Princeton Bridge of Dreams

The early 20th century timber “Bridge of Dreams” across the Tulameen River in the town of Princeton was the final link in the Kettle Valley Railroad and integral in sustaining the development of the British Columbia interior. When the span was decommissioned in the 1960’s, however, the town was left with three large relics – two concrete abutments and a central pier. When the creation of the Trans-Canada Trail was announced, which was to utilize defunct railway corridors, the Trans Canada Trail Society’s local chapter partnered with the town of Princeton in the shared vision of completing their portion of the trail with a crossing which would re-use the existing piers.

Knowing that the owners wished to reference the historic bridge’s strong timber presence in the community, as winner of the design-build tender, we presented two interpretations to honour this. The client unanimously chose the double tied-arch timber scheme, which also complemented the presence of a more recent tied-arch steel highway bridge built immediately adjacent to this site.

The bridge was designed to be a finely detailed kit of parts which would be shop fabricated and assembled to the greatest degree possible before being shipped to site for final assembly. The bridge consists of two spans, 32 meters each, with a shared platform at the center pier. The three-pinned arches were fabricated in halves (17 meters each), laid flat on the ground and joined with a simple vertically-bolted lap splice, then rotated into position about the steel pipe stringers to achieve a rise to span ratio of 0.17. The tie-stringers were welded with connecting tubes to form a vierendeel frame which was suspended from the arches and serves as the primary support structure and diaphragm for the pre-stressed timber deck panels.

A 3D solids computer model was required to establish the geometry of the three-dimensionally curved roof structure and informed all shop and erection drawings for the project. By using a parametric model with accurate geometric and physical properties, the engineer was able to develop an erection sequence that satisfied the weight, stability and safety constraints imposed by the site and crane. The bridge was assembled on the river bank complete with arches, tie-stringers, timber deck, purlins and a temporary railing. X- bracing stabilized the arches until the roof deck and parabolic struts were installed in place. Both bridge halves were installed from the same bank by British Columbia’s largest mobile crane. The lift drew a large crowd who witnessed the installation of both spans, completed in less than four hours.

The Douglas fir Glulam arches feature a tapered cross section optimized to suit the varying axial and bending stresses. This enhances the visual profile, minimizes material, and was achieved economically by working with the laminator to strategically subtract laminations during layup. The arch ends were machined to receive the concealed connection to the tension-tie stringer, also protecting the connection from moisture. In response to durability concerns, a high performance water-based coating was applied to both the arches and timber deck. The coatings were selected for their low VOC content, superior UV protection, and amenability to easy maintenance.

The 4.3 meter wide deck consists of pre-stressed panels of Douglas fir 2x4 on edge, spaced to provide ventilation and drainage through the deck while in service. The members are spaced with shims and tensioned at intervals with threaded rods. The hardness of the rubber shims was selected so that they would compress or relax during the expansion and contraction cycles experienced by lumber exposed to varying moisture and temperature conditions. The panelized system ensures the quality and stability of all deck members and allows for easier maintenance by permitting replacement of individual floor panels as required.

The roof system, comprised of sawn timber purlins, corrugated metal deck, lumber strapping (shop laminated to match the curvature of the bridge) and parabolic shear bracing, was chosen to protect the timber components from direct weathering while acting as the main diaphragm for the structure. The purlins have a curved profile, inducing strong-axis bending of the corrugated deck to facilitate drainage at the sides, eliminating the need for gutters. The parabolic design of the shear bracing was selected for both its structural efficiency and practicality, transferring loads directly to the arch base while providing a safe window of passage for pedestrians, cyclists and equestrians. A carefully detailed guardrail helps make the bridge a true wood-and-steel hybrid.

The community was actively involved in the project, as evidenced by the presence of spectators at every construction milestone. On April 15, 2010 the bridge was officially opened to the public. The celebrations included performances by the local orchestra and the temporary installation of a small steam-driven train which once again carried children and their parents across the “Bridge of Dreams”.