ABSTRACT
Fracture mechanics can be applied to assess cracked bridge members. If crack length and stresses are known the crack driving force (stress intensity factor, K) can be calculated. K was calculated for flange splices and hot-rolled beams, as a function of crack length. In both cases K eventually becomes negative indicating no further crack propagation. However a cracked girder will become compliant and “shed” load to uncracked neighboring members. Our calculations show that the changes in both compliance and load-carrying capacity of the cracked girder are small until the girder is deeply cracked. A finite-element analysis of a cracked girder showed that by determining the bending stresses about one beam depth from the crack it is possible to determine K. Measurement of these stresses was simulated in a field test. The method used small changes in sound speed to determine stress. The ultrasonic transducers used required no couplants and no surface preparation. They were also used to measure liveload stresses for fatigue monitoring and for determining stresses in an integral backwall bridge.
