AISC LRFD 1.pdf

258 CAMBER [Comm. L1.L1. CAMBERThe engineer should consider specifying camber when deflections at the appropriateload level present a serviceability problem.L2. EXPANSION AND CONTRACTIONAs in the case of deflections, the satisfactory control of expansion cannot bereduced to a few simple rules, but must depend largely upon the good judgment ofqualified engineers.The problem is more serious in buildings with masonry walls than with prefabricatedunits. Complete separation of the framing, at widely spaced expansion joints,is generally more satisfactory than more frequently located devices dependentupon the sliding of parts in bearing, and usually less expensive than rocker or rollerexpansion bearings.Creep and shrinkage of concrete and yielding of steel are among the causes, otherthan temperature, for dimensional changes.L3. DEFLECTIONS, VIBRATION, AND DRIFT1. DeflectionsExcessive transverse deflections or lateral drift may lead to permanent damage tobuilding elements, separation of cladding, or loss of weathertightness, damagingtransfer of load to non-load-supporting elements, disruption of operation of buildingservice systems, objectionable changes in appearance of portions of the buildings,and discomfort of occupants.The LRFD Specification does not provide specific limiting deflections for individualmembers or structural assemblies. Such limits would depend on the function ofthe structure (ASCE, 1979; CSA, 1989; and Ad Hoc Committee on ServicabilityResearch, 1986). Provisions that limit deflections to a percentage of span may notbe adequate for certain long-span floor systems; a limit on maximum deflectionthat is independent of span length may also be necessary to minimize the possibilityof damage to adjoining or connecting nonstructural elements.Deflection calculations for composite beams should include an allowance for slipfor short-term deflection calculations, and for creep and shrinkage for long-termdeflection calculations (see Commentary Section I3.2).2. Floor VibrationThe increasing use of high-strength materials and efficient structural schemes leadsto longer spans and more flexible floor systems. Even though the use of a deflectionlimit related to span length generally precluded vibration problems in the past,some floor systems may require explicit consideration of the dynamic, as well asthe static, characteristics of the floor system.The dynamic response of structures or structural assemblies may be difficult to analyzebecause of difficulties in defining the actual mass, stiffness, and damping characteristics.Moreover, different load sources cause varying responses. For example,a steel beam-concrete slab floor system may respond to live loading as anon-composite system, but to transient excitation from human activity as anLRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION