Metals used in old Bridges in Sri Lanka and the Effects of their Material Properties on Capacity Estimations

Most of the metal bridges that were built over 100 years ago are still in use in Sri Lanka. To ensure the safety of these bridges as well as their users, condition assessment of these old structures becomes essential. The details of these bridges such as their mechanical and fatigue properties and the types of materials used in these bridges are very important for assessing their condition. Therefore, this study on metals used in old bridges was carried out using a literature survey and our own experiments. Recommendations were given thereafter for the selection of the mechanical and fatigue properties that are appropriate for assessing the old metal bridges in the country. The recommended values for the ultimate tensile strength of wrought iron and mild steel are in the ranges 284 – 390 N/mm² and 370 – 450 N/mm² respectively; values of yield strength of wrought iron and mild steel are in the ranges 191 – 241 N/mm² and 220 – 280 N/mm² respectively and, mean fatigue strength at 10 million cycles for both wrought iron and mild steel is 190 N/mm². Finally, the importance of selecting appropriate mechanical properties for estimating the current carrying capacity of metal bridges is presented using examples. It is observed that the error in the estimation of the carrying capacity of a bridge resulting from the use of inappropriate mechanical properties could be as high as 33%.