Abstract
A design-based optimisation for pre-engineered steel frames comprising built-up sections is presented in this paper and the effect of unbraced length ratios on flange and web slenderness ratios is elaborated. A wide range of built-up sections was selected with different unbraced length ratios, web slenderness ratios and flange slenderness ratios. Web slenderness and flange slenderness were varied from a fairly compact range to highly slender values. The capacity of the design sections was determined using different design codes under the combined effect of compression and bending forces. The optimised values for flange and web slenderness were determined for each unbraced length ratio and a relationship between unbraced length and optimum flange slenderness values was established. Analysis of the results revealed that the use of appropriate values of flange and web slenderness in accordance with the member unbraced length can significantly reduce the section weight and improves its efficiency.