The simple static and dynamic buckling model (the three-hinge rigid-rod system, constrained laterally by a nonlinear spring) originally proposed by Budiansky and Hutchinson, is modified so that the force of the spring includes both quadratic and cubic terms. Expressions are given for the buckling load of the imperfect structure as function of the imperfection. These formulas generalize the classical expressions for the static buckling load (due to Koiter), and for the dynamic buckling load (due to Budiansky and Hutchinson) for symmetric or asymmetric structures, to nonsymmetric ones.