Structural Engineering & Statics
where the load goes
Concept

Second order — where it appears

Analysis carried out on the deformed structure, so that the forces already present change its stiffness. It is what turns a stability question into a strength one, and it is the difference between a frame that is adequate and one that runs away.

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

02040608000.20.40.60.811.21.41.6brace stiffness (units of EI/L³)effective length factor k23.2 EI/L³ = 1.41 N꜀ᵣ/L0.77 — held1.32 — freepast the threshold the frame buckles in a mode the brace does not hold

The most dangerous day is before it is finished

A structure is analysed once, complete, with every restraint present. It spends weeks in states nobody drew — a beam landed with no deck on it holds 17% of the moment its section is worth, a frame not yet braced buckles at a third of the load it will, and a bolt not yet tightened is a pin where the analysis assumed a fixity.

stability · erection stability 050100150200250300350400020406080100120sideways movement (mm)height (m)bendingracking366 mmroof drift 1 in 328 · worst storey 1 in 296

Two motions with one name

A tall building's sway is two movements added. A frame racks like a stack of parallelograms, worst at the bottom; a cantilever bends about its base, worst at the top. The total at roof level says nothing about which storey is worst, and on this building it is neither.

deflection · drift components

Named alongside it

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

Bending stiffnessBracingCantilever actionCladdingConstruction sequenceDeflected shapeDriftEffective lengthErection stabilityJoint stiffnessLateral restraintLateral system

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