30079_35b

30079_35b - Fig. 35.27 The external load applied to the...

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Fig. 35.27 The external load applied to the joint interface has exceeded the critical load by an amount = A. This suggests that a joint designed to the above equation might have larger and/or more numerous bolts than necessary to support pressure loads the bolts will never see. The ASME Boiler and Pressure Vessel Code takes an even more conservative point of view than that described by the above equation to introduce a factor of safety. This code assumes that the bolts see 100% of external load Lx, not an amount reduced by the stiffness ratio. 35.9 EVALUATION OF SLIP CHARACTERISTICS A slip-resistant joint is one that has a low probability of slip at any time during the life of the structure. In this type of joint, the external applied load usually acts in a plane perpendicular to the bolt axis. The load is completely transmitted by frictional forces acting on the contact area of the plates fastened by the bolts. This frictional resistance is dependent on (1) the bolt preload and (2) the slip resistance of the fraying surfaces. Slip-resistant joints are often used in connections subjected to stress reversals, severe stress fluc- tuations, or in any situation wherein slippage of the structure into a "bearing mode" would produce intolerable geometric changes. A slip load of a simple tension splice is given by Pa, = k/r& T, 1=1 where ks = slip coefficient m = number of slip planes 2 Tt = the sum of the bolt tensions /=! If the bolt tension is equal in all bolts, then Psiip = ksmn Tt n = the number of bolts in the joint The slip coefficient Ks varies from joint to joint, depending on the type of steel, different surface treatments, and different surface conditions, and along with the clamping force Tt shows considerable variation from its mean value. The slip coefficient Ks can only be determined experimentally, but some values are now available, as shown in Table 35.1. 35.10 INSTALLATION OF HIGH-STRENGTH BOLTS Prior to 1985, North American practice had been to require that all high-strength bolts be installed and provide a high level of preload, regardless whether or not it was needed. The advantages in such an arrangement were that a standard bolt installation procedure was provided for all types of con-

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nections and that a slightly stiffer structure probably resulted. Obviously, when a slip-resistant bolted structure was not needed, the disadvantages were the additional cost and inspection time for this type of installation. Since 1985, only fasteners that are to be used in slip-critical connections or in con- nections subject to direct tension loading have needed to be preloaded to the original preload, equal to 70% of the minimum specified tensile strength of the bolt. Bolts to be used in bearing-type connections only need to be tightened to the snug-tight condition. When the high-strength bolt was first introduced, installation was primarily by methods of torque control. Approximate torque values were suggested, but tests performed and field experience con-
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This note was uploaded on 05/02/2010 for the course ME 100 taught by Professor Any during the Spring '10 term at Purdue University.

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30079_35b - Fig. 35.27 The external load applied to the...

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