AISC LRFD 1.pdf

Comm. J3.] BOLTS AND THREADED PARTS 243was initially installed pretensioned or snug-tightened. Recent tests (Johnson, 1996and Murray, Kline, and Rojani, 1992) confirm that the performance of ASTMA325 and A325M bolts in tension not subjected to fatigue are unaffected by theoriginal installation condition. While the equation was developed for bolted connections(Kulak et al., 1987), it was also conservatively applied to threaded partsand to rivets.In connections consisting of only a few fasteners, the effects of strain on the shear inbearing fasteners is negligible (Kulak et al., 1987 and Fisher et al., 1978). In longerjoints, the differential strain produces an uneven distribution between fasteners(those near the end taking a disproportionate part of the total load), so that the maximumstrength per fastener is reduced. The AISC ASD Specification permits connectionsup to 50 in. (1 270 mm) in length without a reduction in maximum shearstress. With this in mind the resistance factor for shear in bearing-type connectionshas been selected to accommodate the same range of connections.The values of nominal shear strength in Table J3.2 were obtained from the equationRn / mAb = 050 . Fu(C-J3-3)when threads are excluded from the shear planes andRn / mAb = 040 . Fu(C-J3-4)when threads are not excluded from the shear plane, where m is the number of shearplanes (Kulak et al., 1987). While developed for bolted connections, the equationswere also conservatively applied to threaded parts and rivets. The value given forA307 bolts was obtained from Equation C-J3-4 but is specified for all cases regardlessof the position of threads. For A325 or A325M bolts, no distinction is madebetween small and large diameters, even though the minimum tensile strength F u islower for bolts with diameters in excess of one inch. It was felt that such a refinementof design was not justified, particularly in view of the low resistance factor ,the increasing ratio of tensile area to gross area, and other compensating factors.7. Combined Tension and Shear in Bearing-Type ConnectionsTests have shown that the strength of bearing fasteners subject to combined shearand tension resulting from externally applied forces can be closely defined by anellipse (Kulak et al., 1987). Such a curve can be replaced, with only minor deviations,by three straight lines as shown in Figure C-J3.3. This latter representationoffers the advantage that no modification of either type stress is required in the presenceof fairly large magnitudes of the other type. This linear representation wasadopted for Table J3.5, giving a limiting tensile stress F t as a function of the shearingstress f v for bearing-type connections. Following a change in the 1994 RCSCLRFD Specification for Structural Joints Using ASTM A325 or A490 Bolts, thecoefficients in the equations in Table J3.5 have been modified for consistency(Carter, Tide, and Yura, 1997).8. High-Strength Bolts in Slip-Critical ConnectionsConnections classified as slip-critical include those cases where slip could theoreticallyexceed an amount deemed by the Engineer of Record to affect the suitabilityfor service of the structure by excessive distortion or reduction in strength or stability,even though the nominal strength of the connection may be adequate. Alsoincluded are those cases where slip of any magnitude must be prevented, for exam-LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION