Concept

Ballast — where it appears

The bed of broken stone a railway track sits in, holding the sleepers in place by the interlock between its own particles. That interlock is lost if the deck beneath it accelerates hard enough to fluidise the stone, which is why a high-speed bridge is governed by an acceleration limit rather than by a stress.

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

A basement is a boat

Every load in this collection presses down and is resisted by strength. Hydrostatic uplift presses up, is resisted by weight, and does not care what is built on it — so the check contains no material property at all. It is a ratio of two weights, and one of them is water.

equilibrium · uplift

The train that arrives in time with itself

A single load crossing a span is a mild problem. A train is not one load — its axles are regularly spaced, so the forcing has a frequency of its own, and where a multiple of it lands on the bridge's frequency each coach arrives exactly in step with the motion the last one left behind.

dynamics · moving load resonance

Named alongside it

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

Equilibrium Free body Acceleration limit Buoyancy Control perimeter Damping Drainage Dynamic amplification Effective depth Factor of safety Flotation Forcing frequency

All concepts