AISC LRFD 1.pdf

172 DESIGN BASIS [Comm. A5.A5. DESIGN BASIS1. Required Strength at Factored LoadsLRFD permits the use of both elastic and plastic structural analyses. LRFD provisionsresult in essentially the same methodology for, and end product of, plasticdesign as included in the AISC ASD Specification (AISC, 1989) except that theLRFD provisions tend to be slightly more liberal, reflecting added experience andthe results of further research. The 10 percent redistribution permitted is consistentwith that in the AISC ASD Specification (AISC, 1989).In some circumstances, as in the proportioning of the bracing members that carryno calculated forces (see Section C3) and of connection components (see SectionJ1.7), the required strength is explicitly stated in the Specification.2. Limit StatesA limit state is a condition which represents the limit of structural usefulness. Limitstates may be dictated by functional requirements, such as maximum deflections ordrift; they may be conceptual, such as plastic hinge or mechanism formation; orthey may represent the actual collapse of the whole or part of the structure, such asfracture or instability. Design criteria ensure that a limit state is violated only withan acceptably small probability by selecting the combination of load and resistancefactors and nominal load and resistance values which will never be exceeded underthe design assumptions.Two kinds of limit states apply for structures: limit states of strength which definesafety against extreme loads during the intended life of the structure, and limitstates of serviceability which define functional requirements. The LRFD Specification,like other structural codes, focuses on the limit states of strength because ofoverriding considerations of public safety for the life, limb, and property of humanbeings. This does not mean that limit states of serviceability are not important to thedesigner, who must equally ensure functional performance and economy of design.However, these latter considerations permit more exercise of judgment on the partof designers. Minimum considerations of public safety, on the other hand, are notmatters of individual judgment and, therefore, specifications dwell more on thelimit states of strength than on the limit states of serviceability.Limit states of strength vary from member to member, and several limit states mayapply to a given member. The following limit states of strength are the most common:onset of yielding, formation of a plastic hinge, formation of a plastic mechanism,overall frame or member instability, lateral-torsional buckling, local buckling,tensile fracture, development of fatigue cracks, deflection instability,alternating plasticity, and excessive deformation. The most common serviceabilitylimit states include unacceptable elastic deflections and drift, unacceptable vibrations,and permanent deformations.3. Design for StrengthThe general format of the LRFD Specification is given by the formula:SgiQi £ fRnLRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION(C-A5-1)