Civil Engineering July 2022 | Vol 30 No 6

28 July 2022 Civil Engineering F or the implementation of the load model in a design code it would be necessary to calibrate the corre- sponding partial factors for different limit states. Large variation currently exists when it comes to the level of reliability in codes across the world and there is a disconnect between the target reliability reference period and design life. This paper approaches the problem from a lifetime reliability perspective, rather than tying reliability indexes to ref- erence periods. This is done in accordance with ISO 2394:2015. The paper addresses model uncertainty from the literature (fib Bulletin 80) and a method is introduced to determine uncertainty in the fitting of distributions to load effects. Comments are made on the validity of the assumed First Order Reliability Method (FORM) sensitivity factors in the literature. A TRAFFIC LOAD MODEL FOR SA As it is not the focus of this paper, the proposed new traffic load model for South Africa is merely summarised here. The single lane model, developed by Van der Spuy and Lenner (2018, 2019) and Van der Spuy (2020) is shown in Figure 1 and consists of a triple axle configuration of 160 kN each with a uniformly distributed load (UDL) of 13 kPa for a 3 m wide lane. This is a characteristic load model based on a 5% probability of exceedance in 50 years, or otherwise a return period of 975 years. This is identical to the values used in EN 1990 (Holicky 2009) and is also the approach of the South African building codes (SABS 2011). It is impor- tant to note that the characteristic values are not tied to target reliability. PARTIAL FACTOR CALIBRATION The design format followed in this work is the semi-probabilistic format. For the imple- mentation of the load model in a design code it is necessary to calibrate partial factors (PFs) for serviceability limit state (SLS) and ultimate limit state (ULS). The characteristic load effects are multiplied by the PFs for SLS and ULS to determine the design load ef- fects. This section provides a brief summary of structural reliability and discusses the derivation of PFs for the live load model. THE RELIABILITY INDEX If the load on a structure is denoted as E, and the resistance as R, then failure will occur when E exceeds R , or E > R. For a probabilistic analysis, both E and R are Prof. Pierre van der Spuy Pr Eng Associate: Zutari Adjunct Associate Professor: Stellenbosch University pierre.vanderspuy@zutari.com pierrevds@sun.ac.za Reliability calibration of a bridge traffic load model A proposed new traffic load model has been derived for bridge design in South Africa, taking into account two important observations which should be considered in the reliability calibration of design codes. 160 kN 40 kN/m 160 kN 160 kN 3 m 2 m 13 kPa 1.2 m Figure 1 New single lane traffic load model for South Africa