Bolted Moment Connection Capacity
Bolted Moment Connection Capacity
Moment connections transfer beam-end moment into a column (or between two members) through a force couple carried by bolt rows above and below the neutral axis, rather than through a single shear plane. Framed moment connections like this appear throughout steel buildings wherever a rigid joint is needed to control drift or carry gravity moment continuity — for example, a beam-to-column connection in a moment-resisting frame.The connection is only as strong as its weakest bolt row, so the tension row is checked explicitly: if the moment demand exceeds what the bolt group can carry in tension, the row can fracture suddenly, which is a brittle, non-ductile failure mode engineers specifically try to avoid in seismic and wind-governed frames.
The tension couple carried by the bolt rows converts the applied moment into a force F = M/d, where d is the lever arm between the tension and compression bolt rows. where M is the applied connection moment, d is the lever arm between the bolt rows, F is the resulting tension force in the critical row, F_bolt_allow is the allowable tension capacity of one bolt, and n_b is the number of bolts sharing that row.
Inputs
M := 80 kN·m=80000 Nm
d := 0.4 m=400 mm
F_bolt_allow := 120 kN=120000 N
n_b := 2=2
Treating the bolt rows as a force couple, the applied moment divided by the lever arm gives the tension force the top row alone must carry.
F := M/d=200000 N
Multiplying the allowable tension per bolt by the number of bolts in the row gives the row capacity, and comparing it with the demand yields the factor of safety.
SF := (n_b·F_bolt_allow)/F=1.2
Results
A factor of safety comfortably above 1 (here around 1.2) shows the tension row has enough reserve, but moment connections are usually governed by several checks at once — bolt tension, prying action, and the column flange or end-plate bending it triggers. Because bolt tension failure is brittle, most codes require a healthier margin here than on ductile yielding limit states, so values close to 1.0 typically call for a redesign rather than being accepted at face value. If SF is too low, adding a bolt to the row or increasing the lever arm (deeper connection) is far more effective than upsizing individual bolts.
Assumptions & limitations
- Bolt row forces come from a linear, rigid-body moment distribution, not a detailed prying or finite-element analysis.
- The compression side is assumed resisted by direct bearing, so only the tension row governs.
- Bolts within the row are assumed to share the force equally.
- Shear on the bolts and any coexisting axial force in the member are checked separately.