AISC LRFD 1.pdf

244 BOLTS AND THREADED PARTS [Comm. J3.ple, joints subject to fatigue, connectors between elements of built-up members attheir ends (Sections D2 and E4), and bolts in combination with welds (SectionJ1.9).The onset of slipping in a high-strength bolted, slip-critical connection is not anindication that the maximum strength of the connection has been reached. Its occurrencemay be only a serviceability limit state. The design check for slip resistancecan be made at two different load levels, factored loads (Sections J3.8a and J3.9a)and service loads (Appendices J3.8b and J3.9b). The nominal slip resistances r strand F v A b to be used with factored loads and service loads, respectively, are based ontwo different design concepts. The slip resistance r str with factored loads is the meanresistance per bolt, which is a function of the mean slip coefficient and the clampingforce. The 1.13 factor in (Equation J3-1) accounts for the expected 13 percentincrease above the minimum specified preload provided by calibrated wrenchtightening procedures. This was used to represent typical installations. The factoredload resistance r str uses the reliability index approach that is used for theother design checks such as tension and bearing. The service load approach uses aprobability of slip concept that implies a 90 percent reliability that slip will notoccur if the calibrated wrench method of bolt installation is used.The Engineer of Record must make the determination to use factored loads, serviceloads, or both in checking the slip resistance of a slip-critical connection. The followingcommentary is provided as guidance and an indication of the intent of theSpecification.In the case of slip-critical connections with three or more bolts in holes with only asmall clearance, such as standard holes and slotted holes loaded transversely to theaxis of the slot, the freedom to slip does not generally exist because one or morebolts are in bearing even before load is applied due to normal fabrication tolerancesand erection procedures. If connections with standard holes have only one or twobolts in the direction of the applied force, a small slip may occur. In this case,slip-critical connections subjected to vibration or wind should be checked for slip atservice-load levels. In built-up compression members, such as double-angle strutsFig. C-J3.3. Three straight line approximation.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION