Structural Engineering & Statics
where the load goes
Concept

Yield — where it appears

The point at which a material stops returning to its original shape when unloaded and begins to deform permanently. In steel it arrives at a stress the material holds while it stretches, so a yielded bolt or bar keeps a permanent elongation and loses whatever preload or prestress that elongation released.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

Three materials pulled until they stop. Three stress-strain curves — mild steel, high-strength steel, aluminium alloy — plotted to a strain of 2.0%. One of them has a plateau, so the stress at which yielding starts is something the specimen does rather than something anyone chooses. The 0.2% offset construction is drawn on the high-strength steel: a line of slope E from a strain of 0.002, cutting the curve at 460 N/mm².

The stress at which nothing in particular happens

One material in six has a yield point that a specimen actually does something at. For all the others the yield stress is a construction — a line drawn at an arbitrary offset — and every strength calculation for those materials depends on it.

materials · Proof stress The base comes back to a different joint. Moment against rotation at the base of a 500 × 400 × 20 mm base plate with two M24 holding-down bolts in each of two rows, preloaded to 150 kN a bolt, through four cycles to ±170 kN·m. It is stiff until the bed lets go at 57 kN·m, softer past it, and near 170 kN·m the bolts on the tension side yield and stretch for good. Coming back through zero the base is not the base it was: its bolts have kept 83 per cent of their preload, and it lets go at 46.0 kN·m instead of 56.6. After the first full cycle the loops lie on top of one another.

The base that lets go and closes again

A column base whose holding-down bolts are preloaded is rigid until the moment on it lifts the plate off its bed, and then softer. Under a storm the moment goes one way and then the other, many times, and the question is whether the preload survives. It does, entirely, as long as no bolt yields — and on an ordinary base the bolts do not yield until the moment is three times what first lifts the plate. Past that the yielded bolts lose part of their preload once, and the loss stops within a cycle or two: the base settles rather than ratchets. What decides how much is lost is how close to its yield the bolt was tightened, so that past a point a larger preload leaves less after the storm.

connections · Base plate

Named alongside it

The objects these essays reach for when they reach for this one.

Base plateConnectionConstitutive lawCyclic loadingDuctilityElastic modulusIdealisationJoint stiffnessPreloadProof stressShakedownStrain hardening

All concepts