Structural Engineering & Statics
where the load goes
Concept

Rotational spring — where it appears

A restraint that supplies a moment proportional to the rotation across it, which is how a real connection is represented in an analysis. Its stiffness rather than its strength is what decides how the frame's moments are shared.

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

Moment against rotation, for three real joints. Three connections on one plot, with the classification boundaries for a beam of EI/L = 14000 drawn as rays through the origin. web cleats is pinned, flush end plate is semi-rigid, extended end plate is semi-rigid. The boundaries are multiples of EI/L, so the same joint is rigid on a short stiff beam and semi-rigid on a long slender one.

Neither pinned nor rigid, which is every real connection

Frame analysis offers two options for a joint and reality supplies a continuum between them. Worse, the boundaries are not properties of the connection at all — the same end plate is rigid on a short stiff beam and semi-rigid on a long slender one.

connections · Joint stiffness A joint is springs in series. The five components of an end-plate joint, with each bar the flexibility it contributes. The column flange in bending is 39.62% of the total on its own, and doubling its stiffness raises the joint's by a factor of 1.25 — while doubling the stiffest component buys 1.05. The joint's rotational stiffness is 25227.71 kN·m per radian.

A joint made of springs in series

A connection's stiffness is not a property of the connection. It is the series combination of the flexibilities of everything the force passes through — so the softest component decides, and stiffening any of the others changes almost nothing.

connections · Component method

Named alongside it

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

ConnectionJoint stiffnessClassificationColumn webComponent methodFlexural rigidityLever armMoment redistributionMoment rotationPryingSemi-rigidSeries stiffness

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