| name | bolt-group-analysis |
| description | Bolt group analysis — in-plane shear (direct + eccentric shear), out-of-plane moment (neutral axis, prying), AISC/RCSC methods, bolt pattern design. |
| metadata | {"priority":7,"promptSignals":{"phrases":["bolt group","bolt pattern","eccentric bolt","bolt shear group","bolt moment","prying force"],"minScore":3}} |
Bolt Group Analysis — Complete Skill
In-Plane Loading (Shear Only)
Applied force passes through centroid of bolt group → equal shear per bolt
Direct shear (no eccentricity):
V_direct = P / n [N/bolt]
Eccentric shear (force offset from centroid):
Two components:
- Direct shear: V_direct = P/n (uniform on all bolts)
- Torsional shear due to moment M = P×e
Torsional shear (elastic method):
V_T = M × r_i / Σr_i²
where r_i = distance from centroid to bolt i
Direction: perpendicular to r_i (tangential)
Resultant: V_i = √(V_direct² + V_T² + 2 V_direct V_T cosθ)
Critical bolt: maximum resultant (usually outermost bolt)
Instantaneous Center of Rotation (IC) method (AISC):
More accurate; accounts for force-deformation compatibility
IC moves away from applied force; bolts closest to IC carry least load
Result tables in AISC Part 7 (coefficients C × V_allow)
Out-of-Plane Loading (Moment in Plane of Faying Surface)
Applied moment M_x about horizontal axis through bolt group
Neutral axis assumption (elastic):
Assume top bolts in tension; bottom in compression (bearing on plate)
T_max = M_x × y_max / Σy_i² [force per bolt]
With prying action:
Prying magnifies bolt tension; depends on flange/plate flexibility
AISC simplified: T_u per bolt = M/(n_t × d_g) where d_g = gauge
With prying: T_total = T_u + Q (prying force Q from plate bending)
Prying force (AISC 360 / Thornton):
Q = T_u × b'/(a') × [1 - ρ/(1+δ(1-ρ)/ρ)] (exact per AISC method)
Simplified: Q ≈ 0.2–0.5 × T_u for typical flange thicknesses
Combined Shear and Tension
AISC LRFD interaction:
(f_v / φV_n)^(5/3) + (f_t / φT_n)^(5/3) ≤ 1.0
Simplified linear:
f_v/φV_n + f_t/φT_n ≤ 1.0 (conservative)
Bolt Capacities (AISC 360-22)
Bearing type (bolts allowed to slip):
φV_n = φ × F_nv × A_b
F_nv: ASTM A325M = 372 MPa; A490M = 457 MPa; A307 = 188 MPa
φ = 0.75 (LRFD)
Slip-critical (friction type):
φR_n = φ μ D_u h_f T_b n_s
μ = slip coefficient: Class A (unpainted): 0.35; Class B (blast-cleaned): 0.50
T_b = minimum bolt pretension from RCSC Table 8.1
φ = 1.13 (serviceability); 0.85 (strength)
Tension:
φT_n = φ F_nt A_b
F_nt: A325 = 620 MPa; A490 = 780 MPa
Bearing Capacity (Plate)
φR_n = φ 2.4 F_u d_b t_p (standard hole, large bearing deformation)
φR_n = φ 1.5 L_c t_p F_u (for deformation-critical or short edge distance)
φ = 0.75
Minimum Edge and Spacing Requirements (AISC)
Edge distance (minimum): 1.5 d_b
Edge distance (preferred): 1.75 d_b
Bolt spacing: 3 d_b minimum; preferred 2.67 d_b for standard holes
Weld vs. Bolt Economy Rule of Thumb
Bolts preferred: field connections, joints needing disassembly
Welds preferred: fatigue joints, waterproof, high moment transfer
AISC allows combined weld+bolt if weld is complete joint penetration (CJP) and both share in-plane shear
Design Procedure
- Identify load type: in-plane shear, out-of-plane moment, combined
- Find bolt group centroid
- Calculate eccentricity e (in-plane) or moment arm (out-of-plane)
- Compute critical bolt force by elastic or IC method
- Check bolt shear, tension, bearing, prying
- Check plate bearing and edge distances
- For LRFD: verify φR_n ≥ R_u
Output
Provide: bolt pattern layout [mm], critical bolt forces V_max [kN] and T_max [kN], prying force Q [kN], bolt utilization ratios (shear + tension interaction), governing failure mode, AISC table coefficient if IC method used.
