Slip-Critical Bolted Connection Capacity
Slip-Critical Bolted Connection Capacity
In a slip-critical (friction-type) connection, high-strength bolts are pretensioned to clamp the connected plates together so tightly that the load is transferred by friction between the faying surfaces rather than by the bolts bearing on the plate holes. These connections are used where any slip — even a small one — would be unacceptable, such as connections subject to load reversal or fatigue, or where oversized/slotted holes are needed for erection tolerance.If the friction resistance is exceeded, the plates slip until the bolts bear against the hole edges, which can cause misalignment, fatigue damage at the new bearing point, or objectionable movement in machinery-supporting or vibration-sensitive structures — which is exactly the scenario slip-critical design is meant to prevent.
The slip resistance is the friction developed by the clamped bolts, R_slip = mu·n_bolt·T_b, the slip coefficient times the total pretension force across all bolts. where mu is the slip coefficient between the faying surfaces, n_bolt is the number of bolts in the connection, T_b is the pretension force per bolt, R_slip is the total slip resistance, and V is the applied shear load.
Inputs
mu := 0.35=0.35
n_bolt := 6=6
T_b := 180 kN=180000 N
V := 250 kN=250000 N
Each bolt clamps the plates with its pretension force, and friction resistance is that clamping force times the slip coefficient, summed over all bolts.
R_slip := mu·n_bolt·T_b=378000 N
Comparing the total slip resistance with the applied shear gives the factor of safety against slip.
SF := R_slip/V=1.51
Results
An SF around 1.5, as found here, gives reasonable margin against slip under service loads, though slip-critical design typically uses a slightly lower resistance factor than bearing-type connections because faying surface condition (rust, paint, galvanizing) is harder to guarantee in the field than bolt shear strength. If SF is marginal, increasing bolt count or pretension (via a higher bolt grade) is more effective than changing the surface treatment alone, since surface class improvements (e.g., blast-cleaned vs. as-rolled) have practical limits. Once slip occurs the connection typically still has bearing capacity as a backup, but that reserve should never be relied on for a connection specified as slip-critical.
Assumptions & limitations
- Faying surfaces are assumed to have a consistent, verified slip coefficient (surface class) across all bolts.
- Full, verified bolt pretension is assumed (installed per a calibrated method, not just snug-tight).
- No coexisting tension on the bolts that would reduce clamping force is included.
- Bearing capacity as a backup after slip is not credited in this check.