Bearing failure of Bolts

Bearing failure

If the connected plates are made of high strength steel then failure of bolt can take place by bearing of the plates on the bolts. If the plate material is weaker than the bolt material, then failure will occur by bearing of the bolt on the plate and the hole will elongate. The beating area is given by the nominal diameter of the bolt times the combined thickness of the plates bearing in any direction. A bolt bearing on any plate subjected to a factored shear force (Vsb) shall satisfy as  Vsb < = Vdb per cl. 10.3.2 of IS 800:2007, where as Vdb = Vdpb = Vnpb/?mb given by cl. 10.3.4 of the code where, ?mb = 1.25 and V_npb = bearing strength of a bolt, calculated as V_npb = 2.5k_bdtf_u where f_u = smaller of the ultimate tensile stress of the bolt and the ultimate tensile stress of the plate, d = nominal diameter of the bolt, t = summation of the thicknesses of the connected plates experiencing bearing stress in the same direction and k_b is smaller of e/3d₀, p/3d₀-0.25, f_ub/f_u, 1.0 where e, p = end and pitch distances of the fastener along bearing direction; d₀ = diameter of the hole; f_ub, f_u = Ultimate tensile stress of the bolt and the ultimate tensile stress of the plate, respectively.

The underlying assumption behind the design of bolted connections, namely that all bolts carry equal load is not true in some cases. In long joints, the bolts farther away from the centre of the joint will carry more load than the bolts located close to the centre. Therefore, for joints having more than two bolts on either side of the building connection with the distance between the first and the last bolt exceeding 15d in the direction of load, the nominal shear capacity Vns, shall be reduced by the factor, ?lj, given by (Cl.10.3.2.1) ?_lj = 1.075 - l_j / (200 d) but 0.75 < ?_lj < 1.0 where, d= nominal diameter of the bolt Similarly, if the grip length exceeds five times the nominal diameter, the strength is reduced as specified in IS 800. In multi-bolt connections, due to hole mismatch, all the bolts may not carry the same load. However, under ultimate load, due to high bearing ductility of the plates considerable redistribution of the load is possible and so the assumption that all bolts carry equal load may be considered valid.