William R. Bennett Bridge

The William R. Bennett Bridge extends across Okanagan Lake, connecting the communities of Kelowna and Westbank on Highway 97, in the interior of British Columbia.

The bridge is one of only eight floating bridges in the world, and the only one of its kind in Canada. It replaces a three-lane floating bridge with a lift span, with one that has five lanes and a navigation channel.

The floating bridge type was chosen partly because the depth of Okanagan Lake is up to 260 metres and the soil conditions are problematic for heavy foundations.

As bridge engineers for WRB Bridge Group, Buckland and Taylor met the difficult challenge of designing a floating bridge combined with fixed sections in a seismic zone.

Floating bridge design

Extending 1,060 metres long in total, the bridge includes a 690-metre string of floating pontoons supporting an elevated deck. There are two 54-m transition spans, and a fixed west approach structure. The latter is 277- metre long and includes an elevated span to allow for an 18-m high by 44-m wide marine passage.

Floating bridges can pitch, roll, yaw, move up and down, and respond dynamically to waves and wind. The Canadian Highway Bridge Design Code (CHBDC) is written for fixed bridges, therefore other codes, particularly the Norwegian Code, were used to help establish the design criteria for vessel collision, pontoon stability, allowable movements and so on. World specialists in floating bridge design, Aas-Jakobsen and Johs Holt of Denmark, were subconsultants.

The transition spans and their bearing and joint arrangements were complex to allow for movement in the floating structure while maintaining the rideability of the roadway surface. Since the lake level fluctuates by several metres every year, the transition spans also had to be long enough to prevent steep grade changes in the roadway.

The east transition span fixes the pontoon string to the east abutment and restrains the floating structure longitudinally during normal operations. Its bearings are energy-absorbing fuse links designed to fail in an earthquake.

Anchoring new to old

One of the most innovative aspects of the project was the method of connecting the new bridge to the existing bridge anchors. The existing bridge was later demolished.

As a new pontoon was floated into place beside the existing bridge, the following sequence took place:

• one or two anchor cables on the north side of the existing bridge were disconnected, shortened and attached to the north side of the new pontoon;

• a cable carrying a suspended weight was connected between the new pontoon and the old bridge. As the lake level fluctuated, the weight raised or lowered keeping the bridge deck in place without overloading the anchors.

• once the new 690-m pontoon string was completed, the pontoon sections from the original bridge were disconnected, along with their submerged weights. Then the south anchor cables were lengthened and connected to the new bridge.

This elaborate sequence was analyzed for a total of 18 steps.

Construction of the $144-million crossing was completed between August 2005 and February 2009, 108 days ahead of schedule.

Project: William R. Bennett Bridge, Kelowna, B.C.

Award-winning firm –bridge engineer: Buckland & Taylor, N. Vancouver (Darryl Matson, P. Eng., Rodger Welch, P. Eng., Joe Leal, P. Eng., Keith Kirkwood, P. Eng., Karsten Veng, P. Eng., Eduardo Pradilla, P. Eng.)

Client/design-builder: WRB Bridge Group (SNC-Lavalin and Vancouver Pile Driving)

Owner: BC Ministry of Transportation & Infrastructure

Owner-operator until 2035: SNC-Lavalin

Owner’s engineer (for BC MoT): Worley Parsons (Westmar)

Other key players: SNC-Lavalin, Aas-Jakobsen, Johs Holt A.S., Ben C. Gerwick, DMD & Assoc., Northwest Hydraulics, Trow Associates