Installing new steel grate deck on Morrison Bridge lift span. It will be filled with concrete this summer.

The failing deck of the Morrison Bridge lift span was replaced in 2017. 

Project Facts

  • Preliminary work began in March 2017, with minimal impact on bridge users. The contractor added weights to the counterweights under the bridge deck, to balance the heavier weight of the new lift span deck. 
  • Four of the six traffic lanes were closed. All lanes and sidewalks are open. 
  • The new deck replaced one that deteriorated prematurely after installation in 2011 - 2012.  
  • Total project cost for the design and construction phases is $10.4 million. 90% of the funds are federal, 10% county.
  • Hamilton Construction was awarded a $6.5 million contract to install the new deck in 2017.
  • The temporary vehicle weight limit of 10 tons on the bridge has been increased to 40 tons now that the new deck is in place.
  • The temporary speed limit of 25 miles per hour will be increased to 35 miles per hour when construction is completed. 

Cross section showing traffic plan during work on north side of bridge.
Traffic Impacts

Construction work in March 2017 focused on strengthening the lift span, which did not impact traffic.  Work to replace the lift span deck closed four traffic lanes from April 3 through October 20, 2017.  Westbound access from the eastside was limited to Interstate  5 north access only.  Westbound bridge users had to use a different bridge from the inner eastside, such as the Hawthorne or Burnside. 

The bridge closed to all vehicle traffic when concrete was poured for the east and west leaves of the lift span. Full bridge closures lasting two weeks occurred in July  and October.  The closures were needed while the concrete was poured and cured, to avoid cracking the concrete from traffic vibrations.

TriMet's #15-Belmont/NW 23rd line has restored service across the Morrison Bridge. 

Project Overview

The solid deck installed on the bridge in 2011 was made of fiber reinforced polymer (FRP). It replaced an open steel grating deck that had lasted 50 years, but provided less traction for vehicles in wet weather.  Choices for the lift span deck type are limited because the deck’s weight needs to be balanced by the weight of two 1.9 million pound concrete counterweights, and supported by the bridge structure.

After the deck began to fail soon after installed, County engineers and consulting engineers from David Evans and Associates investigated replacement deck options with consultants and state and federal agency staff. (See this site for information on why the FRP deck failed.) The process led to the selection of an open steel grating deck that was filled with a top layer of lightweight concrete.  The concrete layer is two inches thick and has a top layer of a polymer asphalt for improved vehicle traction compared to the original deck. This deck type has a long history of use on bascule drawbridges like the Morrison.

The open grating concrete filled deck was not chosen in 2011 primarily due to its heavier weight. The new deck will weigh 44 to 47 pounds per square foot, compared to 20 pounds for the original open steel grating deck and the FRP deck. Analysis confirmed that the bridge structure can support the additional weight.  The weight of the two counterweights will be modified to support the extra weight of the new deck.

The Morrison’s original lift span deck was an open steel grating that had degraded after 53 years of use.  The old deck was replaced with one made of FRP panels similar to decking installed on the county’s Broadway Bridge in 2010.  The Morrison Bridge FRP deck began to show signs of failure in 2012, and an in-depth investigation confirmed that many panels were loosening and deteriorating.  The FRP deck was the subject of litigation that has been completed. County staff regularly monitored and repaired the old deck to ensure the safety of the bridge for vehicle traffic. 

Multnomah County maintains the Morrison Bridge, five other Willamette River bridges, and 300 miles of roads. For information, visit /bridges.