AISC LRFD 1.pdf

257Comm. LCHAPTER LSERVICEABILITY DESIGN CONSIDERATIONSServiceability criteria are formulated to prevent disruptions of the functional use anddamage to the structure during its normal everyday use. While malfunctions may notresult in the collapse of a structure or in loss of life or injury, they can seriously impair theusefulness of the structure and lead to costly repairs. Neglect of serviceability may resultin unacceptably flexible structures.There are essentially three types of structural behavior which may impair serviceability:(1) Excessive local damage (local yielding, buckling, slip, or cracking) that may requireexcessive maintenance or lead to corrosion.(2) Excessive deflection or rotation that may affect the appearance, function, or drainageof the structure, or may cause damage to nonstructural components and their attachments.(3) Excessive vibrations induced by wind or transient live loads which affect the comfortof occupants of the structure or the operation of mechanical equipment.In allowable stress design, the AISC Specification accounts for possible local damagewith factors of safety included in the allowable stresses, while deflection and vibration arecontrolled, directly or indirectly, by limiting deflections and span-depth ratios. In the past,these rules have led to satisfactory performance of structures, with perhaps the exceptionof large open floor areas without partitions. In LRFD the serviceability checks shouldconsider the appropriate loads, the response of the structure, and the reaction of the occupantsto the structural response.Examples of loads that may require consideration of serviceability include permanentlive loads, wind, and earthquake; effects of human activities such as walking, dancing,etc.; temperature fluctuations; and vibrations induced by traffic near the building or by theoperation of mechanical equipment within the building.Serviceability checks are concerned with adequate performance under the appropriateload conditions. Elastic behavior can usually be assumed. However, some structural elementsmay have to be examined with respect to their long-term behavior under load.It is difficult to specify limiting values of structural performance based on serviceabilityconsiderations because these depend to a great extent on the type of structure, its intendeduse, and subjective physiological reaction. For example, acceptable structural motion in ahospital clearly would be much less than in an ordinary industrial building. It should benoted that humans perceive levels of structural motion that are far less than motions thatwould cause any structural damage. Serviceability limits must be determined throughcareful consideration by the designer and client.Serviceability guidelines for low-rise buildings are given in Fisher and West (1990).LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION