V+M Structural Design is the Prime Consultant for the design of Crab Creek Trail Bridge and adjacent trestle, which are located in an environmentally sensitive area.

Owned by Washington State Parks and Recreation Commission and located within a high-desert ecosystem with expansive wetlands, the bridges are a key link in the Palouse to Cascades Trail along the historic Milwaukee electric railway. The previous wooden trestles burned down in 2019 and 2021.

For conceptual design, V+M developed alternatives that looked at structure types, constructability, and cost. During final design, V+M developed the bridge plans, specifications, engineer’s estimate, and tender documents.

Construction access is highly constrained at the site, requiring a 6.5-km one-way trip along the trail from the nearest access point. The remote site is further constrained by the width of the former rail prism with wetlands flanking each side. With all this in mind, V+M’s engineers made constructability a key consideration in the design detailing, including crane size and pick requirements. The design responds to the environmentally sensitive waterway by keeping all works set back behind the highwater mark.

The result is V+M’s unique 3-span bridge design that uses short back spans tied down to abutments, which allows the attractive profiling of the through-girders. The girders were constructed using a cantilevered technique that progressively installed segments out over the water from each abutment. Crab Creek Trail Bridge spans a total length of 56.7m with a clear span of 45m. The adjacent steel trestle utilizes driven H-piles. Both structures have concrete decks comprised of non-composite precast panels.

Crab Creek Trail Bridge responds to the myriad of site constraints through innovative design that results in economical, efficient and aesthetic public infrastructure. The striking bridge architecture complements the surrounding nature with clean lines, weathering steel material.

The construction sequence and associated structural systems respond to access and environmental constraints. Vehicular access was confined to the existing 6m-wide rail prism, which influenced the girder shape and associated splices.

The structures were completed using weathering steel members and full-depth precast panel decks to reduce the need for ready-mix concrete at the remote site, while eliminating the need for concrete pours over the creek and wetlands.

The bridge over the creek was built incrementally out from the abutments. The girders are deepest over the piers and taper down in depth through the central span, with the mesh railing infill providing transparency over the creek. Each girder was divided into 5 segments that could be more easily transported to site. The deep end segments were heavier but required a short reach by the crane. The same crane can lift the intermediate segments, which are lighter but require a longer reach. The central segment is light but the reach is significant, enabling the same crane.

The bridge scheme proved not only to be highly constructable but was also very economical. Much thought was put into the details of the design to ensure it provides users with a memorable experience and complements the beauty of the landscape. The aesthetic girder profile is efficient by responding to the force demands. The tie-downs at the abutments provide uplift resistance with an array of rock anchors grouted into bedrock 6m below grade.

The expressive form and the material (weathering steel) are a natural fit to the desert landscape. The low profile of the girders and trestle reflect the linearity of the trail and as users enter onto the bridge, the girders are their tallest. This sense of compression of space is then followed by a sense of openness as users pass into the central span. The girders drop in height and the transparent railing emerges to create a special experience over the watercourse.