Costs of Shells

COSTS OF SHELL STRUCTURES

Much of the material in this discussion was taken from my paper “Economic Factors in Shell Roof Construction” in the proceedings of the World Conference on Shell Structures, San Francisco, California, 1962, the heyday of shell construction. I have not included any of the prices quoted and have omitted many of the arguments for the use of shell structures. I hope that you will find it of value.

There are three factors to consider in discussing costs of shells:

Designing shells to reduce costs and to use them effectively.

Comparison with costs of other materials.

Accurate estimation of quantities of materials, and subsequent costs.

With respect to designing shells effectively, the word to both architects and engineers is “Keep it Simple. Stupid.” From the point of view of aesthetics, engineering, and construction, there is little to be gained by elaboration. Shells do not require elaboration because they are strong forms in themselves. There is a way to promote shells so that they will be used more often. The first building in the area should be simple in design with relatively simple finishes. When the bids come in low, the architect can prove that shell structures are inexpensive and their economic future is assured.

There are many advantages to shells, but it is difficult to put a price on intangibles. Shells offer uncluttered, clean, light, dust free surfaces, resulting from the smooth undersurface of shells that have no girders, beams or trusses. Other structures may require hung ceilings, that can be avoided with a shell roof. Certainly the increase in efficiency of the workers in a building should have a monetary value.

Comparison of costs with other systems

It should be emphasized that the only accurate method of cost comparison is to make alternate designs and take bids. All other data is useful but is always in question. The difficulty in comparison of shells with other systems of construction, is that the quantity of materials will vary little with the span while others, increase with the span. Here, however, are some of the results of a study made in the late 1950’s, sponsored by the Ideal Cement Company.

Ratio of cost of shell to steel frame, steel deck, and steel purlins
Barrel Shells, 20 ft. spacing, 60 ft. span 1.25

Barrel Shells, 40 ft. spacing. 60 ft. span 1.10

Ratio of cost of shell to steel frame, steel deck, and steel purlins
Barrel Shells, 20 ft. spacing, 60 ft. span	1.25
Barrel Shells, 40 ft. spacing. 60 ft. span	1.10

The obvious conclusion is shells are increasing competitive with longer spans. Another study was made for umbrella hyperbolic paraboloids. In all cases, the area covered (1600 square feet) was the same. Here are some more statistics developed.

Ratio of Cost of 40 ft. square Hypars Compared to Other Systems
Hypars, 40 x40 ft. 1.00

Timber frame 0.91

Steel frame wood joists 0.78

Steel frame wood purlins 0.77

Steel frame, steel purlins, steel deck 1.04

Steel frame, open web joists 0.90

Prestressed Concrete 1.01

Ratio of Cost of 40 ft. square Hypars Compared to Other Systems
Hypars, 40 x40 ft.	1.00
Timber frame	0.91
Steel frame wood joists	0.78
Steel frame wood purlins	0.77
Steel frame, steel purlins, steel deck	1.04
Steel frame, open web joists	0.90
Prestressed Concrete	1.01

Estimating costs
Surprisingly it is less difficult estimate costs of shell roof buildings than it is to estimate costs of other types. The biggest factor is the unit cost of forms, and here the ingenuity and experience of the contractor plays an important part. The area of forms and the volume of concrete are easy to estimate. The quantity of reinforcing does not vary greatly with the spans and is a relatively small percentage of the total cost. Following are some of the results of studies on the quantities of reinforcing steel in the following shell structures. These shells were design for 30 pounds per square foot.

Folded plates, spans to 70 ft., span half the width, slope of plate: 4/12
Barrel shells, spans: 50 to 100 ft, radius of shell 25 ft., width: 30 ft.
Square inverted umbrella hypars, 30 ft. to 60ft., slope: 4/12
Square hypar dome shells, 40ft. to 100 ft., slope: 4/12

Quantities of Reinforcing Steel for Typical Interior Bays of Shell Roof Structures
Type	Thickness, inches	Steel, psf
Folded Plate	3.25	1.71-0.007L
Barrel Shells	3.50	1.10-0.004L
Umbrella Hypars	2.25	1.38-0.003L
Dome Hypars	3.50	1.80-0.002L

Guidance for selecting the forming system is given in the section on construction of shells, In general, unless the forms can be used at least five or six times, it is better to stick to single use forms.

Determination of the cost of forms is the most difficult part of the estimation of the cost of shells. Published costs are virtually worthless. Therefore, it is necessary to go back to fundamentals, to design and price forms for the particular project, and to collect local costs based on systems you know to be satisfactory for the particular job,

Shells of minimum cost

Shells can compete with other structural systems, but a system for construction at a minimum cost must be devised. Four conditions must be fulfilled:

There must be minimum quantities of materials, both steel and concrete.
The formwork must be inexpensive.
The building must be extremely simple, with no extras for light, heat aesthetics, or expensive finish materials.
There must be a real desire on the part of the contractor to reduce costs.

Summary

Shell structures may cost only slightly more than competitive materials, especially if additional costs, for example for hung ceilings, are considered. They are not difficult to estimate. The thickness is based on minimum values, and the reinforcing is only a fraction of the total cost. The most difficult element is the cost of forms. There are many solutions which must be carefully studied to arrive at the most desireable solution.

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