Structural Engineering

Statically Indeterminate

Describes a structure with more unknown reactions or member forces than the equations of equilibrium can find.

A plane structure gives three equilibrium equations per free body. When it has more unknowns than that, it is statically indeterminate, and the excess is its degree of indeterminacy. A propped cantilever, a fixed-fixed beam and a continuous beam over three supports are all indeterminate to the first or second degree.

The extra equations come from compatibility: the deflected shape must fit the supports. The simplest method removes the redundant support, computes the deflection there, and finds the force that cancels it. Indeterminate structures redistribute load and are stiffer, but settlement and temperature change induce stresses in them.

How much of the FE Civil exam is Structural Engineering?