Credit: Photo: KPFF

Seattle's Interstate 90 floating bridge underwent a full-scale load test to see if it could support the weight of a light rail transit line. The test was initiated by Sound Transit and was carried out by WSDOT and KPFF.

Credit: Photo: KPFF

Eight flatbed trucks were used in the full-scale load test on the 17-year-old floating bridge. The test concluded that the bridge could withstand the addition of a light rail transit line.

The short schedule and limited window of opportunity demanded close and proactive coordination between all team members, from the public agencies' project managers and the field technicians responsible for installing instrumentation to the trucking company responsible for preparing the test vehicles. The entire team was dedicated to the project for the 3½ months before the test.


Some of the methods used to ensure necessary communication and interaction included regular team meetings, sidebar meetings, site visits, test area reconnaissance, clearly defined and tracked schedule and milestones, and a clear definition of lines of communication between the consultant team and key stakeholders. Stakeholder involvement and buy-in was required at every stage of planning. Practice runs with the test vehicles were performed in advance to identify flaws in the proposed test procedure.

The result of the planning stage was a step-by-step test script that was strictly followed during performance of the test. A meeting of all participants was held two days before the test to walk through each step of the script. The test script included backup plans in case of equipment failure as well as plans to take advantage of any time remaining for additional testing beyond the basic project scope.

The test was executed smoothly over two nights during the targeted weekend in September. Because the script was completed ahead of schedule, additional tests were performed that were above the basic scope of work. The time and energy spent preparing for the test led to an efficiently operating team of individuals from both the public and private sector.

The communication level between the consultant team, WSDOT, and Sound Transit before and during the test continued during the report writing phase. After a process of review and comment on draft copies of the report, the final report was issued in January 2006. Test results showed close correlation between computer-predicted bridge response and measured response. The main conclusion drawn was that the floating bridge structure could structurally carry the operational loading from Sound Transit's light rail train system.

The successful execution of this project under the constraints of a demanding schedule and a highly technical procedure is a direct result of the ability of all components of the project team to work together. Failure of only one component could have led to disastrous results and money lost. The results of the test program provided WSDOT with the information needed to feel comfortable about adding a new mode of transportation to the unique structure and provided Sound Transit with a critical milestone to achieving their long-range goals for transportation in the Puget Sound region.

— Kuebler is an associate with KPFF Consulting Engineers, and Clarke is a manager with the WSDOT Bridge & Structures Office.

It takes a team

Several public agencies and private companies joined together to execute the full-scale load test on Seattle's Interstate 90 floating bridge.

Regional public transportation authority Sound Transit funded WSDOT to perform the load test. WSDOT in turn contracted Seattle consulting engineering firm KPFF to lead a consultant team to develop, plan, coordinate, and manage the test program as well as report test results. In addition to KPFF, the consultant team included Construction Technology Laboratories of Skokie, Ill., to provide and install bridge instrumentation and Shaughnessy & Co. of Auburn, Wash., to provide and operate test vehicles and equipment. Other entities critical to the project team included WSDOT's survey group to provide GPS instrumentation, WSDOT's traffic control and bridge maintenance groups, and public relations and media staff from WSDOT and Sound Transit.