Optimal design of curved portal frame with consideration of stressed skin action using the equivalent spring theory

Abstract

This article investigates the effect of the roof cover and the stressed skin action on the optimal design of curved portal frames. According to stressed skin effect, diaphragm is carrying part of the horizontal force of structure and the remaining part is carried out by the frames. Therefore, interior frames are designed for lower forces which, in turn, results in a lighter structure. Stressed skin effect is modelled using the equivalent spring theory. A program which links Sap2000 and genetic algorithm was developed that provides the best sections which result in the minimum weight of structure. Designs are performed by using the allowable stress design and stress skin effect. Seven curved portal frames with different span dimensions, lengths, number of frames and column heights are considered and optimally designed for roof cover with thickness of 0.5 mm and 0.7mm. The objective function is defined as the weight of the structure and the problem consists of eight design variables. The results show that consideration of the stressed skin effect results in a reduction of weight by more than 20% and that increasing the height of column results in 10% reduction of the weight. Moreover, it was observed that increasing the length of the span results in a 5% reduction in weight. Effects of increasing the number of frames and changing the length of the structure are negligible and are only about 1 or 2 percent of reduction in weight. Increase in the thickness of the roof cover sheet about 0.2 mm results in a decrease of the weight by about 3%.