AISC LRFD 1.pdf

177Comm. BCHAPTER BDESIGN REQUIREMENTSB2. NET AREACritical net area is based on net width and load transfer at a particular chain.B3. EFFECTIVE AREA OF TENSION MEMBERSSection B3 deals with the effect of shear lag, which is applicable to both welded andbolted tension members. The reduction coefficient U is applied to the net area A n ofbolted members and to the gross area A g of welded members. As the length of connectionl is increased, the shear lag effect is diminished. This concept is expressedempirically by the equation for U. Munse and Chesson (1963) have shown thatusing this expression to compute an effective area, with few exceptions, theestimated strength of some 1,000 bolted and riveted connection test specimens correlatedwith observed test results within a scatterband of 10 percent. Newerresearch (Easterling and Gonzales, 1993) provides further justification for currentprovisions.For any given profile and connected elements, x is a fixed geometric property. It isillustrated as the distance from the connection plane, or face of the member, to thecentroid of the member section resisting the connection force. See Figure C-B3.1.The length l is dependent upon the number of fasteners or equivalent length of weldrequired to develop the given tensile force, and this in turn is dependent upon themechanical properties of the member and the capacity of the fasteners or weld used.The length l is illustrated as the distance, parallel to the line of force, between thefirst and last fasteners in a line for bolted connections. The number of bolts in a line,for the purpose of the determination of l, is determined by the line with the maximumnumber of bolts in the connection. For staggered bolts, the out-to-out dimensionis used for l. See Figure C-B3.2. There is insufficient data to establish a value ofU if all lines have only one bolt, but it is probably conservative to use A e equal to thenet area of the connected element. For welded connections, l is the length of theweld parallel to the line of force. For combinations of longitudinal and transversewelds (see Figure C-B3.3), l is the length of longitudinal weld because the transverseweld has little or no effect on the shear lag problem, i.e., it does little to get theload into the unattached portions of the member.Previous issues of this Specification have presented values for U for bolted orriveted connections of W, M, and S shapes, tees cut from these shapes, and othershapes. These values are acceptable for use in lieu of calculated values and areretained here for the convenience of designers.For bolted or riveted connections the following values of U may be used:(a) W, M,orS shapes with flange widths not less than two-thirds the depth, andLRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION