AISC LRFD 1.pdf

Comm. A3.] MATERIAL 167and Chen, 1993; Chen and Toma, 1994; Chen, Goto, and Liew, 1995; and Leon etal., 1996).The degree of sophistication of the analysis depends on the problem at hand.Usually design for PR construction requires separate analyses for the serviceabilityand ultimate limit states. For serviceability, an analysis using linear springs with asecant stiffness K s is often sufficient. Under factored loads, a more careful procedureis needed to ensure that the characteristics assumed in the analysis are consistentwith those of the connection response. As shown in Figure C-A2.1, theresponse is especially nonlinear as the applied moment approaches the nominalconnection strength, M n . In particular, the effect of the connection nonlinearity onsecond-order moments and other stability checks need to be considered (ASCETask Committee on Effective Length, 1997). The preferable method of incorporatingthese effects in the ultimate limit state check is through a second-order analysisthat models the nonlinear moment-rotation response of the connections explicitly.Alternatively, for regular structures in regions of low to moderate seismicity, properlycalibrated second-order elastic analysis methods or plastic hinge methods(e.g., Leon et al., 1996) can be used. Elastic procedures may, for example, use linearsprings with reduced secant-stiffness values determined to be consistent with themaximum rotations calculated under the factored loads.Prior to this Specification, an analysis procedure was used that ignored the restrainingeffect of connections for gravity loads. However, the effect was taken intoaccount for lateral load resistance to wind loads. This method was referred to as“simple framing”, and required that three conditions be satisfied (Disque, 1964).Today’s approach of using a predictable degree of connection restraint is a moreaccurate representation of the structural behavior.A3. MATERIAL1. Structural Steel1a. ASTM DesignationsThe grades of structural steel approved for use under the LRFD Specification, coveredby ASTM standard specifications, extend to a yield stress of 100 ksi (690MPa). Some of these ASTM standards specify a minimum yield point, while othersspecify a minimum yield strength. The term “yield stress” is used in the Specificationas a generic term to denote either the yield point or the yield strength.It is important to be aware of limitations of availability that may exist for some combinationsof strength and size. Not all structural section sizes are included in the variousmaterial specifications. For example, the 60 ksi (415 MPa) yield strength steelin the A572/A572M specification includes plate only up to 1 1 4-in. (32 mm) in thickness.Another limitation on availability is that even when a product is included inthe specifications, it may be infrequently produced by the mills. Specifying theseproducts may result in procurement delays or require ordering large quantitiesdirectly from the producing mills. Consequently, it is prudent to check availabilitybefore completing the details of a design.Properties in the direction of rolling are of principal interest in the design of steelstructures. Hence, yield stress as determined by the standard tensile test is the principalmechanical property recognized in the selection of the steels approved for useLRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION