Lewis & Clark College is a small college located in SW Portland, Oregon. The college began working with Minarik Architecture, Inc. in 2017 to replace an aging 6-span bridge that connected the Howard Hall area on the south side of the campus to the Reflecting Pool and Rose Garden scenic area of campus. Minarik Architecture, Inc. conceptualized an open-air timber bridge with an “industrial feel” and with the college turned to WWS for design ideas. The bridge was required to span an environmentally sensitive area so upgrading the existing bridge and supports would have led to permitting issues, construction delays, and possibly higher costs. A clear-span bridge was the best viable option for the college and the solution offered by WWS was a 188’ clear-span Parallel Chord Truss bridge with a covered roof system that features Cross Laminated Timber (CLT*).
The new bridge is comprised of pressure treated Industrial Grade Douglas fir Glulam beams and was engineered with (3) splices to facilitate treating and shipping. The steel and hardware for all the member connections was specified as A-588 weathering steel and a weathering steel panelized system with a stainless finish ADA grab rail was used for the rails.
Due to a steep ravine, environmental concerns, and adjacent 100’+ trees, the site was delicate and nearly inaccessible. The bridge design and installation required significant thought and planning to address the constructability issues. The existing bridge was scheduled for demolition but it carried a sewer line that needed to remain in place. It could not be dissembled or shut down even on a temporary basis during construction of the new bridge.
WWS crews assembled a 60’ section of the bridge on the high (north) side of the ravine before swinging it into place. The existing bridge, which WWS crews used as a work platform, was reinforced at one of the existing timber bents allowing a temporary resting spot for this new 60’ section.
The remaining 128’ bridge section was assembled on the low (south) side of the ravine and swung into pace with a 360-ton crane. This crane traveled down a sparsely used road to access the site, leaving just 1” of clearance on either side of the crane. The crane operator did a masterful job of setting the pre-assembled bridge sections and CLT roof panels (see video).
After the existing bridge was demolished and the sewer line was detached and re-attached to the new bridge, WWS crews returned to the site to complete installation of the deck panels. After 2+ years of planning and 6+ months of construction, the new bridge opened for pedestrian use in February of 2019.
WWS offers a special thank-you to Michel George of Lewis & Clark College for teaming with and believing in local engineering and construction firms to tackle this challenging project. Precision Construction Company acted as General Contractor. Thank-you to Paul Drew, Jay Marsh, and Mike Telling for their commitment to this project and for placing their trust in WWS.
*CLT is the relatively new and innovative product of layering solid-sawn members together with each layer orientated perpendicular to adjacent layers, achieving higher levels of rigidity in both X and Y axis directions (think plywood, but 10-15 times thicker). To date, CLT panels have been used for floor and wall systems of both low and high-rise buildings (up to 17 stories so far), as CLT panels are typically fully pre-fabricated prior to shipment to reduce jobsite assembly hours/costs.