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I-90 Floating Bridge


I-90 Floating Bridge. By: Gyan Sinha Darton Riely-Gibbons Betsy Pickrell. Bridge Information. Also known as Lacey V. Murrow Memorial Bridge The bridge was the brainchild of George Lightfoot The second longest floating bridge at 6,620 ft.

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I-90 Floating Bridge

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I-90 Floating Bridge

By: Gyan Sinha

Darton Riely-Gibbons

Betsy Pickrell


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Bridge Information

  • Also known as Lacey V. Murrow Memorial Bridge

  • The bridge was the brainchild of George Lightfoot

  • The second longest floating bridge at 6,620 ft.

  • Crosses lake Washington from Seattle to Mercer Island.

  • Made of reconstructed concrete and floats on Pontoons.

  • Bridge sank on Nov 25, 1990; over Thanksgiving Break.

  • Designed by the engineer Homer Hadley in 1940

  • At the time, The Seattle Times called it "the biggest thing afloat”


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What is floating bridge?

  • A bridge which floats on pontoons on the surface of the water rather than resting on columns or suspended from cables

  • Supported by barge-or-boat-like pontoons to support the bridge deck and its dynamic loads

  • While pontoon bridges are usually temporary structures, some are used for long periods of time

  • Permanent floating bridges are useful for sheltered water-crossings where it is not considered economically feasible to suspend a bridge from anchored piers.


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Clip of Bridge Collapse

I-90 Collapse Video


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Reasons of Collapse

  • The bridge needed resurfacing and was to be widened.

  • Construction done on the old bridge

  • Cut six-foot-high holes into the hollow concrete pontoons, but the wholes were not closed over the weekend.

  • Storm water was able to enter the pontoons which couldn’t handle the load.

  • by the time the construction workers realized the bridge was sinking, it was too late to pump out all the water.


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Reason Of Collapse

  • one pontoon filled and dragged other down

  • the pontoons were temporarily used to store contaminated water from the construction.

  • the contaminated water was released in Lake Washington


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Reason of Collapse

  • The state faulted the contractor for not responding adequately to the accumulation of water in the pontoons.

  • Ben C. Gerwick, Inc hired by Traylor bros to find cause of failure.

  • The failure of the bridge was primarily the result of structural weaknesses that accrued over many years prior to the sinking and as a result of the storm immediately preceding.

  • Bond slip had progressively occurred in the bottom slab of many of the pontoons, leading to cracks which did not close.


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Results of Collapse

  • The public was simply told that A large storm on the Thanksgiving holiday weekend 1990 filled some of the pontoons with rain and lake water causing it to sink. This wasn’t necessarily true.

  • WSDOT (WA state department of transportation), filed a $69 million lawsuit against Traylor Brothers, the contractors hired to repair Lacey bridge.

  • The dispute was resolved in August 1993, through a mediated settlement in which Traylor agreed to pay the state $20 million


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In the End

  • No one was hurt or killed, since the bridge was closed for renovation and the sinking took some time.


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Work Cited

Enlow, Clair. "Architecture & Engineering." DJC.Com. 1 Dec. 2007 <http://www.djc.com/news/ae/05003121.html>.

Vallone, Christine. "Pontoon Bridge-Building Pitfalls Cited." Bnet Research Center. 1 Dec. 2007 <http://findarticles.com/p/articles/mi_hb5249/is_199604/ai_n20045882>

“I-90 Bridge Sinks." Youtube. 1 Dec. 2007 <http://www.youtube.com/watch?v=gm0YQ3vuyyY>.

"Lacey V. Murrow Bridge." Gerwick Inc. 1 Dec. 2007 <http://www.gerwick.net/pdf/inland_waterways/033e_lacey_murrows.pdf>.