Failure mechanism
Bolted connection failure
A bolted connection fails in whichever of its several modes is weakest, and the bolt is frequently not the weak one: the plate tears out to a free edge, the net section ruptures between holes, an unstiffened end plate bends and pries the bolts, or a slip-critical joint slips because it was never pretensioned. The engineering question is not how many bolts are in the group but whether the joint that was installed is the joint that was analysed, and that is where most of the trouble sits.
How this failure works
The modes are countable, and the design strength of the joint is the smallest of them. A bolt shears across its shank, or across its threads where they fall in the shear plane and the capacity is reduced accordingly. The plate bears against it and crushes locally, tears out to a free edge, or ruptures across the net section between holes. Block shear combines a tension plane and a shear plane into one failure that is easy to miss and often governs in a short bracket. A bolt in tension fails at the thread while the plate around it yields and the washer pulls into the plate. Each mode has its own capacity and its own dependence on geometry, which is why a joint that looks generous in bolt count can still be governed by an end distance nobody checked.
Bearing type and slip-critical are different assumptions about the same detail, and they cannot be mixed. A bearing joint is designed for load after slip: the holes are standard clearance, the load is carried in shear and bearing, and the limit state allows the clearance to be taken up. A slip-critical joint relies on friction between prepared faying surfaces, clamped by a pretension well above the working tension, so that the plies do not move in service — which is what a fatigue-loaded or load-reversing connection requires. Specifying slip resistance and then leaving mill scale, an unblasted plate or the wrong paint between the plies removes the friction the calculation was built on, and nothing done afterwards with a spanner restores it.
Torque is an indirect and unreliable measure of tension. Most of the torque goes into overcoming friction in the threads and under the head, so the tension actually reached depends on lubrication, plating and the condition of the threads and the hole. A generic torque table applied with an uncalibrated wrench produces a joint whose state nobody can describe. The defensible options are a wrench calibrated against the bolts actually supplied, a direct tension indicator under the head, or the turn-of-nut method from a defined snug-tight condition. Whether the joint is snug-tight or pretensioned has to be stated on the drawing, because a snug-tight joint carries shear but develops no friction and has no fatigue resistance.
Two geometric effects are routinely left out of the calculation. Prying adds tension beyond the applied load whenever an unstiffened end plate or tee-stub flexes away from the supporting face, so the bolt carries the direct tension plus a prying force that grows as the plate thins. Group eccentricity is the other: when the line of action of the load does not pass through the centroid of the bolt group, the group carries a moment as well as a direct shear, and the distribution between bolts is not uniform — least of all when the plates are flexible enough that the rigid plate assumption behind the simple distribution does not hold. Oversize and slotted holes make both effects worse: they reduce the bearing area, they need plate washers, and they allow more movement before the bolts pick up load.
Illustration
Conceptual illustration: a bolted lap joint with bolt shear, plate bearing, edge tear-out and net section rupture drawn as four separate failure modes of the same bolt group.
Typical causes
- Bolt grade and hole class not as specified, with a lower class substituted without rechecking bearing and tear-out on the plate.
- A connection designed for direct shear while the real load path brings tension, or combined shear and tension, from an eccentric or inclined member.
- Prying at an unstiffened end plate or tee-stub not allowed for, so the bolts carry considerably more tension than the applied load.
- A slip-critical connection made up over painted, scaled or contaminated faying surfaces, so the friction the design assumed is absent.
- Tightening by an uncalibrated wrench against a generic torque table, with no record of the method or the tension achieved.
- End and edge distances shorter than the design required, so tear-out or block shear governs before the bolt does.
- Holes that did not line up, reamed out or enlarged on site, and then made up with oversized holes without plate washers.
Warning signs
- Rust streaks running from inside a joint, or a nut that turns under a podger spanner.
- A washer cupped under a head, or a bolt head standing off the plate, showing prying or a plate that has already deformed.
- Paint cracked in a line across the faying surface, or a scribed witness line that has shifted, indicating that the joint has slipped.
- Elongated holes, a raised burr at a hole edge, or a head pulled part-way into the plate.
- Mixed bolt lengths or markings within one group, or bolts missing from a group that was designed as a set.
- A plate visibly bowed between bolt rows, or a gap at the faying surface along one edge of the joint.
Inspection methods
- 01Check bolt markings for grade against the bolt schedule, and record what was found rather than that it was checked.
- 02Check hole type and fit, end and edge distances, and whether any hole has been reamed or enlarged since fabrication.
- 03Check the faying surface preparation and paint system in slip-critical joints against the slip factor the design assumed.
- 04Verify pretension by the specified method — calibrated wrench, direct tension indicator or turn-of-nut — and keep the record for the batch.
- 05Inspect unstiffened end plates for flexure and prying, and measure plate thickness against the drawing.
- 06Check the corrosion protection at and around the joint, and the drainage path, because a joint that holds water corrodes from the inside out.
Preventive measures
- Check every mode: bolt shear, plate bearing, tear-out, block shear, net section rupture, combined shear and tension, and prying.
- State the bolt grade, hole type, tightening method, and whether the joint is bearing or slip-critical, on the fabrication drawing.
- Detail against prying — a thicker end plate, a stiffener, or bolts placed closer to the loaded flange.
- Specify the faying surface treatment for slip-critical joints and protect those surfaces until the joint is made up.
- Use a calibrated wrench or a tension-indicating method, sample beyond the minimum, and keep the records with the joint.
- Set end and edge distances from the tear-out and block shear checks rather than from a nominal rule of thumb.
- Keep threads out of the shear plane where the capacity assumes the shank, and provide the washers the design assumed.
Relevant standards
The codes and specifications a design is normally judged against. Naming them is not a claim that AKEN certifies, inspects or approves against them — the design authority for a project does that.
- IS 800 — general steel construction: bolted connection design, end distances and slip resistance.
- IS 4000 — high strength bolted joints, including pretension and the tightening methods.
- IS 2062 — structural steel, including the plate grades the bearing and net section checks depend on.
- AISC 360 — bolt and plate strength, prying and slip-critical provisions, used where a client specifies it.
- Eurocode 3 (EN 1993-1-8) — the European basis for bolted connection design.
Related AKEN services
Scope note
This article explains a failure mechanism for engineering education. It is general guidance, not a design check, an inspection report or a certification for any particular structure. Assessments of an actual structure must be carried out by the responsible design authority against the project’s own specifications.