Reliability performance of bridges designed according to TMH7 NA load model

Abstract

The current code of practice for the design of highway bridges in South Africa is called Technical Methods for Highways 7 (TMH7). It was first published in 1981 and is largely based on the loading provisions of the BS5400. The purpose of this study is to carry out an investigation into the reliability performance of new highway bridges designed according to TMH7. Past studies have identified deficiencies in the traffic load model. A case study considering isolated traffic load effects and sagging moment on a simply supported bridge indicates that NA loading generally performs well for spans ranging from 15 to 50 metres. However, a poor reliability performance is seen for short-span narrow bridges, especially for 5 m and 10 m spans where the number of traffic lanes is equal to the number of notional lanes for NA loading. To study the reliability performance of a critical element, flexural behaviour of a 20 m twin-spine simply supported deck is then investigated. The results agree with the first case study. It is, however, recommended to verify the presented findings by investigation based on data from other heavy-freight routes and to account for continuous structures and shear load effects.

Target reliability for new road bridges in South Africa

“…A CoV of V R = 0.175 is used here for the structural resistance, which includes provision for model uncertainty. Lenner et al (2021) and Van der Spuy (2020) concluded that the load effect of traffic loading in SA is best modelled by a censored generalised extreme value distribution. In previous research by Steenbergen et al (2018) and Fischer et al (2019), however, it was found that the combined load effect of permanent (normal distribution) and variable loading (extreme value distribution) was well represented by a lognormal distribution for V R , V S < 0.3.…”

“…This is due to the fact that at the time of compilation of TMH7, circa 1980, the use of reliability as a basis for structural design was still in its infancy. Recent research (Lenner et al 2021;Van der Spuy 2020) indicates that the currently achieved levels of reliability from TMH7 may not be in line with those from SANS 10160-1. Irrespective, TMH7 is now outdated and needs considerable revision or replacement.…”