Launching a Steel Arch Bridge with a King Post

May 31, 2018

It has been just over a year since the Vedder Bridge in Chilliwack, British Columbia was replaced. The old bridge had deteriorated and did not meet current seismic codes. The new Vedder Bridge is an attractive 80 m long steel arch bridge with hanger rods in a ray arrangement. The innovative design included a king post and cable system for launching the bridge into place over the Vedder River - a world first for a steel arch bridge. The same system was re-used to remove the old bridge for demolition. The new bridge has created better access for motorists, cyclists and pedestrians, while improving the river’s path and minimizing the project’s environmental footprint.

We were part of the design-build team with Emil Anderson Construction (EAC) and others. We led the structural, geotechnical and hydrotechnical engineering design of the new bridge.

As project owner, the City of Chilliwack, wanted a unique and attractive bridge for the popular and highly visible location in their community. The project also included an upgrade of the approach roads and a new roundabout intersection to improve travel capacity and safety.

A fundamental goal in construction of the new bridge, and demolition of the old bridge, was to avoid any work within the river. This was important from both an environmental standpoint in protecting the salmon-bearing river, and a construction scheduling standpoint because the fisheries windows for working in the river were very short. Avoiding work within the river also reduced construction safety risks, and so assembling the bridge along the north side of the river and launching it into place was the preferred installation method. The arch span meanwhile required temporary strengthening for the launch sequence, which was provided by a KCB-designed cable king post support system.

King post launch of steel arch - a world first

The Vedder Bridge is believed to be the world’s first steel arch bridge launched into place using a king post support system. Arch bridges are usually either constructed in place and temporarily supported by “falsework”, until the arch at each end is supported by permanent abutments, or floated into place on barges. Neither method was practical on the Vedder River.

Moving the 80 m long bridge across the river without support on one end of the arch posed an interesting challenge. The design-build team decided to assemble the steel superstructure on site and launch it in one piece across the river. Using the king post, the bridge was picked up from its temporary supports and propelled across the river in one metre increments using hydraulic jacks on four travelling skid shoes. The critical part of the launch – the first 60 metres when the bridge was cantilevered over the river and unsupported at one end – was successfully completed in one day before a large audience of local residents. The final 20 metres was completed over the next few days and involved several resets of the jacks to place the end of the bridge on its permanent abutment.

In addition, precast deck panels were used as a launching counterweight, eliminating the cost of hauling other types of counterweight to the site. The precast deck panels also acted as formwork for the cast-in-place deck topping, avoiding construction of timber formwork over the water. These design efficiencies further minimized the environmental footprint of the project.

The new bridge has “integral” piers and abutments and a continuous bridge deck, which avoided the need for expansion joints and enabled a simple and consistent fabrication method for the entire bridge. It has pathways for pedestrians and cyclists, and supports utilities for water, sewer, gas, electrical power and fibre-optic communication cables which are suspended under the bridge.

The demolition of the old through-truss bridge would ordinarily require temporary supports in the river. This would have been very difficult at this environmentally-sensitive site and would have extended the project schedule and significantly increased costs. The solution was to modify and re-use the king post system to remove the old bridge.

The community held a Hello-Goodbye Vedder Bridge event in late-summer 2017 when locals got an up-close look at the new $12.5 million structure. The new bridge is a safe and convenient access point for tourists and residents to the popular lakes in the area, contributing to the development of the community. It also blends into the local environment and reduced risk of the Vedder River flooding and overtopping the bridge.

The project recently won an ACEC-BC Award for Engineering Excellence in the Transportation & Bridges category.