COMPARATIVE ASSESSMENT OF STRUCTURAL RELIABILITY ANALYSIS AND THE PARTIAL SAFETY FACTORS METHOD IN THE DESIGN OF STEEL BEAMS

Abstract

Structural design involves accounting for inherent uncertainties in material qualities and load quantities. Methods of accounting for uncertainties in steel beams design, is the main focus of this study. The objective is to acknowledge the relevance of factors of safety in accounting for uncertainties in structural design; consider the economic advantage of improved design methods, recognise the effects of span variation on samples, and identify the effect of the degree of fixity of end restraints (support conditions). Comparison was made between the traditional partial factors of safety method – where constants were factored into quantities based on experience; and a more recent method known as Structural Reliability Analysis (SRA) - where solving for uncertainties was done from first principles, using theories of probability and statistics. A total of ten (10) sampled steel beams were modeled for this study, in the lenses of Moment Capacity (Mc) as the limit state function for the demand-capacity (or load-capacity) problem. Two software types, namely StaadPro-V8i and Comrel-TI were used for designs, for factors of safety method and structural reliability analysis method respectively. Results show that savings of more than 20% are achievable in unit weight and sectional-area of steel for a moment capacity limit state, in reliability-based designs. Fixed ended beams showed an average saving of 25.9% in unit weight and 26.5% in sectional area, while propped cantilevered beams showed an average saving of 22.1% and 21.9% in unit weight and sectional area respectively. The results further show that, for reliability-based design of the 8m span fixed ended beam, the resultant partial safety factors for the three variables namely loading span and capacity, were approximately 1.2, 1.33 and 0.93 respectively. When the same beam was cantilevered, the resultant partial safety factors for the same variables were approximately 1.18, 1.31 and 0.92 respectively. With material savings of over 20%, the implication of these findings is that, as far as moment capacity is the critical limit state of consideration in a steel design venture, structural reliability analysis method should be preferred over the popular partial safety factors method. It is therefore recommended that this method should be adopted for design, based on the cost saving in materials.