CTLGroup was contracted to design, install, and operate a structural monitoring system to measure expansion joint movement on the Homer Hadley Bridge, as part of the preliminary design phase of the Sound Transit East Link Expansion project. The Homer Hadley Bridge carries Interstate 90 across Lake Washington linking Mercer Island and Seattle. The bridge is a floating structure consisting of prestressed concrete pontoons connected at articulated joints. The purpose of the monitoring system was to facilitate analysis of movement at the selected joints, which was an important consideration in the design of the light rail transit track system that was to be constructed in the existing high occupancy vehicle lanes of the Homer Hadley Bridge.
CTLGroup’s monitoring system consisted of displacement and tilt sensors at selected bridge joints. The sensors were used to measure relative longitudinal displacement and horizontal and vertical rotation of the expansion joints at the ends of the transition steel spans. A weather station also provided temperature, wind speed and direction to correlate with joint response over the monitoring period. An integrated wireless data acquisition system was used to collect and transmit data through cellular modems to a server at CTLGroup. The data were made available to key stakeholders through a password protected website. Bridge movement and weather data were communicated and reported continuously between 2008 and 2011.
In a later phase of the project, CTLGroup was re-engaged in 2014 to recover additional data from the monitoring system, which had continued to operate after the initial three-year study. Our scope during this phase involved the statistical characterization and analysis of the joint movement and weather data to facilitate future operational decision making for the Homer Hadley Bridge. The work was a first step toward evaluating the usefulness of the present monitoring system for the purposes of determining when to close or restrict traffic on the floating bridge during bad weather.