AISC LRFD 1.pdf

Comm. L5.] CORROSION 259orthotropic composite plate. Nonstructural partitions, cladding, and built-in furnituresignificantly increase the stiffness and damping of the structure and frequentlyeliminate potential vibration problems. The damping can also depend on the amplitudeof excitation.The general objective in minimizing problems associated with excessive structuralmotion is to limit accelerations, velocities, and displacements to levels that wouldnot be disturbing to the building occupants. Generally, occupants of a building findsustained vibrations more objectionable than transient vibrations.The levels of peak acceleration that people find annoying depend on frequency ofresponse. Thresholds of annoyance for transient vibrations are somewhat higherand depend on the amount of damping in the floor system. These levels depend onthe individual and the activity at the time of excitation (ASCE, 1979; ISO, 1974;CSA, 1989; Murray, Allen, and Ungar, 1997; and Ad Hoc Committee on ServiceabilityResearch, 1986).The most effective way to reduce effects of continuous vibrations is through vibrationisolation devices. Care should be taken to avoid resonance, where the frequencyof steady-state excitation is close to the fundamental frequency of the system.Transient vibrations are reduced most effectively by increasing the damping inthe structural assembly. Mechanical equipment which can produce objectionablevibrations in any portion of a structure should be adequately isolated to reduce thetransmission of such vibrations to critical elements of the structure.3. DriftThe LRFD Specification does not provide specific limiting values for lateral drift.If a drift analysis is desired, the stiffening effect of non-load-supporting elementssuch as partitions and infilled walls may be included in the analysis of drift.Some irrecoverable inelastic deformations may occur at given load levels in certaintypes of construction. The effect of such deformations may be negligible or serious,depending on the function of the structure, and should be considered by thedesigner on a case by case basis.The deformation limits should apply to structural assemblies as a whole. Reasonabletolerance should also be provided for creep. Where load cycling occurs, considerationshould be given to the possibility of increases in residual deformationthat may lead to incremental failure.L5. CORROSIONSteel members may deteriorate in particular service environments. This deteriorationmay appear either in external corrosion, which would be visible upon inspection,or in undetected changes that would reduce its strength. The designer shouldrecognize these problems by either factoring a specific amount of damage toleranceinto the design or providing adequate protection systems (e.g., coatings, cathodicprotection) and/or planned maintenance programs so that such problems do notoccur.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION