This chapter presents basic issues that affect the design of building structures and presents an overall view of the materials, products, and systems used to achieve them.
All physical objects have structures. Consequently, the design of structures is part of the general problem of design for all physical objects. It is not possible to understand fully why buildings are built the way they are without some knowledge and understanding of the problems of their structures. Building designers cannot function in an intelligent manner in making decisions about the form and fabric of a building without some comprehension of basic concepts of structures.
Life safety is a major concern in the design of structures. Two critical considerations are for fire resistance and for a low likelihood of collapse under load. Major elements of fire resistance are:
Combustibility of the Structure. If structural materials are combustible, they will contribute fuel to the fire as well as hasten the collapse of the structure.
Loss of Strength at High Temperature. This consists of a race against time, from the moment of inception of the fire to the failure of the structure—a long interval increasing the chance for occupants to escape the building.
Containment of the Fire. Fires usually start at a single location, and preventing their spread is highly desirable. Walls, floors, and roofs should resist burn-through by the fire.
Major portions of building code regulations have to do with aspects of fire safety. Materials, systems, and details of construction are rated for fire resistance on the basis of experience and tests. These regulations constitute restraints on building design with regard to selection of materials and use of details for building construction.
Building fire safety involves much more than structural behavior. Clear exit paths, proper exits, detection and alarm systems, firefighting devices (sprinklers, hose cabinets, etc.), and lack of toxic or highly flammable materials are also important. All of these factors will contribute to the race against time, as illustrated in Figure 1.1
The structure must also sustain loads. Safety in this case consists of having some margin of structural capacity beyond that strictly required for the actual task. This margin of safety is defined by the safety factor, SF, as follows:
Thus, if a structure is required to carry 40,000 lb and is actually able to carry 70,000 lb before collapsing, the safety factor is expressed as SF = 70,000/40,000 = 1.75. Desire for safety must be tempered by practical concerns. The user of a structure may take comfort in a safety factor as high as 10, but the cost or gross size of the structure may be undesirable. Building structures are generally designed with an average safety factor of about 2. There is no particular reason for this other than experience.
For many reasons, structural design is a highly imprecise undertaking. One should not assume, therefore, that the true safety factor in a given situation can be established with great accuracy. What the designer strives for is simply a general level of assurance of a reasonably adequate performance without pushing the limits of the structure too close to the edge of failure.
There are two basic techniques for assuring the margin of safety. The method once used most widely is called the allowable stress design or service load method. With this method stress conditions under actual usage (with service loads) are determined and limits for stresses are set at some percentage of the predetermined ultimate capacity of the materials. The margin of safety is inferred from the specific percentage used for the allowable stresses.
A problem encountered with the allowable stress method is that many materials do not behave in the same manner near their ultimate failure limits as they do at service load levels. Thus prediction of failure from a stress evaluation cannot be made on the basis of only a simple linear proportionality; thus using an allowable stress of one-half the ultimate stress limit does not truly guarantee a safety factor of 2.
The other principal method for assuring safety is called the strength design or load and resistance factor (LRFD) method. The basis of this method is simple. The total load capacity of the structure at failure is determined and its design resistance is established as a percentage (factored) of the ultimate resistance. This factored resistance is then compared to an ultimate design failure loading, determined as a magnified (factored) value of the service load. In this case the margin of safety is inferred by the selected design factors.
Although life safety is certainly important, the structural designer must also deal with many other concerns in establishing a satisfactory solution for any building structure.
Structures are real and thus must use materials and products that are available and can be handled by existing craftspeople and production organizations. Building designers must have a reasonable grasp of the current inventory of available materials and products and of the usual processes for building construction. Keeping abreast of this body of knowledge is a challenge in the face of the growth of knowledge, the ever-changing state of technology, and the market competition among suppliers and builders.
Feasibility is not just a matter of present technological possibilities but relates to the overall practicality of a structure. Just because something can be built is no reason that it should be built. Consideration must be given to the complexity of the design, dollar cost, construction time, acceptability by code-enforcing agencies, and so on.
Buildings cost a lot of money, and investors are seldom carefree, especially about the cost of the structure. Except for the condition of a highly exposed structure that constitutes a major design feature, structures are usually appreciated as little as the buried piping, wiring, and other mundane hidden service elements. Expensive structures do not often add value in the way that expensive hardware or carpet may. What is usually desired is simple adequacy, and the hard-working, low-cost structure is much appreciated.
However vital, the building structure usually represents a minor part of the total construction cost. When comparing alternative structures, the cost of the structure itself may be less important than the effects of the structure on other building costs.
Building designers often are motivated by desires for originality and individual expression. However, they are also usually pressured to produce a practical design in terms of function and feasibility. In many instances this requires making decisions that constitute compromises between conflicting or opposing considerations. The best or optimal solution is often elusive. Obvious conflicts are those between desires for safety, quality of finishes, grandeur of spaces, and general sumptuousness on the one hand and practical feasibility and economy on the other. All of these attributes may be important, but often changes that tend to improve one factor work to degrade others. Some rank ordering of the various attributes is generally necessary, with dollar cost usually ending up high on the list.
Good structural design requires integration of the structure into the whole physical system of the building. It is necessary to recognize the potential influences of the structural design decisions on the general architectural design and on the development of the systems for power, lighting, thermal control, ventilation, water supply, waste handling, vertical transportation, firefighting, and so on. The most popular structural systems have become so in many cases largely because of their ability to accommodate the other subsystems of the building and to facilitate popular architectural forms and details.
1.2 Architectural Considerations
Primary architectural functions that relate to the structure are:
Need for shelter and enclosure
Need for spatial definition, subdivision, and separation
Need for unobstructed interior space
In addition to its basic force-resistive purpose, the structure must serve to generate the forms that relate to these basic usage functions.
Shelter and Enclosure
Exterior building surfaces usually form a barrier between the building interior and the exterior environment. This is generally required for security and privacy and often in order to protect against various hostile extern...