TAILIEUCHUNG - Mechanical Engineer´s Handbook P50

Fig. 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. EVALUATION OF SLIP CHARACTERISTICS. | INSTALLATION OF HIGH-STRENGTH BOLTS 1153 Fig. 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. 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 fluctuations 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 i k rn T i i where k. slip coefficient m number of slip planes n 2 Tf the sum of the bolt tensions i i If the bolt tension is equal in all bolts then Aiip ks m n Tt where n the number of bolts in the joint The slip coefficient K varies from joint to joint depending on the type of steel different surface treatments and different surface conditions and along with the clamping force T 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 . INSTALLATION OF HIGH-STRENGTH BOLTS Prior to 1985 North American practice

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