Explain the unit load method for calculating deflections.

The unit load method (virtual work method) calculates deflection by applying a virtual unit load at the point where deflection is needed. Deflection = integral of (M * m / EI) dx over the structure, where M is moment due to actual loads and m is moment due to unit load. For trusses: deflection = sum of (F * f * L / AE), where F is actual force and f is force due to unit load. This energy method works for any determinate or indeterminate structure.