Concept

Stability — where it appears

Whether a disturbed structure returns to where it was, which is a question about the shape of its equilibrium path rather than its strength.

Named by 3 essays across one field — each of them below, with the objects they name alongside it.

050100150200250300350-150-100-50050100150movement of the apex (mm)load (kN)limit point: 133.4 kNit jumps 260 mmas builtat the limitafter it goes

The roof that jumps

Every stability failure in this collection so far has been a bifurcation — a straight thing discovering it can be bent. A shallow frame does something else entirely. It stays perfectly symmetric, deforms steadily, and at some point the load it can carry starts to fall while it is still moving in the direction it was pushed.

stability · snap through
stable symmetric — a columnan imperfection is a nuisancecritical89%unstable symmetric — a shellan imperfection is a demolitioncritical77%asymmetric — a frameand it matters which waycritical

A third of what the theory promised

A column with a small crookedness reaches almost its full Euler load. A cylinder with the same relative crookedness reaches a third of its classical one, and the theory is not wrong — what separates them is the slope of the path just past the critical load, which no calculation of the critical load itself can see.

stability · imperfection sensitivity
stiffeners at 1000 mmthe band at 22.5°the truss it has becomestiffener in compression,web in tensionbuckles at 383 kN · carries 696 kN · a stocky web would reach 953 kNσ in the band 252 N/mm² over 541 mm

The panel that carries more after it has failed

Everywhere else in this field a critical load is where the argument ends. A thin web is the exception — it buckles visibly, in waves anybody can see, and then goes on to carry nearly twice as much again by turning itself into a truss nobody drew.

stability · tension field

Named alongside it

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

BifurcationBucklingEquilibrium pathPost bucklingCritical loadEfficiencyGeometric nonlinearityImperfection sensitivityKnockdown factorLimit pointLoad pathLocal buckling

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