PROBABILISTIC VERIFICATION OF THE PARTIAL SAFETY FACTORS IN THE DESIGN OF REINFORCED CONCRETE STRUCTURAL ELEMENTS

Abstract

Reliability is defined as the probability that a system or component will perform its intended functions under certain conditions for a specified period of time. In this study, reliability analysis of reinforced concrete solid slabs and rectangular beams was carried out. Solid slabs and beams with different boundary conditions were analysed and live loads of 4 kN/m2 and 10 kN/m were assumed for slabs and beams respectively. Analysis was carried out based on the ultimate limit state design according to BS8110 (1997). Limit state functions were generated for each of the structural elements with span, live and dead loads, characteristic strength of concrete, and cross-sectional geometries treated as random variables. The analysis was carried out using the First Order Reliability Method (FORM) incorporated into a coded algorithm (CalREl), which is suitable for computing multidimensional integral of failure surfaces. The partial safety factors for live and dead loads (αG and αQ) and resistance (γR) were varied from a lower limit (αGl, αQl and γRl) to upper limit (αGu, αQu and γRL). The reliability indices obtained for each structural element were plotted against varying load ratio and the results show that the reliability index decreases with increasing load ratio and increasing partial factor of safety. For a continuous beam, the middle span appears safer than the end spans. With an increase in the partial safety factor to the upper limit the reliability index becomes higher than the assumed target reliability index βT. An assumed target reliability index βT of 3.0 was used and all reliability indices were compared to the target reliability index, the findings show that only the two way slab adequate satisfies the required level of safety with maximum load ratio of 1.0. It was therefore recommended that before the structural use of a reinforced concrete structure will be changed a complete risk based appraisal of such structure must be carried out to determine the level of risk involved.