bolt-group-analysis

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:

1. Direct shear: V_direct = P/n (uniform on all bolts)
2. 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

1. Identify load type: in-plane shear, out-of-plane moment, combined
2. Find bolt group centroid
3. Calculate eccentricity e (in-plane) or moment arm (out-of-plane)
4. Compute critical bolt force by elastic or IC method
5. Check bolt shear, tension, bearing, prying
6. Check plate bearing and edge distances
7. 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.