Structural Engineering & Statics
where the load goes
Concept

Soft storey — where it appears

A storey markedly less stiff or less strong than those above it, usually because its walls or infills were omitted for an entrance or parking. It concentrates the whole structure's inelastic drift into one level, and every global measure of the building improves as it does so.

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

Three modes of a five-storey frame. The first three mode shapes of a five-storey shear frame, from the eigenvalue problem rather than sketched. Mode 1 has a period of 0.6 s, no node and carries 88.0% of the mass; Mode 2 has a period of 0.21 s, one node and carries 8.7% of the mass; Mode 3 has a period of 0.13 s, two nodes and carries 2.4% of the mass. The nth mode crosses the axis n−1 times, which is a theorem rather than a drawing convention.

A structure has more than one period

One mass on one spring has one period. A building with eight floors has eight, each with a shape of its own, and the second one bends the building into a curve nobody drew. They are not harmonics, they do not interfere, and each behaves as though the others were not there.

dynamics · Mode shapes Where the drift went. Storey drift at the target displacement, for the same frame with and without a soft ground storey at 50 per cent of the others' stiffness and strength. The regular frame spreads 219 mm over every storey; the soft one reaches 266 mm and puts 5.58 per cent of it into the ground storey against 0.53 next to it — a concentration of 5.9 against 1.7. The roof goes 22 per cent further, and where that extra displacement lands is the whole of the difference between the two buildings.

Weaker in one place, and better on every average

Take an eight-storey frame and make its ground storey half as stiff and half as strong. Its ductility demand falls, its first mode carries more of the mass, and its period lengthens into a gentler part of the spectrum. Three global numbers all improve, and the building is the one that collapses.

dynamics · Pushover

Named alongside it

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

Mode shapeBase shearCapacity curveCollapse mechanismDegrees of freedomDuctility demandEigenvalueModal analysisModal massNatural periodOrthogonalityParticipation factor

All concepts