AISC LRFD 1.pdf

Comm. J3.] BOLTS AND THREADED PARTS 245in trusses, a small slip in the end connections can significantly reduce the strengthof the compression member so the slip-critical end connection should be checkedfor slip at the factored-load level, whether or not a slip-critical connection isrequired by a serviceability requirement.In connections with long slots that are parallel to the direction of the applied load,slip of the connection prior to attainment of the factored load might be large enoughto alter the usual assumption of analysis that the undeformed structure can be usedto obtain the internal forces. The Specification allows the designer two alternativesin this case. If the connection is designed so that it will not slip under the effects ofservice loads, then the effect of the factored loads acting on the deformed structure(deformed by the maximum amount of slip in the long slots at all locations) must beincluded in the structural analysis. Alternatively, the connection can be designed sothat it will not slip at loads up to the factored load level.Joints subjected to full reverse cyclical loading are clearly slip-critical joints sinceslip would permit back and forth movement of the joint and early fatigue. However,for joints subjected to pulsating load that does not involve reversal of direction,proper fatigue design could be provided either as a slip-critical joint on the basis ofstress on the gross section, or as a non-slip-critical joint on the basis of stress on thenet section. Because fatigue results from repeated application of the service loadrather than the overload load, design should be based upon service-load criteria.For high-strength bolts in combination with welds in statically loaded conditionsand considering new work only, the nominal strength may be taken as the sum of theslip resistances provided by the bolts and the shear resistance of the welds. SectionJ1.9 requires that the slip resistance be determined at factored load levels. If onetype of connector is already loaded when the second type of connector is introduced,the nominal strength cannot by obtained by adding the two resistances. TheGuide (Kulak et al., 1987) should be consulted in these cases.Slip of slip-critical connections is likely to occur at approximately 1.4 to 1.5 timesthe service loads. For standard holes, oversized holes, and short-slotted holes theconnection can be designed either at service loads (Appendix J3.8b) or at factoredloads (Section J3.8a). The nominal loads and factors have been adjusted accordingly.The number of connectors will be essentially the same for the two proceduresbecause they have been calibrated to give similar results. Slight differences willoccur because of variation in the ratio of live load to dead load.In connections containing long slots that are parallel to the direction of the appliedload, slip of the connection prior to attainment of the factored load might be largeenough to alter the usual assumption of analysis that the undeformed structure canbe used to obtain the internal forces. To guard against this occurring, the design slipresistance is further reduced by 0.8 when designing at service load (AppendixJ3.8b) and by setting to 0.60 in conjunction with factored loads (Section J3.8a).While the possibility of a slip-critical connection slipping into bearing under anticipatedservice conditions is small, such connections must comply with the provisionsof Section J3.10 in order to prevent connection failure at the maximum loadcondition.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION