Movable Bridges

1.4.5.1 Johnson Street Bridge

In 2009 the city of Victoria decided to replace two existing Strauss bascule bridges located on the edge of the city’s downtown core. Before making this decision, investigations and preliminary designs were prepared to compare the cost of rehabilitating and strengthening the existing bridges with the cost of building a new architecturally significant bridge. It was found that either option would have similar costs and that new construction would have fewer unknowns than a rehabilitation project. The deci- sion to replace the existing bridges was endorsed by a referendum. Important requirements for the new bridge included:

• Realignment of the approach roads to eliminate an “S” curve and eliminate pedestrian/cyclist/ vehicular conflicts at the east bridge head

• Construction of a new bascule bridge to carry a 5 m multiuse trail, three 3 m lanes of traffic, two 1.8 m wide on-street cycle lanes, and a 2.5 m wide pedestrian sidewalk

• Lifeline seismic performance

• Increase of the navigation channel width to meet the requirements of the Navigable Waters Act • Integration with the existing and proposed adjacent path and trail systems

• Design in accordance with current accessibility requirements • Provision for a future 5 m wide rail corridor

• Decommissioning of the existing bridges

The replacement bridge design (see Figure 1.27) was developed in the context of the city’s Old Town Design Guidelines as well as with respect to the historic nature of the site. The new bridge design was developed to provide view corriors of the old town, the Upper Harbor, and the Inner Harbor that are currently blocked by the existing bridge superstructure and counterweights. The design of the new bridge reflects the truss and heavy construction of traditional railway bridges and as such provides a memory of this important historical element of the site while providing for a new, modern design.

In developing the replacement bridge concept, it was decided that a single leaf bascule bridge would be appropriate from an urban and architectural perspective. Three concepts were developed and presented to the public. Concepts included a cable-stayed option, an option with an overhead counterweight, and a truss (see Figure 1.28). The truss option was selected for design and implementation.

The replacement bridge will have three spans: a west approach, the bascule span, and the east approach. An inclined rest pier is proposed, while the east pier provides the counterweight pit and houses the bearings and motors required to drive the bridge. A corbel on the east pier provides support for the east approach span. The bascule superstructure comprises a tapered truss connected to a Φ 12 m wheel that provides for the required rotating movement.

The bascule spans needs to provide for a 41 m wide navigation channel. Thus, a clear distance of 45 m was established between substructures. To accommodate these dimensions, the bascule span measures 51 m between its tip and the center of rotation and about 65 m between the tip and the end of the counterweight. The counterweight is located below the deck and attached to the wheel. Traffic drives directly over the counterweight. The wheels are connected under the deck to provide lateral stiffness. Bearings mounted in the counterweight pit support the wheel and allow it to rotate. The motors and drives are attached to the tail of the counterweight and “walk” down a FIGURE 1.27 Proposed Johnson Street Bridge. (Image courtesy of Wilkinson Eyre Architects.)

rack that is mounted in the counterweight pier to open the bridge (see Figure 1.29). In addition to being architecturally important, the “lobe” on the wheel is necessary to adjust the bridge’s center of gravity.

The bridge deck is separated into three distinct decks: the road deck, the multiuse path deck, and the sidewalk (see Figure 1.30). The road deck on the bascule span will be constructed using a steel ortho- tropic plate supported by transverse floor beams while on the approaches a concrete deck will be used. The orthotropic deck will be provided with an epoxy asphalt wearing surface. Aluminum planks will be used for the path and sidewalk decks. In addition to the orthotropic deck, a number of deck options were considered for the bascule span. These included fiber-reinforced plastic (FRP), exodermic, concrete- filled grillages, and open grating decks. An evaluation based on weight, initial cost, and life cycle cost was made and the orthotropic solution was identified as most appropriate.

The approach spans were designed to be similar to the bascule span from an architectural perspective. This was achieved by using the same transverse floor beam arrangement as used for the bascule span. Steel edge beams are used to bring floor beam loads to the abutments and piers.

FIGURE 1.29 Johnson Street Bridge mechanical and counterweight arrangements. (Images courtesy of Wilkinson

Eyre Architects.)

The site is underlain by silts and clays that were deposited on bedrock. The bedrock elevation varies from being at-grade at the bridge’s west abutment to being about 20 m below water at the middle of the navigation channel. As such, spread footings are required for the west abutment and pier whereas drilled, large-diameter shafts are required for the east pier and east abutment. The bridge is currently being designed for the city of Victoria by MMM Group, together with Wilkinson Eyre Architects and Stafford Bandlow. Construction is scheduled to be complete in 2016.