Structural Engineering & Statics
where the load goes
Concept

Ductility demand — where it appears

How far a member must travel past its yield displacement to absorb what a dynamic event delivers, expressed as a multiple of that yield displacement. It is the quantity a blast or an earthquake actually asks for, and it is met by detailing for deformation rather than by adding strength.

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

0123400.20.40.60.81period (s)spectral acceleration (g)fixed baseisolatedbase shear ÷ 10.20123400.050.10.150.20.25period (s)spectral displacement (m)fixed baseisolatedmovement × 2.5five per cent damped, faint; 20 per cent damped, bold

Made weaker on purpose

Everything else in this collection resists a load by being stiff or strong enough for it. A base-isolated building resists an earthquake by refusing to hear it — a layer of bearings under the whole structure with a lateral stiffness a twentieth of the frame's, bought with almost no strength at all.

dynamics · base isolation 00.511.522.500.511.52load duration ÷ natural periodpeak ÷ static deflectiontwice the static answerthe static answera load that rises linearly, then staysa rectangular pulse, then nothing

The load that is over before it has moved

A blast delivers an enormous pressure for a few milliseconds. Everything else in this field asks what force a structure can carry; a load that has come and gone before the structure has travelled any distance is not asking that question, and the answer turns out to depend on the mass and the ductility with the strength barely in it.

dynamics · blast

Named alongside it

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

DampingHysteresisNatural periodBase isolationBlastBound theoremsDynamic amplificationEnergyEnergy dissipationFree bodyImpulseLoad path

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