Modular expansion joints are commonly used on bridges with potential movements larger than approximately 5 inches. Single support bar modular systems with 48 inches of movement capability were used for the third Lake Washington Floating Bridge. Within 18 months after the bridge was opened to traffic, cracks were noted in the centerbeams of these large, modular systems. Additional cracks have occurred.
This research program was a study into the causes of the observed cracking. The work was divided into two tasks. The first task was a literature review and evaluation of existing methods for fatigue design of modular joint systems. The second task consisted of a wide range of finite element analyses of the particular joint, and correlation of the computed results in existing design models and observed behavior.
The results show that the cracking has been caused by fatigue due to repeated wheel loading. However, existing design methods do not appear to be reliable indicators of the fatigue behavior because the behavior is influenced by the stiffness and dynamic response of the individual joint system. The variable span lengths complicate the evaluation process. The work shows that there is no reliable information for the wheel load spectrum for U.S. traffic on joints of this type. However, extension of past behavior of this joint indicates that centerbeams of the large joint will require replacement before its expected design life of 25 to 30 years has been achieved.