Bolt Pattern Calculator — in, lbs

How to Calculate Bolt Pattern Capacity

This calculator determines the shear and tension capacity of bolt groups per AISC 360-22 (Specification for Structural Steel Buildings). Enter the bolt size, grade, connection type, and pattern layout. The calculator uses nominal bolt area and AISC design strengths with φ=0.75 for bearing and tension, φ=1.0 for slip-critical connections (Du=1.13, hf=1.0, Tb per Table J3.1).

Formulas

Bolt area: Ab = π/4 × d². Bearing shear: φRn = 0.75 × Fnv × Ab × ns. Tension: φRn = 0.75 × Fnt × Ab. Slip-critical: φRn = 1.0 × μ × Du × hf × Tb × ns (μ=0.35 Class A, Du=1.13, hf=1.0). A325/F3125 Fnv(N)=54, Fnv(X)=68, Fnt=90 ksi. A490 Fnv(N)=68, Fnv(X)=84, Fnt=113 ksi. F1554 anchor rods (J3.6) Fnt=0.75·Fu, Fnv=0.45·Fu (threads in plane); anchor rods are not pretensioned so slip-critical is not available. Minimum spacing 2⅓·d (3·d preferred) per J3.3; minimum edge distance per Table J3.4.

FAQ

What is the difference between N-type and X-type? N-type assumes threads are in the shear plane (lower capacity, Fnv=54 ksi for A325). X-type excludes threads from the shear plane (Fnv=68 ksi). When in doubt, use N — it is the safer assumption and most shop drawings do not control thread location.

What is slip-critical? Slip-critical connections resist load through friction between faying surfaces with pretensioned high-strength bolts (Tb per Table J3.1). They are required for fatigue, oversized or slotted holes loaded transverse to the slot, and built-up tension members — see AISC 360-22 J3.8.

Are bolt bearing on the plate and net section checked? No — this tool sizes the bolt group only. Verify bolt bearing/tear-out (J3.10) and net section after you fix the gusset or base plate thickness, then confirm the member it frames into with the beam and column calculators. For cast-in F1554 anchor rods into a concrete pedestal, detail the reinforcement with the rebar calculator and check pier volume on the concrete beam calculator.

On install day

Three things fabricators forget. Snug-tight vs pretensioned — bearing-type joints (Group A standard) can be snug-tight, but slip-critical, fatigue-loaded, and direct-tension connections must be pretensioned by turn-of-nut, calibrated-wrench, twist-off, or DTI washer per RCSC Section 8. Faying surface — for slip-critical, Class A (μ=0.35) requires clean mill scale or blast-cleaned surfaces; paint, oil, mill primer, or galvanizing without the right Class B procedure can drop φRs in half. Edge and end distance — for ¾″ bolts in a sheared edge plate, J3.4 wants 1¼″ minimum; rolled edges can go to 1″. Typical 2025 material: ASTM F3125 A325 ¾″ × 2½″ HDG runs $0.90–1.40 per bolt assembly delivered, A490 about 1.4× that, F1554 Gr 36 anchor rods $4–9 per linear foot depending on length and finish.