Structural Engineering & Statics
where the load goes
Concept

Cable net — where it appears

A surface made of cables in two directions, whose stiffness against load is very largely the tension in it rather than the stretch of it.

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

00.20.40.60.811.2020406080100120140160midspan sag (m)total load on the cable (kN)the design load, 150 kNsolved 0.740 m1.125 mtangent here 341.2 kN/mk₀ = 8T₀/L = 133.3 kN/mthe flat-cable law

The stiffness that comes from the shape

A cable has no bending stiffness whatever, and it still holds up a roof. What resists the load is the change of its own geometry, so its stiffness is a function of the tension already in it — and prestress buys stiffness that no change of material could.

structures · cable stiffness a square with both diagonalss = 1 · m = 0Maxwell's count: +1nothing to stiffentwo bars in a straight lines = 1 · m = 1Maxwell's count: +0prestress stiffness +1.000

The forces that are there with nothing applied

Maxwell's count is the difference between two dimensions, and it knows neither of them separately. A frame can satisfy it exactly and still both fold and be prestressable — and when it does, the second of those is what stops the first.

structures · self stress

Named alongside it

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

Geometric stiffnessPrestressCompatibilityDeterminacyEquilibrium matrixFunicularHorizontal thrustInfinitesimal mechanismMaxwell countMechanismNull spaceRank

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