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The Cost of Building Structure

1. Introduction
The art of architectural design was characterized as one of dealing comprehensively with a complex set of physical and nonphysical design determinants. Structural considerations were cast as important physical determinants that should be dealt with in a hierarchical fashion if they are to have a significant impact on spatial organization and environmental control design thinking.
The economical aspect of building represents a nonphysical structural consideration that, in final analysis, must also be considered important. Cost considerations are in certain ways a constraint to creative design. But this need not be so. If something is known of the relationship between structural and constructive design options and their cost of implementation, it is reasonable to believe that creativity can be enhanced. This has been confirmed by the authors’ observation that most enhanced. This has been confirmed by the authors’ observation that most creative design innovations succeed under competitive bidding and not because of unusual owner affluence as the few publicized cases of extravagance might lead one to believe. One could even say that a designer who is truly creative will produce architectural excellence within the constraints of economy. Especially today, we find that there is a need to recognize that elegance and economy can become synonymous concepts.
Therefore, in this chapter we will set forth a brief explanation of the parameters of cost analysis and the means by which designers may evaluate the overall economic implications of their structural and architectural design thinking.
The cost of structure alone can be measured relative to the total cost of building construction. Or, since the total construction cost is but a part of a total project cost, one could include additional consideration for land(10～20percent),finance and interest(100～200 percent),taxes and maintenance costs (on the order of20 percent).But a discussion of these so-called architectural costs is beyond the scope of this book, and we will focus on the cost of construction only.
On the average, purely structural costs account for about 25 percent of total construction costs, This is so because it has been traditional to discriminate between purely structural and other so-called architectural costs of construction. Thus, in tradition we find that architectural costs have been taken to be those that are not necessary for the structural strength and physical integrity of a building design.
”Essential services” forms a third construction cost category and refers to the provision of mechanical and electrical equipment and other service systems. On the average, these service costs account for some 15 to 30 percent of the total construction cost, depending on the type of building. Mechanical and electrical refers to the cost of providing for air-conditioning equipment and he means on air distribution as well as other services, such as plumbing, communications, and electrical light and power.
The salient point is that this breakdown of costs suggests that, up to now, an average of about 45 to 60 percent of the total cost of constructing a typical design solution could be considered as architectural. But this picture is rapidly changing. With high interest costs and a scarcity of capital, client groups are demanding leaner designs. Therefore, one may conclude that there are two approaches the designer may take towards influencing the construction cost of building.
The first approach to cost efficiency is to consider that wherever architectural and structural solutions can be achieved simultaneously, a potential for economy is evident. Since current trends indicate a reluctance to allocate large portions of a construction budget to purely architectural costs, this approach seems a logical necessity. But, even where money is available, any use of structure to play a basic architectural role will allow the nonstructural budget to be applied to fulfill other architectural needs that might normally have to be applied to fulfill other architectural needs that might normally have to be cut back. The second approach achieves economy through an integration of service and structural subsystems to round out one’s effort to produce a total architectural solution to a building design problem.
The final pricing of a project by the constructor or contractor usually takes a different form. The costs are broken down into (1) cost of materials brought to the site, (2)cost of labor involved in every phase of the construction process, (3)cost of equipment purchased or rented for the project, (4)cost of management and overhead, and(5) profit. The architect or engineer seldom follows such an accurate path but should perhaps keep in mind how the actual cost of a structure is finally priced and made up.
Thus, the percent averages stated above are obviously crude, but they can suffice to introduce the nature of the cost picture. The following sections will discuss the range of these averages and then proceed to a discussion of square footage costs and volume-based estimates for use in rough approximation of the cost of building a structural system.
2. Percentage Estimates
The type of building project may indicate the range of percentages that can be allocated to structural and other costs. As might be expected, highly decorative or symbolic buildings would normally demand the lowest percentage of structural costs as compared to total construction cost. In this case the structural costs might drop to 10～15percent of the total building cost because more money is allocated to the so-called architectural costs. Once again this implies that the symbolic components are conceived independent of basic structural requirements. However, where structure and symbolism are more-or-less synthesized, as with a church or Cathedral, the structural system cost can be expected to be somewhat higher, say, 15and20 percent (or more).
At the other end of the cost scale are the very simple and nonsymbolic industrial buildings, such as warehouses and garages. In these cases, the nonstructural systems, such as interior partition walls and ceilings, as will as mechanical systems, are normally minimal, as is decoration, and therefore the structural costs can account for60 to 70 percent, even 80 percent of the total cost of construction.
Buildings such as medium-rise office and apartment buildings(5～10 stories)occupy the median position on a cost scale at about 25 percent for structure. Low and short-span buildings for commerce and housing, say, of three or four stories and with spans of some 20 or 30 ft and simple erection requirements, will yield structural costs of 15～20 percent of total building cost.
Special-performance buildings, such as laboratories and hospitals, represent another category. They can require long spans and a more than average portion of the total costs will be allocated to services (i.e., 30～50 percent), with about 20 percent going for the purely structural costs. Tall office building (15 stories or more) and/or long-span buildings (say, 50 to 60 ft) can require a higher percentage for structural costs (about 30to 35percent of the total construction costs),with about 30 to 40 percent allocated to services.
In my case, these percentages are typical and can be considered as a measure of average efficiency in design of buildings. For example, if a low, short-span and nonmonumental building were to be bid at 30 percent for the structure alone, one could assume that the structural design may be comparatively uneconomical. On the other hand, the architect should be aware of the confusing fact that economical bids depend on the practical ability of both the designer and the contractor to interpret the design and construction requirements so that a low bid will ensue. Progress in structural design is often limited more by the designer’s or contractor’ slack of experience, imagination, and absence of communication than by the idea of the design. If a contractor is uncertain, he will add costs to hedge the risk he will be taking. It is for this reason that both the architect and the engineer should be well-versed in the area of construction potentials if innovative designs ate to be competitively bid. At the least the architect must be capable of working closely with imaginative structural engineers, contractors and even fabricators wherever possible even if the architecture is very ordinary. Efficiency always requires knowledge and above all imagination, and these are essential when designs are unfamiliar.
The foregoing percentages can be helpful in approximating total construction costs if the assumption is made that structural design is at least of average (of typical) efficiency. For example, if a total office building construction cost budget is ﹩5,000,000,and 25 percent is the ”standard” to be used for structure, a projected structural system should cost no more than ﹩1,250,000.If a very efficient design were realized, say, at 80 percent of what would be given by the ”average” efficient design estimate stated above the savings,(20 percent),would then be﹩250,000 or 5 percent of total construction costs ﹩5,000,000.If the ﹩5,000,000 figure is committed, then the savings of ﹩250,000 could be applied to expand the budget for ”other” costs.
All this suggests that creative integration of structural (and mechanical and electrical) design with the total architectural design concept can result in either a reduction in purely construction design concept can result in either a reduction in purely construction costs or more architecture for the same cost. Thus, the degree of success possible depends on knowledge, cleverness, and insightful collaboration of the designers and contractors.
The above discussion is only meant to give the reader an overall perspective on total construction costs. The following sections will now furnish the means for estimating the cost of structure alone. Two alternative means will be provided for making an approximate structural cost estimate: one on a square foot of building basis, and another on volumes of structural materials used. Such costs can then be used to get a rough idea of total cost by referring to the ”standards” for efficient design given above. At best, this will be a crude measure, but it is hoped that the reader will find that it makes him somewhat familiar with the type of real economic problems that responsible designers must deal with. At the least, this capability will be useful in comparing alternative systems for the purpose of determining their relative cost efficiency.
3. Square-foot Estimating
As before, it is possible to empirically determine a ”standard” per-square-foot cost factor based on the average of costs for similar construction at a given place and time. more-or-less efficient designs are possible, depending on the ability of the designer and contractor to use materials and labor efficiently, and vary from the average.
The range of square-foot costs for ”normal” structural systems is ﹩10 to ﹩16 psf. For example, typical office buildings average between ﹩12 and ﹩16 psf, and apartment-type structures range from ﹩10 to ﹩14.In each case, the lower part of the range refers to short spans and low buildings, whereas the upper portion refers to longer spans and moderately tall buildings.
Ordinary industrial structures are simple and normally produce square-foot costs ranging from ﹩10 to ﹩14,as with the more typical apartment building. Although the spans for industrial structures are generally longer than those for apartment buildings, and the loads heavier, they commonly have fewer complexities as well as fewer interior walls, partitions, ceiling requirements, and they are not tall. In other words, simplicity of design and erection can offset the additional cost for longer span lengths and heavier loads in industrial buildings.
Of course there are exceptions to these averages. The limits of variation depend on a system’s complexity, span length over ”normal” and special loading or foundation conditions. For example, the Crown Zellerbach high-rise bank and office building in San Francisco is an exception, since its structural costs were unusually high. However, in this case, the use of 60 ft steel spans and free-standing columns at the bottom, which carry the considerable earthquake loading, as well as the special foundation associated with the poor San Francisco soil conditions, contributed to the exceptionally high costs. The design was also unusual for its time and a decision had been made to allow higher than normal costs for all aspects of the building to achieve open spaces and for both function and symbolic reasons. Hence the proportion of structural to total cost probably remained similar to ordinary buildings.
The effect of spans longer than normal can be further illustrated. The ”usual” floor span range is as follows: for apartment buildings,16 to 25 ft; for office buildings,20 to 30 ft; for industrial buildings,25 to 30 ft loaded heavily at 200 to 300 psf; and garage-type structures span,50 to 60 ft, carrying relatively light(50～75 psf) loads(i.e., similar to those for apartment and office structures).where these spans are doubled, the structural costs can be expected to rise about 20 to 30 percent.
To increased loading in the case of industrial buildings offers another insight into the dependency of cost estimates on ”usual” standards. If the loading in an industrial building were to be increased to 500psf(i.e., two or three times), the additional structural cost would be on the order of another 20 to 30 percent.
The reference in the above cases is for floor systems. For roofs using efficient orthotropic (flat) systems, contemporary limits for economical design appear to be on the order of 150 ft, whether of steel or prestressed concrete. Although space- frames are often used for steel or prestressed concrete. Although space-frames are often used for steel spans over 150 ft the fabrication costs begin to raise considerably.
At any rate, it should be recognized that very long-span subsystems are special cases and can in themselves have a great or small effect on is added, structural costs for special buildings can vary greatly from design to design. The more special the form, themore that design knowledge and creativity, as well as construction skill, will determine the potential for achieving cost efficiency.
4. Volume-Based Estimates
When more accuracy is desired, estimates of costs can be based on the volume of materials used to do a job. At first glance it might seem that the architect would be ill equipped to estimate the volume of material required in construction with any accuracy, and much less speed. But it is possible, with a moderate learning effort, to achieve some capability for making such estimates.