Bolt Shear (Lap Joint)
Shear capacity of a bolted lap joint per AISC J3 — LRFD and ASD strengths plus utilization against your applied load.
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The engineering
Bolt shear is one of the three limit states you check at every bolted lap joint (the others being bearing/tearout on the plate and net-section rupture). AISC 360 Section J3.6 gives the nominal strength as the nominal shear stress Fnv times the gross bolt area, per shear plane, per bolt. A lap joint puts each bolt in single shear; a symmetric butt splice doubles up with two planes per bolt.
The Fnv values in Table J3.2 already bake in the threads question — no separate root-area reduction. Group A (A325) bolts: 372 MPa (54 ksi) with threads in the shear plane (N), 457 MPa (68 ksi) threads excluded (X). Group B (A490): 457 MPa (68 ksi) N, 579 MPa (84 ksi) X. Sanity check: a single 3/4" A325-N bolt in single shear should come out near φRn ≈ 17.9 kips (79.5 kN) — the value straight out of AISC Manual Table 7-1.
This card checks bolt shear only. For joints longer than about 38 in. between end fasteners, Fnv takes a further reduction, and bearing or tearout on thin plate frequently governs before the bolt does — run those checks separately.
Where this math comes from
Structural steel was riveted for a century — hot rivets driven by four-man gangs, hammered while glowing. The case for bolts started in 1934 when C. Batho and E. H. Bateman reported to the UK Steel Structures Research Committee that high-strength bolts, torqued hard, could clamp plates tightly enough to replace rivets. American practice caught up after the war: in 1947 the Research Council on Riveted and Bolted Structural Joints formed to put numbers on it, and its 1951 specification made the blunt, field-changing claim that one A325 bolt could substitute one-for-one for a rivet of the same diameter.
Two workers with wrenches replaced a four-man rivet gang, and by the time the Mackinac Bridge went up in 1957 the transition was effectively over. The φ = 0.75 resistance factor on this card arrived with AISC's first LRFD specification in 1986, calibrated against decades of lap-joint shear tests; the Fnv values are the current Table J3.2 numbers carried in AISC 360-16, the specification behind the 15th edition Manual.
- 1934C. Batho & E. H. BatemanReport to the UK Steel Structures Research Committee that high-strength bolts can develop clamping force equal to rivets.
- 1947RCSCResearch Council on Riveted and Bolted Structural Joints founded to standardize bolted connection design.
- 1951RCSCFirst specification permits A325 bolts to replace rivets one-for-one.
- 1986AISCFirst LRFD specification sets φ = 0.75 for bolt shear.
- 2016AISCAISC 360-16 Table J3.2 (15th ed. Manual) pins the Fnv values this card uses.
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