Force Method for Indeterminate Structures - Intro to Structural Analysis

Learn how to calculate the reaction forces for indeterminate structures using the Force Method (sometimes called the flexibility method).

The force method is an intuitive way of computing reactions for systems with low degrees of indeterminacy. Supports are removed until the resulting structure is determinate, and then forces are applied at the removed supports to ensure that the displacements at those locations are zero, thereby satisfying the original constraints. These external forces are equivalent to the reaction forces.

For the two examples conducted here, the necessary displacements are available from common deflection tables. However, for more complex problems, the setup for the force method lends itself nicely to using the principle of virtual work (PVW) to find all the displacements. See here for how to compute displacements using PVW:
https://youtu.be/JAD-8ATyu6k