Abstract
A simplified procedure for the analysis and optimum design of frames with rigid or flexible connections is presented. The procedure is based on the elastic analysis and the allowable stress design of AISC specifications. The analysis procedure is simplified by using the secant stiffness as the average connection stiffness to approximate the connection behaviour in the analysis. A computer program is developed to find the optimum design of flexibly-jointed frames, as well as rigidly-jointed frames. A predictor-corrector scheme is employed to arrive at the optimum design. The predicted design vector is expressed in terms of true member forces by an implicit scheme. Dynamic scaling and a unique process of step size reduction are employed on the predicted design vector to avoid bypassing an optimum during the search procedure. A portal frame numerical example is employed to compare the optimum design of a flexibly-jointed frame to that of the same frame when assumed to be rigidly-jointed. The comparison is accomplished through the graphical presentation of the constraints and the objective function contours of each case in the design space. The safety of approximating the analysis of flexible frames as rigid is discussed. The global optimum solution, which is always safe for rigid or flexible analysis, is also presented. All these points are also addressed when employing a larger frame example.
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