Bolt Shear Capacity Calculator

Bolt shear capacity in AISC 360 is deliberately expressed as a fraction of the bolt's ultimate tensile strength acting on the nominal, unreduced shank area: Rn = Fnv times Ab. The fraction absorbs everything the simple model ignores. For a shear plane passing through the threads, Fnv is 0.450 times Fu; for a plane through the plain shank it is 0.563 times Fu. The roughly 20 percent penalty reflects loss of area at the thread roots, since the tensile stress area of a UNC bolt is only about 75 to 80 percent of nominal. Both coefficients also carry a further reduction of about 0.9 that accounts for uneven force distribution along a line of bolts in a long joint. For A325 bolts with Fu of 120 ksi this gives Fnv of 54 ksi threads included and about 68 ksi excluded; A490 at 150 ksi gives about 68 and 84 ksi. Two safety formats wrap the same nominal number. LRFD applies a resistance factor phi of 0.75 against factored loads, while ASD divides by a safety factor omega of 2.00 against unfactored service loads. Doubling the number of shear planes doubles capacity, which is why a gusset plate sandwiched between two angles is so efficient. The important limitation is scope: this is a shear check only. Bearing and tearout at the hole, block shear through the connected material, bolt tension under prying action, and slip in slip-critical joints are separate limit states under AISC 360 Chapter J, and any one of them can govern long before the bolt itself shears.

Structural steel connections were riveted for most of a century, and shear was checked against the driven rivet filling the hole. Interest in bolting rose after Great Britain's Steel Structures Research Committee sponsored work in the 1930s, including the demonstration by C. Batho and E. H. Bateman that high strength bolts tightened to a high preload could clamp a joint tightly enough to transfer load by friction rather than by bearing. Postwar America turned that finding into practice quickly. The Research Council on Riveted and Bolted Structural Joints was formed in 1947, ASTM issued the A325 high strength bolt specification in 1949, and the council published its first specification for bolted joints in 1951. Field bolting was faster, quieter, and safer than heating and driving rivets, and by the mid 1960s riveting had essentially disappeared from American building construction. The A490 grade followed for higher strength alloy steel bolts. The nominal shear stress coefficients used today were calibrated against extensive shear test programs, much of it at the Fritz Engineering Laboratory at Lehigh University, and summarized in the Guide to Design Criteria for Bolted and Riveted Joints by Fisher, Kulak, and Struik. AISC restructured the checks into load and resistance factor format with its 1986 LRFD Specification, and the 2005 unification merged the LRFD and ASD approaches into the single Chapter J used today.