Design of this type of connection assumes that the fastener, under high initial¬†tensioning, develops frictional resistance between the connected parts, preventing¬†slippage despite external load. Properly installed A307 bolts provide some friction,¬†but since it is not dependable it is ignored. High-strength steel bolts tightened nearly¬†to their yield strengths, however, develop substantial, reliable friction. No slippage¬†will occur at design loads if the contact surfaces are clean and free of paint or have¬†only scored galvanized coatings, inorganic zinc-rich paint, or metallized zinc or¬†aluminum coatings.
The AISC ‚Äė‚ÄėSpecification for Structural Steel for Buildings,‚Äô‚Äô ASD and LRFD,¬†lists allowable shear for high-strength bolts in slip-critical connections. Though¬†there actually is not shear on the bolt shank, the shear concept is convenient for¬†measuring bolt capacity.
Since most joints in building construction can tolerate tiny slippage, bearingtype¬†joints, which are allowed much higher shears for the same high-strength bolts¬†when the threads are not in shear planes, may, for reasons of economy, lessen the¬†use of slip-critical joints.
The capacity of a slip-critical connection does not depend on the bearing of the¬†bolts against the sides of their holes. Hence, general specification requirements for¬†protection against high bearing stresses or bending in the bolts may be ignored.
If the fasteners B in Fig. 7.41b are in a slip-critical connection, the bolts above¬†the neutral axis will lose part of their clamping force; but this is offset by a compressive¬†force below the neutral axis. Consequently, there is no overall loss in¬†frictional resistance to slippage.
When it is apparent that there may be a loss of friction (which occurs in some¬†type of brackets and hangers subject to tension and shear) and slip under load¬†cannot be tolerated, the working value in shear should be reduced in proportion to¬†the ratio of residual tension to initial tension.
Slip-critical connections subjected to eccentric loading, such as that illustrated¬†in Fig. 7.41, are analyzed in the same manner as bearing-type connections (Art.¬†7.32).