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Torshear-type high-strength bolt assemblies: tightening and acceptance essentials for steel structures

2026-09-28

Compiled by JIALI Industrial (164.hk) — a Hong Kong fastener supplier specialising in screws, bolts, nuts and selection know-how.

Why steel structures specify torshear-type bolts — preload accuracy decides the joint

Friction-type high-strength bolted joints carry load through the friction between plies, so preload directly sets joint capacity: under GB 50017 the slip factor and the design pretension P are a matched pair — tighten ten percent short and the joint loses ten percent of capacity, while over-tightening drives the bolt toward yield and accelerates relaxation. The difficulty with ordinary hex-head assemblies is that preload is controlled through torque, and the torque coefficient wanders with thread lubrication, washer rotation and operator technique; site scatter of 15-30% is common, and our August 21 torque reference article covers exactly that method. Torshear-type high-strength bolt assemblies (GB/T 3632, often called TC bolts) build the torque control into the part itself: the spline and break-neck groove at the bolt tail are dimensioned for the target torque, the electric shear wrench loads the bolt until the spline shears off, and within one batch the final-pretension scatter stays inside about ±10%. Whether the spline is gone is visible at a glance, so aerial acceptance never requires bolt-by-bolt torque re-testing — the reason torshear assemblies are the default specification for steel buildings, bridges and towers.

Choosing the assembly: grade, size, pretension and matched-batch supply

A torshear assembly is supplied as a set — bolt, nut and washer — to GB/T 3632, in grades 10.9S and 8.8S, with bolt steels such as 20MnTiB and 35VB, covering M16-M30. Design pretension follows GB 50017: for 10.9S, M16 / M20 / M22 / M24 take 100 / 155 / 190 / 225 kN respectively, and 8.8S takes 80 / 125 / 150 / 175 kN; the mean fastening force from batch retest should fall inside the GB/T 3632 band of roughly 0.9-1.1 times the design value. The single most important purchasing rule is matched sets from one batch: axial-force behaviour depends on the combined surface treatment and lubrication of all three components, which the mill guarantees through phosphate plus soap processes — mixing batches breaks the calibration, and drift of up to twenty percent is realistic. Order assemblies priced and delivered as complete sets under one batch number, and demand the batch fastening-force retest report. Against the hex-head system (GB/T 1231), the torque-coefficient mean must sit between 0.110 and 0.150 with a standard deviation no greater than 0.010; the two systems are never cross-ordered — when the drawing says 10.9S torshear, quote to GB/T 3632 as a set.

Tightening and acceptance: initial pass, final pass and the spline check

Work to GB 50205: on arrival, retest the assemblies per batch — torshear type takes eight sets for the fastening-force test — before any installation. Temp-bolt the joint, square up the faying surfaces, then swap in the high-strength bolts and tighten in two steps: an initial pass at about half the final pretension, a re-pass on large joints, then the final pass with the dedicated electric wrench shearing the spline in one continuous pull — the break marks that the design pretension is reached. Never substitute a hand wrench with a cheater bar. Where site access makes the shear wrench unusable for a few bolts, finish those by the hex-head torque method with a written record for each bolt; none may be left untouched. Accept on three checks: every spline sheared off with a clean, square fracture; washers unworn and 2-3 threads proud of the nut; and tightening records matching the retest report batch numbers. Storage decides the outcome too: issue assemblies against a daily limit, use them the same day, keep washers and threads clean and unbumped — damp storage changes lubrication and pulls the axial force off target. Need matched torshear assemblies, retest reports or substitute selections? Contact JIALI Industrial (164.hk) with sizes and joint conditions for a complete quotation.

Torshear-type high-strength bolt assemblies: tightening and acceptance essentials for steel structures

Why steel structures specify torshear-type bolts — preload accuracy decides the joint

Choosing the assembly: grade, size, pretension and matched-batch supply

Tightening and acceptance: initial pass, final pass and the spline check

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