Deflections and deformations are vital measures of the performance of a structure. How structures move and deform affects their use and their suitability for different applications. For example, a floor system must be stiff enough so that it does not deflect excessively. Excessive deflections can be perceived by building occupants and make them uncomfortable, even if the floor is otherwise perfectly safe. We must also ensure that structures don't deflect too much laterally when subjected to wind loading, because excessive lateral deflections can actually result in increased overturning moments (the so-called `P-Delta effect'). Such structural deflection limits (vertical and lateral) may be found in design codes from around the world, including the National Building Code of Canada.
This chapter will introduce four different methods that can be used to calculate the deflections for determinate structures: integration of the curvature diagram, use of moment area theorems, the conjugate beam method and use of virtual work.