Structural Engineering & Statics
where the load goes
Concept

Classification — where it appears

Sorting a member into a category that decides which method of analysis may be used on it. It replaces a continuum by a small number of cases, which is convenient at the boundaries and misleading just inside them.

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 What the joint does to the beam. End moment as a fraction of the fixed-end value wL²/12, against the joint's rotational stiffness, for a beam of EI/L = 14000. At the rigid boundary of 112000 kN·m/rad the joint delivers 80% of it and at the pinned boundary 20%. Everything between the two lines is a redistribution nobody chose and every analysis assumed away.

The redistribution nobody chose

A beam designed as simply supported, on connections that are not pins, has end moments the analysis never predicted and a mid-span moment smaller than it was sized for. Usually that is safe. It is never intentional, and there is one direction in which it is not safe at all.

connections · Joint classification

Named alongside it

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

ConnectionJoint stiffnessMoment redistributionSemi-rigidBeam-columnDeflectionFlexural rigidityMoment rotationRotational springServiceability

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