Structural Engineering Expertise to Improve Resiliency on a Historic Recreation Crossing

• Rope access inspection to determine condition and guide rehabilitation approach
• 3D model for structural analysis developed by reverse engineering existing bridge
• Durable timber decking incorporated during design after federal policy changes

For more than a century, the 220-foot Umtanum Suspension Bridge has connected the banks of the rugged Yakima River Canyon near Yakima, Washington. Once a vital passageway for homesteaders and railroad pioneers over the river, the structure now accommodates upward of 100,000 visitors each year as part of the Bureau of Land Management’s (BLM) popular Umtanum recreation site.

By 2016, the bridge had significantly deteriorated from years of service, neglect in a remote setting and windstorm damage, posing safety concerns for those crossing it. Faced with indefinitely closing the popular visitor site, the BLM sought to restore the structure and revamp the nearby recreation area and parking site to maintain accessibility for outdoor recreationalists.

Initially, a simple pressure-treated deck and railing replacement effort, the project transformed into a complete structural assessment and modernization of the bridge. Selected as the lead designer and engineer of record, HDR worked with the BLM to evaluate the bridge’s integrity, explore alternatives to extending the bridge’s life beyond a deck revamp and redesign the overall site.

Understanding the Existing Bridge

HDR was originally tasked with evaluating the bridge and developing a cost-effective design to replace the wood deck and railing on a compressed 130-day timeline. A physical inspection using a mix of traditional and rope access revealed incongruent geometry, including main cable slippage, sag and misalignment at midspan, bent hanger rods, loose wood plank panels and twisted towers. Complicating efforts to understand the structure were a lack of as-built plans, foundation details, anchor block information and condition assessments. As a result, our team reverse-engineered the structure and was tasked with a more complete structural rehabilitation.

A survey was conducted to complement the inspection, defining geometry and member sizes, leading to a 3D model that allowed the design team to explore the bridge’s structural integrity and response to loading and proposed modifications. This allowed engineers to program the material properties, make proposed changes to the geometric form, and then watch in real time as the structure relaxed and found equilibrium. This method enabled the team to further grasp the structure’s internal stresses and geometry so that viable rehabilitation schemes could be developed to improve stability, provide a practical design configuration with respect to the bridge’s limited load capacity, and meet historical guidelines and expectations.

Designing a Modern Structure

With improved understanding of the condition, deficiencies and constraints of the existing bridge, the team developed a cost-sensitive and viable design.

Challenging the structure were frequent windstorms during winter months and intense rainfall. A self-anchored, pre-tensioned cable railing system was chosen to minimize lateral wind drag. The team’s vibration assessment confirmed the railing would improve stiffness and stability under pedestrian vibrations and could be anchored by a rebuilt ramp to avoid lateral loading to the existing tower foundations.

At 90% design, a federal environmental policy change prohibiting pressure-treated wood required the design team to seek viable alternatives. We collaborated with University of Oregon researchers studying the commercial viability of treatment-free timber, and chose Alaska Yellow Cedar for its known durability and reliability.

Much of the original structure was maintained but restored, including mainline cables and towers, take-up anchors, a positive camber and primary load path for new wire-rope hangers. The final design included a fresh coat of paint for the towers and existing steel floor framing elements.

Recreation Site Overhaul

Aligning development of overall site improvements with available funding and procurement required separating the project into two parts. Once a bridge rehabilitation plan was created, our civil design team collaborated with the BLM to redevelop the adjoining parking site and bring it up to modern standards for access, stormwater runoff and improved maintenance.

The existing gravel parking lot and approach roadway was designed to be paved with upgraded parking and wayfinding striping. With accessibility a BLM priority, our design increased the number of ADA parking stalls and created a stabilized gravel trail around the property for use by all recreationalists. To improve drainage, swales were integrated into the site to prevent contaminants from entering the Yakima River — Washington State’s only Blue Ribbon trout stream. Permeable pavers were added to reduce gravel erosion and the need for routine maintenance like concrete. Boulders on site were repurposed to line swales. Signage, wood barricades at the bridge entrance and parking blocks were reused.

The $1 million bridge rehabilitation was completed on a revised schedule after a delay in searching for a treatment-free timber, reopening to visitors on July 4, 2021.