A methodology is provided for evaluating the design quantities necessary for designing membrane elements of concrete shearwalls and shells with multidirectional reinforcement against in-plane forces. The mode of failure-ductile, ductile-brittle, or brittle-is determined, as are the critical loads and associated stresses in the reinforcement and concrete and the extent of cracking. Toward this end, a set of equations, each involving one unknown parameter, is applied iteratively to convergence, which is shown by example to be rapid. The basis for omitting shear at the crack surface and for using continuum strain equations in the cracked concrete matrix is demonstrated. An illustrative example is included.