AISC LRFD 1.pdf

248 DESIGN RUPTURE STRENGTH [Comm. J4.This can be explained by the two extreme examples given in Figure C-J4.2. In Case(a), the total force is resisted primarily by shear, so shear rupture, not shear yielding,should control the block shear tearing mode; therefore, use Equation J4-3b. ForCase (b), block shear cannot occur until the tension area ruptures as given by EquationJ4-3a. If Equation J4-3b (shear rupture on the small area and yielding on thelarge tension area) is checked for Case (b), a smaller P o will result. In fact, as theshear area gets smaller and approaches zero, the use of Equation J4-3b for Case (b)would give a block shear strength based totally on yielding of the gross tensile area.Block shear is a rupture or tearing phenomenon not a yielding limit state. Therefore,the proper equation to use is the one with the larger rupture term.J5. CONNECTING ELEMENTS2. Design Strength of Connecting Elements in TensionTests have shown that yield will occur on the gross section area before the tensilecapacity of the net section is reached, if the ratio A n /A g 0.85 (Kulak et al., 1987).Since the length of connecting elements is small compared to the member length,inelastic deformation of the gross section is limited. Hence, the effective net area A nof the connecting element is limited to 0.85A g in recognition of the limited inelasticdeformation and to provide a reserve capacity.J6. FILLERSThe practice of securing fillers by means of additional fasteners, so that they are, ineffect, an integral part of a shear-connected component, is not required where a connectionis designed to be a slip-critical connection using high-strength bolts. In suchconnections, the resistance to slip between filler and either connected part is comparableto that which would exist between the connected parts if no fill were present.Filler plates may be used in lap joints of welded connections that splice parts of differentthickness, or where there may be an offset in the joint.J8. BEARING STRENGTHThe LRFD Specification provisions for bearing on milled surfaces, Section J8, followthe same philosophy of earlier AISC ASD Specifications. In general, thedesign is governed by a deformation limit state at service loads resulting in stressesnominally at 9 10 of yield. Adequate safety is provided by post-yield strength asdeformation increases. Tests on pin connections (Johnston, 1939) and on rockers(Wilson, 1934) have confirmed this behavior.As used throughout the LRFD Specification, the terms “milled surface,” “milled,”and “milling” are intended to include surfaces which have been accurately sawed orfinished to a true plane by any suitable means.J9. COLUMN BASES AND BEARING ON CONCRETEThe equations for resistance of concrete in bearing are the same as ACI 318-99except that AISC equations use = 0.60 while ACI uses = 0.70, since ACI specifieslarger load factors than the ASCE load factors stipulated by AISC.See DeWolf and Ricker (1990) for guidelines on the design of column base plates.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION