Structural Engineering & Statics
where the load goes
Concept

Racking — where it appears

Shear deformation of a frame panel, in which the members bend in double curvature and a rectangle becomes a parallelogram. Its stiffness is one storey of column bending in series with one bay of beam bending, and a diagonal brace replaces that bending with axial action.

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

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 -15-10-551015020406080across the face (m from the centre)column stress (N/mm²)11 columns at 3.0 mcorner 69.418.2 in the middleplane sectionseffective width 51% · stiffness 72% of gross

The corner columns take more than their share

A framed tube is a hollow cantilever, and a hollow cantilever's flange ought to be uniformly stressed. It is not, and the reason is that the only route the axial force has into a column in the middle of a face is the in-plane shear of the frame — one bay at a time, from the corner inwards.

structures · framed tube

Named alongside it

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

DriftLateral systemStiffnessBending stiffnessBracingCantileverCantilever actionCladdingCorner columnDeflected shapeEffective widthFramed tube

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