Bolted connections

Connections are where most steel structures succeed or fail. Bolts join members quickly on site with simple tools, and their behaviour is well understood. Two main types are used. Bearing-type connections let the plates slip slightly until the bolt shank bears against the hole; the bolt then carries load in shear and the plate in bearing. Friction-grip (slip-critical) connections use high-strength bolts tightened to a high pre-tension, so friction between clamped surfaces carries load without slip, which is needed where fatigue or load reversal makes slip unacceptable.

In a bearing connection each bolt is checked for shear on each shear plane, with the threaded area used if threads cross the plane, and for bearing on the thinnest plate, which depends on edge and pitch distances. The weaker value is the bolt value. Long joints and thick packing reduce bolt shear strength. Bolts in tension, such as in end-plate connections, are checked for tension and for combined shear and tension, and prying action can add force when the end plate bends.

Minimum pitch, edge and end distances prevent plate tearing; maximum spacing prevents gaps that let moisture in and plates buckle between bolts. Holes are normally 2 mm larger than the bolt for smaller sizes. Bolt groups loaded eccentrically, as in brackets, share direct shear equally and resist the moment by forces proportional to distance from the group centroid.