AISC LRFD 1.pdf

262Comm. NCHAPTER NEVALUATION OF EXISTING STRUCTURESN1. GENERAL PROVISIONSThe load combinations referred to in this chapter reflect gravity loading because itis the most prevalent condition encountered. If other loading conditions are a consideration,such as lateral loads, the appropriate load combination from ASCE 7(ASCE, 1998) or from the applicable building code should be used. Guidelines forseismic evaluation are available in other publications (FEMA, 1997a and 1997b).The Engineer of Record for a project is generally established by the owner.N2. MATERIAL PROPERTIES1. Determination of Required TestsThe extent of tests required depends on the nature of the project, the criticality of thestructural system or member evaluated, and the availability of records pertinent tothe project. Thus, the Engineer of Record is required to determine the specific testsrequired and the locations from which specimens are to be obtained.2. Tensile PropertiesSamples required for tensile tests should be removed from regions of reducedstress, such as at flange tips at beam ends and external plate edges, to minimize theeffects of the reduced area. The number of tests required will depend on whetherthey are conducted to merely confirm the strength of a known material or to establishthe strength of some other steel. Guidance on the appropriate minimum numberof tests is available (FEMA, 1997a).It should be recognized that the yield stress determined by standard ASTM methodsand reported by mills and testing laboratories is somewhat greater than thestatic yield stress because of dynamic effects of testing. Also, the test specimenlocation may have an effect. These effects have already been accounted for in thenominal strength equations in the Specification. However, when strength evaluationis done by load testing, this effect should be accounted for in test planningbecause yielding will tend to occur earlier than otherwise anticipated. The staticyield stress, F ys , can be estimated from that determined by routine application ofASTM methods, F y , by the following equation (Galambos, 1978 and 1998):Fys= R( Fy−4 )(C-N2-1)( Metric: Fys= R( Fy−27 )) (C-N2-1M)whereF ys = static yield stress, ksi (MPa)F y = reported yield stress, ksi (MPa)LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION