Structural Engineering & Statics
where the load goes
Concept

Envelope — where it appears

The outline of the greatest and least values a quantity takes over every arrangement of load considered. No single arrangement produces it, so it is a set of states rather than a state, and checking against it is checking against something the structure never experiences all at once.

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

02468101214161820-200-100100distance along the beam (m)bending moment (kNm, sagging up)the sagging envelopethe hogging envelopeeach case exact to 3e-16 · the envelope out by 23%

The envelope is not a structure

A continuous beam whose imposed load may sit on any span has eight load cases, and every one of them is a genuine state of equilibrium. The curve the design is made against is not one of them — it is assembled from different cases at different stations, and it fails the identity all eight satisfy exactly.

equilibrium · load arrangement wind0.85 m3.4 m of driftbalanced 0.48 kN/m²peak 2.60 kN/m² — 5.4× the balanced load20 m of roof upwindvertical scale exaggerated 4×

The load that arrives where the wind stops

Snow is the one load a structure is given rather than subjected to. What falls is spread evenly over a whole region; what a member carries is whatever the wind left above it, and the wind piles it against whatever gets in the way. The heaviest patch on a roof is usually a quarter of the snow on it.

equilibrium · snow drift

Named alongside it

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

Bending momentContinuityFree bodyLoad arrangementServiceabilityConservation of massDistributed loadDriftEquilibriumFetchImposed loadIndeterminacy

All concepts