stiff backs, which in turn will take the stresses and enable production of an undamaged product. The same principles must apply when analyzing loading conditions during transportation. As a second step in the design, the hoisting and the erection conditions must be taken into account. I would like to emphasize especially the erection conditions which, for this very short period of time compared to the lifetime of the structure, require an entirely different analysis. Due to applied temporary bracing, the support conditions are different from the final one; nevertheless, the elements must be capable of taking applicable wind and earthquake forces without any damage. As a final stage in the structural design, the service conditions must be analyzed. Besides its own weight or gravity load, the elements must take vertical floor and roof loads and the horizontal forces due to wind and earthquake, if applicable. In some designs the wall elements must be capable of transferring all or part of the horizontal loads to other structural elements. Careful analysis and design is required due to the seePCI Journal

Fig. 2. Typical industrial building

ondary effects such as creep, shrinkage and thermal movements. Typical situations. As far as structural schemes are concerned, I would like to show a few basic arrangements. For single-story industrial buildings, depending on the wall section and the foundation conditions, the load-bearing wall panel can be made fixed at the base and the roof element freely supported on it (Fig. la). Or, depending on the shape of the building, the flexural stresses can be reduced on the wall elements simply by freely supporting them on the foundation and providing shear wall bracings at the ends or across the building to insure lateral stability (Fig. lb). For medium rise buildings up to two or three stories high, the wall elements are considerably taller, but they can be freely supported at the base with the floor and roof elements fixed to the wall components to provide a rigid or semi-rigid frame structure (Fig. 3a). As an alternative, the floor and roof elements can be freely supported with shear wall bracings to insure lateral stability of the structure (Fig. 3b). In multi-story or high-rise buildings, the most commonly used arJuly-August 1971ҟ

(b) Fig. 3. Medium-rise buildings 37

rangement is a cast-in-place core, which can incorporate the mechanical components and elevator shafts and take all horizontal loads imposed on the building, with wall

Fig. 5. Multi-story building 38ҟ

components acting as columns to receive vertical floor and roof loads and transfer them to the foundation. The floor and roof components are freely supported with hinged connections at the wall and core intersections (Fig. b). This arrangement can be economical, not only from a structural design point of view but also from the viewpoint of over-all construction because, while the core is being cast and the mechanical components installed, the precast manufacturer can proceed with the fabrication of the precast components and install them after the castin-place work has been completed. Within this framework, depending on the shape and use of the building, a number of structural sub-systems can be worked out which will satisfy all the structural requirements of the building. Insulated panels. In order to achieve maximum economy, load-bearing wall panels can be made as sandwich-wall elements incorporating the required insulation at the time of manufacturing. Fig. 7 shows two PCI Journal

heat-flow charts which represent the temperature variation through the wall section due to two extreme outside temperature conditions. In Fig. 7a, the 6 in. structural component is on the inside protected by 2 in. insulation and a 2 in. concrete surface. In Fig. 7b, the conditions are re-

versed with the 6 in. structural component on the outside and the 2 in. insulation and 2 in. protective layer applied on back surface. As can be seen from these charts, the first condition gives a very small temperature variation of approximately 25 deg. F. in the structural component,

F.

+I^oo°

4 I00 1+0

i Ibo

^

R t57

^ +IreO

.. s9'

140

120 loa $

M

mo

a

i2

`moo

40

4o

So

zo

ze

S4

vs

59

I /,^

°

ca,

7tiickne5s, in. Conduc^ivi1y

U

to

}

= 0.104

g.

2.,

os to

B+u.so. ^^. /r.

L

-19

I

7tiicknessin. Ccnduoi vy

0 0.25

IC

U = o. io (b)

Fig. 7. Heat-flow gr adients through insulated panels July-August 1971

39



Ro n l rrier, n..pre ?•rfOraied /'ube rexpansiec chamb.,

PrecasF wall po„el Air seal ; closed cell spo„e r b6.rl

Fkxlble rain screen (with serrated ed9as) e„ded ,-cm ^I ^pf»^,^ ab.^e E ccnc;o„ ehanber

P gnash co„cre+e ^ pa„el eficaf fnbe )v- se.r

^

F

ode

^Q°

ii!: (b)

(a)

Ccpa„sion chamber s Neororene sfrip

Fig. 8. Typical vertical joint details

ҟ

oaa^•^a^

,;

Rra^ra^

i`

.

bloror rllin9 (()

as against the second condition where a maximum temperature variation is affecting the component. Therefore, if possible, the structural element of a sandwich-wall panel should be kept on the warm side to reduce thermal stresses due to temperature variation. A final stage in the design of load-bearing wall panels is consideration of the joint design. A good joint must be made weathertight, air-tight and, in certain conditions, capable of transferring load from one component to the other. The sealing material must also be able to take movements due to load-

Joint design.

40

ing and/or secondary effects. In Fig. 8 are three joint designs which can satisfy most of the conditions met in load-bearing panels; these joint arrangements have proven satisfactory in curtain wall applications as well. The designs shown are normally known as two-stage joints making use of the rain barrier principle. A rain barrier (Fig. 8a) prevents most water from penetrating into the wall and the small amount which may enter is collected in the expansion chamber and drained out of the building at every floor level. Behind the chamber is an air seal, which can be a closed-cell sponge rubber allowing 50 percent compression of its PCI Journal

5 -J_^.b wait

original dimension, to seal off the building from the outside. The other designs show similar arrangements where the rain barrier consists of a flexible screen. A satisfactory horizontal joint detail for load-bearing sandwich-wall panels is shown in Fig. 9. MANUFACTURE OF LOAD - BEARING PANELS

The manufacture of load-bearing wall panels must combine the infinite care of architectural precast concrete production with the rigid discipline demanded when producing structural concrete products. Manufacturing can be broken down into eight steps: 1. Form preparation or construction 2. Preparation of reinforcing steel, metal hardware and miscellaneous components 3. Set-up of beds or forms 4. Casting the elements 5. Curing the elements 6. Stripping the elements 7. Finishing 8. Storage Forms can be made out of wood, steel, concrete or fiberglass. Dimensional tolerances of these forms must be the same as that demanded for architectural precast concrete. When the elements are produced in conventional steel forms for structural units, such as double tees or single tees, the forms must be verified for dimensional tolerances, cleanliness, possible joint marks and straight chamfers since any imperfection will be accentuated and much more visible on wall panels than on floor or roof units. The same high quality is required of any component which is used as a wall panel but made on standard long-line prestressed production, therefore the bulkheads, eiJuly-August 1971ҟ

I."

F

Neetr^ene

^v

sroҟ

Sanowrcn wo1( panel

Fig. 9. Typical horizontal joint detail

ther standard or custom made, must fit snug in the form and perhaps be covered with a sponge rubber seal to prevent leakage or honeycombing at these points. Similarly, the prestressing equipment, such as jacks, pumps and gauges, must be sensitive enough to allow partial prestressing or very lighttensioning of the strands depending on the requirement. When custom made moulds are used for production, the same basic requirements must be followed as for high quality architectural precast concrete production. In addition, the forms must be sturdy enough to withstand not only the required number of castings, but also the extra weight which might be imposed on them due to the size of the load-bearing precast elements. The reinforcing steel cages must be rigidly constructed, preferably tack-welded, on custom made or standard steel jigs. Depending on the shape and structural requirements of the precast product, specially designed cages of welded wire fabric, either smooth or deformed, can be used economically in conjunction with conventional reinforcing bars. 41

The miscellaneous metal components, such as plates, bolts, anchors, and the like, should be fabricated, based on clear and well-defined specifications, taking into account the grade of the steel, the radius of the bends, the size of the weld, the type of welding rods, and the sequence of welding of the various components. After fabrication, they must be checked, not only for conformance with the specifications and drawings prepared by the designers, but also for brittleness or breakage. If these components are to be galvanized, the same verification process must also be done after galvanizing. When all components are fabricated and verified, they must be set up in the form on a trial basis. At this stage of production, a careful, critical inspection must be made to verify all components against the shop drawings and make certain that they are going to perform as planned. Cages should have the specified cover and spacing of bars. The various anchors attached to the metal components should overlap the cages or be attached to them. Any additional components, such as reglets or window inserts, must be checked for tolerances or interference with other components. Any discrepancies which might be found at this inspection stage should be reported to the designer who can verify the problem and, if necessary, modify the set-up in order to satisfy the basic requirements. After the components are well fitted and attached to the form or additional supports, the concrete casting operation can proceed. Casting can be done in any conventional way, using either external or internal vibrators, vibrating tables, or other techniques for consolidation. 42ҟ

If different mixes are used in the same unit, either for architectural or economic reasons, special attention is required to avoid cold joints between the mixes which might jeopardize the structural behavior of the unit. In the case of sandwich-wall panel construction, when the insulation is applied to the wet concrete and covered with an additional protective layer, the metal ties must be well embedded in both layers as planned in order to avoid any possibility of separation. Similarly, the insulation must be well butted together in order to avoid thermal bridges through the finished product. The curing of the product, depending on the facilities and type of production, can be done either with live steam or by heating the moulds or the curing area, but in any case, the structural requirements govern and must be strictly followed. When the product has reached the designed stripping strength, the stripping operation can proceed. During this operation, special attention and care is required to avoid damaging, cracking or possibly breaking the units. If the wall units are made in a long-line prestressing operation, they are normally prestressed with zero eccentricity, and hence they are not self-releasing from the forms when strands are cut. Therefore, the adhesion between the product and the form is much greater and, consequently, a slow jacking operation might be necessary prior to using overhead cranes or other lifting equipment. Large, conventionally reinforced products might have to be stripped with specially designed stiff backs or frames, or the use of a series of lifting hooks and a spreader beam, to insure equal tension on the lifting cables. Even PCI Journal

so, the adhesion might be so great between the form and the product that jacking or other releasing methods might have to be used before other equipment can take over. Depending on the shape and size of the elements, tilting tables or forms can be used economically on a large repetitive production. These types of forms and stripping methods are most desirable. After stripping and plant handling, finishing the elements can follow, in which case the architectural requirements are the main consideration. The same procedure and care must be followed as for any high quality architectural precast concrete product. Until the elements are ready to leave the yard, they must be stored in a vertical position if the units are conventionally reinforced. During this vertical storage period, the elements must be supported on two points only and laterally guided, preferably at both ends, in order to avoid bowing or warping (Fig. 10). In addition, the elements must be located in a way that the hot rays of the sun do not overheat one side which may create deformation due to the thermal stresses. In case of long prestressed elements, the units must be stored in a way that the positive and negative bending moments are equalized and the units will not deform during storage. ERECTION OF LOAD-BEARING PANELS

The erection of load-bearing wall panels can be broken down into six basic steps: 1. Project evaluation and site survey 2. Shipping 3. Hoisting 4. Assembling 5. Joint application July-August 1971ҟ

Fig. 10. Proper storage of precast wall units

6. Final finish Prior to the erection of precast units, the erection personnel must be fully aware of the basic design requirements and have a full undef standing of the function of the components in order to insure the design behaviour of the individual elements and the building as a whole. Further, the job site must be checked for truck crane access so that, even in congested areas, they will not run into unforeseen difficulties which may result in serious delays and unnecessary expense prior to or during erection. The receiving elements, such as foundation walls or pile caps, must be checked for correct elevations and line in order to guarantee that the precast components will have proper position and elevation, and so that the assembly can be done without any serious delay which could jeopardize the economy of construction. When the survey has verified satisfactory conditions, the proper layout and sequencing of the precast components must be made, including the 43

I i + BM = - SM

0. •

L.

Fig. 11. Transportation of long prestressed load-bearing units joint details, which in turn will facilitate and speed up erection. Any discrepancies which might affect the structural behaviour of the components should be reported back to the designer who will verify the overall behaviour of the elements and the building. With these necessary precautions, erection can proceed

smoothly according to the carefully analyzed sequence and plan. Shipping the elements should also be planned in advance. The shipping personnel must be fully aware of the basic requirements or special equipment which might have to be used in order to prevent any damage during transportation. The general rule

C^ 1 rr0 — -------- ---

Fig. 12. Transportation of reinforced concrete multi-story panels

44ҟ

PCI Journal

Fig. 13. Transportation of single-story panels

is that every element should be shipped on only two supports. Comparatively long prestressed elements, such as double tees or single tees which are serving as loadbearing components, should be shipped on pole trailers in a way that the positive and negative bending moments are nearly equal and not exceeding the capacity of the members (Fig. 11). If these basic requirements cannot be satisfied, auxiliary bracing, such as stiff backs or space frames that can take either total or partial load during transportation, must be attached to the elements prior to loading. Multi-story, normally reinforced elements which are thin compared to their length or width, should be shipped on the edges (Fig. 12), taking advantage of the large moment of inertia, but supported on only two points in order to avoid any chance of bridging over a support during transportation. Single-story elements, which are commonly used in industrial buildings and high-rise construction, should be shipped in a standing position on their edges (Fig. 13) from which they can be hoisted directly into place. July-August 1971ҟ

Fig. 14. Trailer-mounted A-frames provide lateral support during transportation

The best equipment for shipping these components are the specially designed A-frames which are tall enough to provide lateral support during transportation (Fig. 14). When the elements arrive on the job site, they must be checked for damage during transportation, especially for damage which might affect structural behaviour when installed. If satisfactory, the elements should be hoisted into position, preferably directly from the trucks to avoid site storage and double handling on a 45

Fig. 15. Hoisting a one-story panel directly from truck to building

congested job site which might result in damage to the elements (Fig. 15). If these requirements cannot be met, the components must be stored in the same way as they were stored in the yard, avoiding distortion, cracking or chipping. Long elements should be hoisted

by two cranes or, if this is not necessary from a capacity point of view, then by one crane using two falls by which the elements can be picked up in a horizontal position and gradually tilted into a vertical position for installation (Fig. 16). Relatively flat elements which were shipped on the side edges, should be picked up in the same position and tilted to a vertical standing position by means of two falls, carefully avoiding tipping or twisting during this operation (Fig. 17). For unusually shaped elements, special erection rigs, such as rotating sheaves, spreader beams or specially designed equipment, should be used in order to avoid damaging the elements during hoisting and installation. In assembling precast elements, the connections and temporary bracing play a key role, therefore, these

Line ',' 1

Line

Step ° 1

NO

Line x° 2

5^e1 ua 2

M03

Fig. 16. Hoisting long precast or prestressed concrete units 46ҟ

PCI Journal

Rolling shie—s.^

_sf

f

STe!] ? ?

Se a No 3

Fig. 17. Hoisting and turning multi-story units

must be designed in such a way as to allow final alignment of the elements from their temporary positions. Here again, I would like to emphasize the importance of full understanding by the erection personnel of the role of connections and their behaviour in order to guarantee the design performance of the building. Any deviation or error which may affect the functioning of the components must be reported back to the designer so that he can verify their adequacy or, if necessary, carry out needed modifications at this early stage of erection. Temporary connections, bracing and guy wires must be fully secured to take the necessary loads before the crane can release the element, as well as all horizontal loads which might occur during the erection stage before the components are fully assembled. If cast-in-place concrete is used to connect the components or build up the connection, the bracing must remain until the concrete has reached its full strength and is ready to transfer all vertical and/or horizontal loads imposed on the connection. In July-August 1971

the case of site post-tensioning, bracing should remain in place until the tensioning is done and the tendons are fully grouted. Once the elements are in position and the connections are completed, the joints have to be sealed as planned. If mortar joints are used as load transferring elements, special care is required to carry out this work with maximum efficiency. Under cold weather conditions, the building might have to be closed in and heated temporarily before this operation can proceed in order to prevent the mortar from freezing. If pre-moulded weather seals are used to seal the joints, care is required in order to insure continuity, especially in multi-story construction, to avoid water penetration or air leakage at gaps. If field-moulded material is used to seal the joints, the manufacturer's specifications must be fully followed which means that, in certain cases, the joints might have to be sealed temporarily before the final application of the specified material, due to critical weather conditions. 47

As a final stage of erection, the building and its components must be inspected, all connections and joints verified and, if warranted, field tested before the conventional final wash-down, repairs or touch-ups. ECONOMY OF LOAD -BEARING WALL PANEL APPLICATIONS

No technical study can be complete without analyzing its economical advantages or disadvantages. In this presentation I have discussed the design, manufacturing and erection requirements of the load-bearing wall panel, from which the reader may draw the conclusion that it is a rather sophisticated product which is difficult to produce and, therefore, he may question whether it is worthwhile to use it. My answer is definitely, yes! The concrete elements normally used for cladding or curtain wall applications, due to their desired architectural effect, have a substantial structural reserve which is not always recognized and utilized. With very little addition to the standard or specially designed shapes, the ele-

ments can easily be used as structural components, thus taking full advantage of the concrete section and the embedded reinforcing steel which otherwise is needed only for handling purposes. Nevertheless, before using a concrete element as a load-bearing wall component, the whole design group —architect, structural engineer, mechanical engineer and precast manufacturer—must recognize the advantages and limitations of the product and design the building accordingly. Only through such a coordinated team effort can the maximum economy be achieved. Once the building has been designed, the general contractor takes over the coordination and execution of the work and a well-defined and planned schedule has to be developed and broken down into step-bystep operations which will allow assembly of the precast components with maximum efficiency and economy. By recognizing all these factors, we should be able to design and build buildings more effectively and economically.

Discussion of this paper is invited. Please forward your comments to PCI Headquarters by Nov. 1 to permit publication in the Nov.-Dec. 1971 issue of the PCI JOURNAL. 48ҟ

PCI Journal