In this video tutorial, we learn how to calculate the deflection of a frame or beam using the Principle of Virtual Work. Also known as the Unit Load Method, this basis of this method is two analyze two structures:

(1) Real System - The structure with the design loads applied.
(2) Virtual System - The structure with a single load of magnitude one applied at the location where deflection is being calculated.

We combine the results of these two analyses by balancing out the external and internal work (i.e., strain energy) to compute the deflection of the frame at the location of interest.

In general, we can combine the deflection contributions from axial, shear, and moment demands. In practice, the moment dominates the deflections of most beams. Shear is important only for very deep beams, and axial may be important in structures with slender columns or very large axial loads.