Abstract:
The design of structurally adequate buildings at minimum cost involves considering several design alternatives. However, due to limitation of appropriate software, engineers typically generate and analyse very few design alternatives during the conceptual stage of a project. The vast majority of Computer-Aided Design (CAD) tools available do not allow structures to be represented parametrically. Hence, many structural designs are done without comprehensive consideration for achieving optimum design. To achieve minimum mass optimization, mathematical model was developed and subjected to British Standard (BS 5950) code requirements for structural integrity as constraints. Visual basic application (VBA) codes were written into a spreadsheet environment to implement the model for four (4) case studies. 1) For a given span b, frame spacing S, and height from eaves to apex h, the height from ground to eaves H was varied and corresponding masses of steel for purlin, rafter and stanchion were established. 2) For a given span b, frame spacing S and height from ground to eaves H, the height from eaves to apex h was varied by varying the slope of the rafters. The corresponding masses of steel for purlin, rafter and stanchion were established. 3) For a given span b, heights H and h, the frame spacing S was varied and corresponding masses of purlin, rafter and stanchion were determined and 4) For a given height to eaves H, height from eaves to apex h and optimal spacing S of 6.1m, span b was varied and corresponding masses of purlin, rafter and stanchion were determined. The minimum masses of steel for a fixed plan area of the buildings were obtained for each of the four scenarios. The research work established relationship between heights (height to eaves and height from eaves to apex), steel frame spacing and mass of framework steel of fixed feet and pin feet single span single storey portal frame buildings. The best parameter ratio of height to length to breadth obtained is 1:1:1. Pin feet frames were found to have smaller masses of steel than fixed feet frames as breadth (span) of the portal frames increase while fixed feet frames were smaller in masses than the pin feet frames as height of the portal frames increased. It is recommended that design engineers should consider varying major frame parameters such as frame spacing and heights at pre-design stages in order to obtain optimal values of parameters which will ensure economical structures.
