The central Puget Sound region is home to Fortune 500 corporations such as Microsoft, Boeing and Starbucks, while serving as a primary gateway for the movement of goods to and from East Asian markets through its world class ports and terminal facilities. The region has only two transportation facilities crossing Lake Washington: I-90 and SR 520 Floating Bridges. The Puget Sound area is faced with a growing population and increased congestion on these key regional links. The Central Puget Sound region has a steadily growing population with an estimated 3.5 million people in 2005 and is projected to grow to over 4.6 million by 2030 with notable growth assumed on the east side of Lake Washington.
For the I-90 Corridor, past studies and regional agreements have identified I-90 as the preferred corridor for high capacity transit, light rail. The I-90 roadway and floating bridges link the City of Seattle with the island community of Mercer Island and communities on the east side of Lake Washington. During an average weekday the I-90 roadway carries approximately 133,000 vehicles per day. It is for these reasons that Sound Transit is preparing the corridor to accommodate high capacity transit in the form of light rail across the I-90 floating bridge.
The Homer Hadley Floating Bridge was designed in the early 1980s. The design for the bridge was supported by the 1976 Memorandum Agreement signed communities and jurisdictions along the I-90 corridor to support the development of high capacity transit in the center roadway, the Homer Hadley Floating Bridge. As part of the bridge design process, the design included analysis of the bridge for light rail (LRT) loads which had different axle spacing for loads than current Sound Transit LRT loading standards. This previous analysis assumed that the center roadway HOV (South side) lanes would be converted to LRT. The effect of the large movement transition span joints at each end of the bridge on the LRT rails were recognized as an issue but were not addressed. It was assumed that rail/expansion joint issue would be addressed during design of LRT on I-90 across Lake Washington.
Governor’s Blue Ribbon Panel, Sinking of I-90 Lacey V. Murrow Bridge
- On November 25, 1990, the old I-90 Lacey V. Murrow Bridge that spanned Lake Washington sunk prompting then-Governor Booth Gardner to investigate what happened and reasons why the bridge sank. The panel reached several conclusions and made recommendations for the operation and maintenance of the (4) four remaining floating bridges in the state.The panel recognized that structures are build to withstand certain natural forces, but that it is not economical with current knowledge to build for extreme conditions. While the Hood Canal Bridge sank under a force in excess of a “100 year” storm, the panel did not question the need for design limits.
- When planning for construction on the bridge, the panel recommended an independent third party review at several milestones of a reconstruction or major maintenance project. The use of outside experts is especially needed for major reconstruction.
- Construction of floating bridges requires marine construction practices that are not the same as road construction practices. Floating bridges should be treated as a marine vessel.
- Construction practices and procedures should be conducted with a mind-set of “whatever it takes to keep the bridge afloat” and take extra precautions to assure this is the case. The watertight status of the bridge must be maintained.
- Establish contractual provisions with help from outside marine construction experts on minimum standards to prevent the flooding of bridges during construction. i.e., limiting major reconstruction and rehabilitation during storm-season months.
Modeled and Simulated Load Tests
Beginning in 2001, studies and tests were initiated to evaluate the effects of LRT on the floating bridge utilizing current Sound Transit LRT loads. These structural feasibility studies performed by the consulting firm, KPFF assessed LRT in the center roadway and adding an HOV lane to the outer westbound roadway. The analysis showed LRT conversion modifications were structurally feasible with weight mitigation measures on the bridge and limitations on track system weight.
In 2005, fully loaded large trucks were run across the Homer Hadley Bridge to simulate an LRT system based on current Sound Transit train and track standards. The bridge was fully instrumented to record pontoon deflections and stresses during the test. The data from the load test demonstrated close correlation to the simpler computer model used in the 2001 Structural Feasibility Studies with minor modifications. LRT loads were combined with original design load combinations like wind, wave, temperature, dead load and pre-stress.
The analysis showed that LRT loading combined with the 1-year storm loads produced stresses that were 97% of the allowable stresses becoming the controlling case for operational limitations of LRT. The allowable stress protects for projected bridge fatigue and does not represent the ultimate factor of safety.
In 2006, Governor Christine Gregoire restated the state’s previous commitment to dedicate the center roadway to light rail or light rail convertible bus rapid transit. During this year, the Sound Transit Board also identified light rail as the preferred mode for high capacity transit across the I-90 Bridge.
During summer and fall 2007, Sound Transit prepared preliminary concept studies for:
- Rail Expansion Joints Across The Transition Spans Joints
- LRT-Induced Vibrations
- Overhead Catenary System (OCS)
- Stray Current Issues (Structures and Utilities)
- Instrumentation of Transition Spans Joints For Current In-Service Motions
Sound Transit intends to expand structural analysis of light rail and mitigation to the Homer Hadley Bridge during the design phase of East Link, following the funding of light rail across Lake Washington from Seattle to Redmond.