AISC DESIGN GUIDE 3
Serviceability Design Considerations for Steel Buildings
|Publication Date:||1 March 2004|
Serviceability is defined in the AISC Specification as "a state in which the function of a building, its appearance, maintainability, durability, and comfort of its occupants are preserved under normal usage". Although serviceability issues have always been a design consideration, changes in codes and materials have added importance to these matters.
The shift to a limit-states basis for design is one example. Since 1986, both the AISC LRFD and AISC ASD Specifications have been based upon the limit-states design approach in which two categories of limit states are recognized: strength limit states and serviceability limit states. Strength limit states control the safety of the structure and must be met. Serviceability limit states define the functional performance of the structure and should be met.
The distinction between the two categories centers on the consequences of exceeding the limit state. The consequences of exceeding a strength limit may be buckling, instability, yielding, fracture, etc. These consequences are the direct response of the structure or element to load. In general, serviceability issues are different in that they involve the response of people and objects to the behavior of the structure under load. For example, the occupants may feel uncomfortable if there are unacceptable deformations, drifts, or vibrations.
Whether or not a structure or element has passed a limit state is a matter of judgment. In the case of strength limits, the judgment is technical and the rules are established by building codes and design specifications. In the case of serviceability limits, the judgments are frequently non-technical. They involve the perceptions and expectations of building owners and occupants. Serviceability limits have, in general, not been codified, in part because the appropriate or desirable limits often vary from application to application. As such, they are more a part of the contractual agreements with the owner than life-safety related. Thus, it is proper that they remain a matter of contractual agreement and not specified in the building codes.
In a perfect world the distinction between strength and serviceability would disappear. There would be no problems or failures of any kind. In the real world all design methods are based upon a finite, but very small probability of exceedance. Because of the non-catastrophic consequences of exceeding a serviceability limit state, a higher probability of exceedance is allowed by current practice than for strength limit states.
The foregoing is not intended to say that serviceability concerns are unimportant. In fact, the opposite is true. By having few codified standards, the designer is left to resolve these issues in consultation with the owner to determine the appropriate or desired requirements.
Serviceability problems cost more money to correct than would be spent preventing the problem in the design phase. Perhaps serviceability discussions with the owner should address the trade-off between the initial cost of the potential level of design vs. the potential mitigation costs associated with a more relaxed design. Such a comparison is only possible because serviceability events are by definition not safety related. The Metal Building Manufactures Association (MBMA) in its Common Industry Practices (MBMA, 2002) states that the customer or his or her agent must identify for the metal building engineer any and all criteria so that the metal building can be designed to be "suitable for its specific conditions of use and compatible with other materials used in the Metal Building System." Nevertheless, it also points out the requirement for the active involvement of the customer in the design stage of a structure and the need for informed discussion of standards and levels of building performance. Likewise the AISC Code of Standard Practice (AISC, 2000) states that in those instances where the fabricator has both design and fabrication responsibility, the owner must provide the "performance criteria for the structural steel frame."
Numerous serviceability design criteria exist, but they are spread diversely through codes, journal articles, technical committee reports, manufacturers' literature, office standards and the preferences of individual engineers. This Design Guide gathers these criteria for use in establishing serviceability design criteria for a project.