Fully-Stressed Design for Alternative Loads

The fully-stressed method is used to optimize structures whose members are subjected to bending, such as beams and frames, under the action of alternative load systems. In a fully-stressed design, at every section the section modulus is chosen so that the maximum allowable stress is reached under at least one of the alternative load systems. For statically indeterminate structures, the technique adopted for achieving the design criterion is iterative in nature. The forces and moments in the structure are adjusted to satisfy the conditions of equilibrium between the external and internal forces and at the same time satisfy fully-stressed condition. A mathematical statement of the design problem is presented for the case of a two-span indeterminate beam under alternative load systems. A sample problem illustrates the application of the method to obtain an optimum design of the indeterminate beam. Also, the procedure is outlined for obtaining a fully-stressed design of an indeterminate portal frame.