Overview
Tucked below the Leif Erikson trail in Forest Park, sits the Thurman Bridge. Also known as the Balch Gulch Bridge, the 116-year-old bridge transports cars, bikes, and pedestrians on Thurman Street over Lower Macleay Park and Balch Canyon.
The Thurman Bridge sits above Lower Macleay Park in northwest Portland.
Build History
The bridge was built in 1905 for the Lewis and Clark Centennial Exposition and World’s Trade Fair. It helped transfer fair attendees to the 400-acre fairgrounds in the northwestern part of the city. Built by a West Virginian company, Riverside Bridge Company, the bridge was 160 feet long with a pratt type pin connected deck truss.
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A 1904 depiction of the proposed bridge published in the Oregonian. In subsequent years, much of the canyon under the bridge was filled in as part of an effort to create more buildable lots in the area. Image from ODOT. Explore Kathleen Tucker's dissertation for more information on the history of the gulch.
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View of the fairgrounds from the bridge. Image from ODOT.
In Need of an Update
From 1954 onward, the iconic bridge began to show significant wear and tear on its structural elements. The steel floor beams started to deteriorate and caused a host of problems: cracks along the deck of the bridge, increased potholes, and a reduced weight capacity. As the bridge aged, the surrounding community felt the impacts of the weakening structure. Fire trucks became too heavy for the bridge, and the one-mile detour added four minutes to their response time. TriMet buses continued to cross, but they had to slow down significantly. Pedestrians ventured carefully over the damp and slippery wooden planked sidewalk.
In 2012, the city began funding efforts for a bridge rehabilitation project that would fix the growing structural issues while preserving the bridge's unique characteristics. The work kicked off in 2014 following a $3.4 million funding approval from the Federal Highway Administration Highway Bridge Program and $390,000 from the City of Portland. Great care was taken to update the sidewalk, deck, rail system, and steel beams, all while maintaining the iconic trusses and aesthetic of the bridge.
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Pre-construction image shows deteriorated bridge deck. Image from ODOT.
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First bay and floor beam construction. Image from ODOT.
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Rehabilitation required dismantling the bridge to its structural skeleton. Image from ODOT.
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The bridge deck was fully rebuilt. Image from ODOT.
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A view of the completed project. Image from ODOT.
Bridge Update Completed
Portland Bureau of Transportation sent out monthly newsletters to update the community on the bridge construction progress. On October 10, 2014, the bridge reopened to the public and the first fire truck in 20 years safely crossed the bridge in celebration.
The first fire truck drives across the refreshed bridge in 2014. Image from ODOT.
Modeling Process
One of the biggest challenges during the bridge modeling process in SketchUp was the need for a set of key bridge "components" to speed up the workflow and ensure consistency throughout the structure. These components were necessary to quickly repeat certain structural elements, but also had to be dynamic as not each component required the same length.
An example of this, was the diagonal cross supports. These were built as dimensional rectangles with a cross bar texture painted on to a face. Using a texture allowed for the appropriate amount of transparency through the bars and also made it easy to apply to structural pieces of different lengths and orientations.
In companion to a library of key bridge components, the bridge structure was also broken down into key sections. Once certain sections were modeled in SketchUp, they could be adjusted and repeated to create the rest of the structure.
Putting the many pieces together required constant reference to photos of the bridge, Google Street View and old concept drawings. Adding environmental context in Twinmotion brought the SketchUp model to life through shadows, landscaping, people, and weather.
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Model of the bridge using SketchUp
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Render of the bridge model using Twin Motion
The details on the bridge are impressive: complex steel framing, pin-jointed truss, and ornate railings. Additionally thrilling is the accessibility of the underside of the bridge. Trudging down the stairs from Thurman street, or driving to Lower Macleay Park, one can view the structure of the bridge up close. From below, the crisscrossing emerald steel feels spacious and wispy: a historic feat of engineering among the trees.
The bridge was modeled in SketchUp and then pulled into Twinmotion for some finishing touches and provided the final image renders.
