By Milo S. Ketchum
The construction of a reinforced concrete shell involves many problems, the design and construction of forms, reinforcement selection and placing, concrete materials and placing, and curing and decentering. All of these problems must be understood in order to make the structure safe and economically feasible. More than almost any other structural system, shells depend upon the ability of the architect and engineer to foresee the design problems and upon the ingenuity of the contractor to solve the mechanical problems of construction. To build a satisfactory shell requires a detailed study of the methods of construction, well prepared plans, and good supervision.
The normal standards for the construction of concrete structures are, of course, absolutely necessary for shells. Particularly, the production, placing, and curing of concrete must be under firm control. Only the highest standards should be acceptable for shell structures. These are outlined in the codes, standards, and publications of the American Concrete Institute.
By the term economy, we mean the design and construction of the best building at the least cost. This criterion is not always useful, because it is difficult to define the best building, especially if there are intangibles that cannot be evaluated in terms of money.
Shells require a minimum of structural materials. The volume of concrete in the roof, is usually less than the concrete in the floor slab. It is fairly easy to estimate quantities. For example, for hyperbolic paraboloid (HP) umbrella shells, the average thickness per square foot of projected area is about 3.5 inches and for a gabled HP shells or saddle shells, 4.5 inches including the edge members. The weight of steel for an inverted umbrella is about 3 pounds per square foot of horizontal area. For square gabled hypars or saddle shell it is about 17 pounds per square foot of horizontal area. These quantities vary little with the span. Other types of shells have similar quantities.
The factor next in importance is the cost of formwork; the contractor must use his ingenuity to devise form systems that can be erected, moved forward and re-erected at minimum cost. The next factor is the time to erect forms, set reinforcing, place and cure concrete, dismantle forms, and be ready to the next cycle. The contractor must have a well-planned organization with good supervision, to achieve acceptable results and costs.
There are intangible advantages to shell structures that are difficult to estimate, but are nevertheless real. The undersurface is uncluttered, clean, light, and dust free. Other structural systems may require large additional costs for hung ceilings. There are certain industries, particularly food handling and processing, where such dust free surfaces are most desirable. Light interior surfaces may reduce the cost of lighting and bright interiors may be better for the morale of the workers.
It is difficult to put a price on intangibles, especially if they are not absolutely necessary. An increase in the efficiency of the workers should have a monetary value. The entire cost of the building, must be considered, and not just the cost of the structure. For example, the use of a shell structure may raise or lower the cost of heating, lighting, and finish materials, so the cost of the structure alone is no measure of the economy. These factors are not always properly evaluated by owners and architects, and the engineer should point them out.
Comparison with other materials.
For short spans, costs may be greater than other materials; the cost of shells of 50 to 100 foot span for roof systems for industrial or commercial use may be less than the cost of other structural systems such as steel or timber. There are many factors that make this possible. The quantity of materials is almost independent of the span, so the cost of longer spans may be only a little higher than for short spans. The weight and cost of steel structures. however, increases with the span. For this type of shell structure, it is important that the forming system be movable and that the construction be planned so that steel setting, concrete placing, concrete curing, and moving ahead is rapid.
Long span shells.
The economy of long span shells depends on many factors. The aesthetics and functional solution of the architectural problems often have a more important effect on the selection of the structure than have the costs. The overriding element in the cost, is the forming system. A large shell will require a single use form unit unless there are many repeated uses. Without the savings of multiple use forms, the economy may be lost. On the other hand, alternate materials are also expensive. One cannot make an out-of- hand judgment that shells are too expensive for a particular design. The only accurate way to determine the least cost of alternates, is to make separate, complete, designs, and to obtain accurate formal bids from contractors. The costs obtained will apply only to a particular bidding climate and do not necessarily hold for some other bidding situation.
The design and construction of forms is a major consideration of costs, and involves a significant proportion of the total cost. It is important to understand the various types of forming and their advantages and disadvantages.
Single use forms.
The entire roof is formed at one time, and the forms are not reused. This method is satisfactory, either for small or large shells, where there is a single structural element. It requires a large amount of forming materials in comparison to the final area of the shell. An advantage is that it is not necessary to have an elaborate schedule of sequential forming, reinforcement pacing, concrete placing, curing, and decentering. All of the forming can be done at one time, then the reinforcing, and so forth. On large shells, patented steel scaffolding is often used and may be rented. Single use forms should be considered if there are fewer than, say, 4 units.
The form is constructed with panels supported by shores arranged so that the shores are built into and support the shell directly. Then some of the panels can be taken down and moved ahead in one or two days without disturbing the shores. A crew can be kept continuously busy removing and reerecting panels and shores. It is important to have available more panels and shores than required for each concrete placing operation, otherwise this system will have no particular advantage in terms of labor efficiency. Some of the built-in shores should remain in place until adequate concrete strength precludes excessive deflection from overloaded young concrete. The contractor and the engineer should fully agree upon the schedule for removal of panels and shores. This method is most useful when, say, three or four structural units are to be built which require some mutual support.
Movable forms - small decentering
For inverted HP umbrella shells, for example, forms for one unit can be constructed in quadrants so it is necessary to decenter only a few inches. Then the quadrants may be separated and moved to the next unit to be constructed. Fortunately this type of shell is structurally self-supporting so it is not necessary to leave shores in place until the next element is joined to the structure. However, for umbrella shells, the corners may tend to sag, so re-shores should be used at these places. If large numbers of elements are to be built, as for an industrial building, then it will pay to design these form units so they can be raised and lowered by hydraulic or mechanical jacks and have wheels to move them around.
Movable forms - large decentering.
On shells other than umbrellas, it may be necessary to decenter the forms for a major portion of the full height of the shell. This method requires a considerable investment in mechanical equipment such as long hydraulic jacks or the repeated use of short jacks. The mechanical ingenuity of the contractor is very important for proper design of this type of forming system.
Precasting has the advantage that material and construction conditions are under the best control, forms may be constructed for repeated use, and concrete materials may be better controlled. The disadvantage is that it is usually necessary to transport the shell units over a considerable distance if the are built in a precasting yard. If they are precast on the construction site, then the transportation is easier, but it is necessary to have large cranes to move and to lift them into place. A structural problem is the connection of these shells to the supporting columns. Precasting should be considered only for small units.
A number of shells have been built by using earth as a forming material. The surface of the earth mound is covered with a suitable contact material such as plywood to make the under surface of the concrete acceptable. After casting the concrete, the earth is excavated. Most of the structures built in this manner have been domes. Shells have also been built without forms by using a close grid of reinforcing bars with the concrete placed by shotcrete
The form surface.
The most convenient and least expensive material for the surface of the forms is plywood. Loose boards may look better to the architect, but they are considerably more expensive, and must be replaced more often. Usually the curvature of the surface is such that plywood can be twisted to the required shape, if not, then four foot by eight-foot sheets must be cut into two-foot widths. It will be necessary either to trim the large sheets slightly to obtain double curvature, or provide some method of closing the gaps between sheets. It is expensive to trim all the sheets, and afterward the are difficult to use again. Strips of plywood, say four inches wide, placed on the top of the plywood, have been used to cover these gaps. The underside of the shell will show as rectangular panels if additional strips are placed at the middle of the eight-foot length. This trick makes an interesting under surface of the shell.
Except for very steep slopes and thick walls, top forms are unnecessary. Candela has built these shells with practically a vertical slope. The thickness was only 1.5 inches, and there was a grid of reinforcing bars to support the concrete. The shell was virtually plastered. It top forms become necessary, the most convenient method is to use wire mesh or metal lath panels that can be removed as soon as the concrete has been placed, so that the surface can be finished with the rest of the roof.
For movable forms, the cost of forms per square foot is reduced if the forms are used repeatedly during construction. The optimum number of uses appears to be from 6 to 10, depending on the total length for the time of construction, and the curing time between uses. By this number of uses, the original total cost will be divided by the number of uses, and the form surface may not have to be rebuilt at an additional cost.
Small to medium size shells may be placed at one time and construction joints may not be a problem. On the other hand, a shell may be so large, that is not possible to place all of the concrete at one operation, and construction joints become necessary. These joints should be planned and specified by the engineer, and indicated on the construction drawings, together with the details of the method of stress transfer such as the inclusion of keyways, special reinforcing, and possible thickening of members. If the shell membrane is thin, usually the stresses are fairly small, and no special reinforcing is required at construction joints. The most important detail is that the screed at the edge of the placing area is carefully fitted to the reinforcing, and that careful preparation of the surface for the next concrete placing is provided. In edge members, the usual good construction methods for beams, girders, and columns should be followed.
In many cases there is a grid of reinforcing bars that must be held in place. On a sloping surface, steel setters find it more convenient to place the lower bars vertically. If this is done, then the slab bolsters will run horizontally. Fresh concrete on slopes tends to slide downward, and if the bolsters are horizontal, a gap will form in the concrete below the bolster. On steep slopes, this may be unsightly and be an unsatisfactory structural condition. The reinforcing should be detailed so the bolsters are vertical. It is important to provide adequate lapping of the bars, or if the reinforcing is heavily stressed, the bars may be welded. It must be specified and detailed by the engineer, and may be expensive. With curved surfaces, it is easy to underestimate the length of bars.
The compression stresses in shells are usually quite low, so the concrete strength is not the most important element in designing the concrete mixture. However, there are some cases where high early strength concrete becomes necessary in order to move forms rapidly for maximum production. Placeability and low shrinkage are important. The use of admixtures that make the concrete more fluid for pumping may cause the concrete on steep slopes to move downward, so that the engineer in writing the specifications must consider this problem. For slanting surfaces, the water cement ratio tends to become adjusted to the optimum value. It there is too much water, the concrete will run down the slope, and if there is too little, the concrete becomes unplacable. The use of lightweight concrete for shells may save some weight, but this saving will result in little reduction in stresses, so there will be little reduction in the quantity of reinforcing. If soil conditions are marginal, the reduced weight on the footings may save some concrete.
The objective of concrete placing is the production of a smooth dense solid texture on the under surface of the shell with no pockets or honeycombing. Shells are so thin that the under surface will not have the advantage of the weight above the surface as in beams or columns, so extra care must be taken. Placing concrete in shells is hard work for the placing crew who must work on a sloping surface and often shovel heavy concrete uphill, so every effort should be made to make the operation simple and convenient, with an adequate number of workers and finishers. It is important that the concrete be placed on the form at the place where it is required. Otherwise the placing will be greatly slowed.
There is a general agreement among engineers that concrete should be placed from the bottom of the shell upward. Then the concrete will not sag downward and cause pockets. There is considerably more work for the crew to place the concrete from the bottom up than from the top down, because gravity is a very convenient concrete mover. Both the contractor and the crew may resist placing from the top down. There should be a thorough understanding, both in the specifications and in the supervision on this point.
There are several methods to establish the thickness of the shell as the concrete is placed. One method is to use long screed rails set on blocks on the form, so the reinforcing can be placed underneath. They may run either horizontally or vertically, and should be close enough together so that the screed board will rest on the rails. The screed rails are removed as soon as the surface is established, and the depressions left by the boards and blocks are filled up so there are no marks on the under surface. Another method is to use concrete blocks or short posts nailed to the form and placed close enough together so the finisher can establish the proper thickness by eye. On a large production job, mechanical screeds are desirable.
In order to produce a smooth dense texture, the vibration of the concrete must be under careful control. Shells are thin and vibrations are transmitted only short distance, the vibrator must cover so every square foot of the surface. If this is not done, an air pocket or rough texture will result. One method that has been used, is to construct a rectangular grid of 3/4 inch square wood strips fastened with s single nail or bolt at the intersections. The vibrator should be placed at the center of each square for only a short time. One touch of the vibrator is required to achieve the desired result. This grid will fold into a compact unit so it can be moved from spot to spot. The vibrator operator must be given precise directions.
The selection of the type of placing equipment, whether by pumping, by bucket and mobile crane, by movable wood runways and carts, should be the decision of the contractor, based on factors such as the equipment available, the steepness of the slopes, the form and shape of the shell, and the distance above the ground.
The accepted standards for the curing of concrete apply to the construction of shells. They are thin and do not generate or retain heat, so cold weather protection, both above and below the surface is essential. It is often possible to build enclosures under the forms that can be insulated and heated. On relatively small inverted umbrellas, where movable forms are used, the enclosure may be part of the forming system. In hot weather, the thin surface is susceptible to plastic cracking, so precautions must be taken.
Time of decentering.
In general, for any concrete structure, the longer the forms remain in place, the better the results. Too early removal of forms may not affect the strength, but may have serious consequences for deflections, especially for thin cantilevers at the corners of an umbrella shell. However, rapid decentering may be directly involved with the economy of the project, especially for industrial buildings where large areas are to be covered. Special precautions must be made, if rapid movement is involved. A decision must be made on the minimum cylinder strength allowable in the concrete before decentering, so it can be written into the specifications. Then the contractor can make his decision on the type of concrete and the rapidity of movement of the forms. It may be possible to move forms on a 24 hour curing schedule if high early strength concrete is used, and the critical thin shell element are reshored. On large projects, the time of decentering is sometimes controlled by deflection tests to establish an acceptable modulus of elasticity at which forms may be moved.
SUMMARY AND CONCLUSIONS
Shells have a great economic potential for the construction of low-cost industrial or commercial buildings to cover large areas. Long spans are more expensive to build because they must usually be constructed with a single use form. However, they have other advantages that may outweigh the initial cost of the structure. The only answer to the economic question is to take bids from contractors on several competitive systems.
The construction of shells is not difficult, but it requires teamwork and the cooperation of the contractor and engineer. The latter must design the structure so it is easy to build, and must show sufficient details so the contractor can construct it economically. The engineer must be aware of the economic factors in shell construction. The specifications must be carefully written to reflect the problems in thin shell construction. The contractor should study the plans in order to solve the construction problems before they become difficult in the field. He should use a competent superintendent, one with mechanical ingenuity and perseverance. The engineer should, during construction, be free to make decisions on plans and specifications that will expedite the construction without any reduction in quality. The joint efforts of the contractor and engineer will create a structure bringing pride and a sense of accomplishment to all parties.