Countersunk Bolts Technical Note

When countersunk bolts are used in a connection, the bearing capacity of the bolt against a plate is going to be less than that of a non-countersunk bolt. BS 5950 Pt1 clause 6.3.3.2 states that the thickness of the plate in consideration should be reduced by half the depth of the countersinking when calculating the bearing capacity of the bolt against the plate.

Note that this reduction is not half the diameter of the countersunk bolt, but slightly different, as outlined below.

A similar statement is contained as a note in EC3-1-8 table 5.7.

Explanation

The BS states in clause 6.3.3.2 that you reduce the plate thickness by half the countersink in the plate. If you take the countersink depth of the bolt head to be 0.5d + t, where t is the depth of the small vertical portion at the head of the bolt, then with the bolt countersunk flush with the top of the plate, the half-depth of the countersink is (0.5 x t) + (0.5 x BoltDia) – (0.25 x HoleDia) where the hole diameter is usually 1mm or 2 mm greater than the bolt diameter.

For example - countersinking is at 45 deg and a 20 mm bolt will have a 40mm head.

The countersunk depth reduction will not be D/4, but slightly different:-

MasterSeries Output

For a 20 dia countersunk bolt in a 22 dia hole and 15mm thick outer plate.

Therefore the reduced plate thickness in bearing for a 15mm plate is 15mm less 6.0mm giving 9.0mm.