A zero curvature method to estimate the limit load is introduced in detail, and a uniform computing process for computerization of the zero curvature method is presented. A new protection criterion against plastic collapse is proposed. A lot of examples are performed and discussed. The zero curvature load determined by the zero curvature method is a preferable definition of the limit load for perfectly plastic materials or the plastic load for strain strengthening materials. Both advantages of the ASME VIII-2 code and the EN-13445 standard are absorbed in the new protection criterion.