Block Shear Capacity of a Bolted Connection
Block Shear Capacity of a Bolted Connection
Block shear is a failure mode unique to bolted connections in which a chunk of the connected plate tears out along a path that combines shear yielding on one face with tension fracture on the perpendicular face — for example, at the end of a coped beam or a gusset plate with a tight bolt pattern. It is a genuinely different failure mode from simple bolt shear or plate tension, and it can govern even when the individual bolts and the gross plate area both check out fine.Block shear failures tend to occur suddenly with little visible warning, which is why steel design codes (like AISC) give it its own explicit check rather than folding it into ordinary shear or tension rupture provisions; connections with short edge distances or tight bolt spacing near a member end are the classic locations where it governs.
The block shear resistance combines shear yielding on the gross shear plane with fracture on the net tension plane, R_n = 0.6·F_y·A_gv + F_u·A_nt. where F_y is the steel yield strength, F_u is the steel ultimate tensile strength, A_gv is the gross area on the shear plane, A_nt is the net area on the tension plane, R_n is the combined block shear resistance, and V is the applied load on the connection.
Inputs
A_gv := 1200 mm^2=1.2e-03 m^2
A_nt := 300 mm^2=3e-04 m^2
F_y := 350 MPa=350 MPa
F_u := 450 MPa=450 MPa
V := 250 kN=250000 N
The shear plane yields over its gross area while the tension plane fractures over its net (bolt-hole-reduced) area, so the two terms are added to get the total block shear resistance.
R_n := 0.6·F_y·A_gv + F_u·A_nt=387000 N
Comparing the block shear resistance with the applied load gives the factor of safety against tear-out.
SF := R_n/V=1.55
Results
An SF around 1.5, as found here, is a healthy margin for a failure mode that codes treat conservatively because of its brittle, sudden nature. When block shear governs over ordinary bolt shear or gross-section yielding, the usual remedy is increasing edge distance and bolt spacing to enlarge both A_gv and A_nt, rather than adding more bolts in the same tight pattern. This simplified one-term combination is a common textbook approximation; production code checks (like AISC J4.3) take the lesser of two rupture/yield combinations and apply additional resistance factors.
Assumptions & limitations
- Simplified single-term combination of shear yield plus tension fracture (not the full code two-case check).
- Net areas already account for bolt hole deductions, which are not modeled explicitly here.
- Load is assumed to act concentrically along the failure block.
- Standard bolt shear and plate gross-section checks are performed separately.