| The increased number of structurally deficient bridges as well as the limited available funds to rehabilitate or replace these bridges has proved to be among the most difficult dilemmas facing bridge engineers. With the current trend of extending the service life of bridge structures, fatigue and fracture are becoming prominent problems. For steel bridges, one of the most frequently occurring fatigue cracks is due to out-of-plane distortion of the web in the region of the diaphragm connections. Two approaches may help alleviate the bridge dilemmas: (1) applying simple retrofit solutions for bridge defects, and (2) adopting a nondestructive evaluation method that is automated, continuous, and simple to use on bridges.; In the first approach, an experimental evaluation was conducted using a simple retrofit solution for cracking due to the out-of-plane distortion of the web plates in multiple girder bridges with X-type or K-type diaphragms. The retrofit consists of loosening the bolts that connect the diaphragm diagonals to the webs. Local and global responses were measured at various locations in five bridges. Implementing this retrofit resulted in substantial reductions in the diaphragm diagonal forces, and consequently, in the out-of-plane distortion, strains, and stresses in the web at the diaphragm connections. The overall results for steel bridges with X-type diaphragms were favorable. A design check, however, is needed before implementing the retrofit method.; In the second approach, one particular nondestructive evaluation method, modal testing, was evaluated for global evaluation of bridge structures. Experimental modal tests were performed on a typical multiple steel girder bridge using truck excitation to study the sensitivity of the modal parameters to environmental changes, excitation sources, simulated structural damage, and deck rehabilitation. Additionally, the effects of two types of cracking were examined theoretically. The results indicate that while frequencies of vibration are global parameters, mode shapes are local parameters with the mode shape changes being more sensitive to damage than frequency shifts. Both mode shapes and frequencies of vibrations are capable of detecting major damage to the bridge structure. |
