Abstract A method to find the minimum weight design of steel trusses under multiple loading conditions and subject to stress and side constraints is presented. The problem is first simplified by replacing the ‘separable’ nonlinear stress constraints by their polygonal approximations. A gradient method for problems with separable functions is then used to find the minimum weight design. In each iteration, the direction vector is found by the simplex method with a restricted basis entry procedure, while the step size is found such that the constraints are satisfied and the objective function is minimized. The efficiency of the proposed method is illustrated by optimizing the weight of a classical three-bar truss.