HEALTH AND RESIDUAL LIFE ASSESSMENT OF STEEL BRIDGES

Abstract

Presently there are more than thousands of bridges in India which were built more than 100 years ago. Now the need is felt to check the safety of these bridges for current loading conditions as well as to predict the remaining fatigue life of these very old bridges by bridge engineers all over the world. To accomplish this task standard procedures are followed in developed countries. If any old steel bridge where corrosion is not a problem fails it is not due to its inadequacy in load carrying capacity but its consumed fatigue life after which formation and propagation of fatigue cracks takes place. Fatigue is a problem where live loading forms major part of total loading such as railway steel bridges. In this report study of the truss type bridges and their member connections in respect with their load carrying capacity and endurance to fatigue loading is made. The critical parts of the bridge which are most likely to be damaged due to fatigue loading are identified. A general procedure to assess the present health and remaining fatigue life of the bridge is presented. The important factors that affect the fatigue resistance of a connection or member are discussed Load rating of bridges is a procedure which enables engineer to take into account all the factors which are otherwise not possible. Checking the safety of the bridge for higher than permitted loadings requires a mathematical model to be calibrated which simulates the actual field readings. The procedure of load rating and model calibration is explained for selected truss bridge. Visual inspection of the strain history data obtained from actual load rating of bridge gives great insight and qualitative information about the structure, those observations are listed. The Effects of member end.conditions and its deterioration on the forces in truss members are studied. A 60 m span steel truss bridge constructed about 100 years ago is selected for residual life assessment. The mathematical modelling is done in STAAD.pro package. Standard allowable truck loadings are assumed and applied as moving loads over the bridge model. The stress histories are plotted for various members of the main truss bridge. The obtained stress histories are decomposed into number of stress ranges using Rain-flow counting method. The plot of number of stress cycles and stress range called stress histogram is plotted. The Palmgren Miner's linear damage rule is used to calculate the damage due to single passage of vehicle and then depending upon frequency total accumulated damage in a member in a day is calculated. The member or detail is supposed to have consumed all its fatigue life when Miner's summation equals unity.