Incremental Launching Without a Nose: Case Study #3 Sombrio Bridge
- Raj Singh
- Sep 24
- 3 min read
As part of our Incremental Launching Blog Series, the Sombrio Bridge on Highway 14 in Port Renfrew, British Columbia, shows how careful evaluation by the construction engineer can transform erection methodology. Built in 2012, the project replaced a 55-year-old timber bridge with a new two-lane composite steel girder crossing, 122 meters long with spans of 40m and 82m.
The Challenge
The longer 82m span crossed a deep ravine with limited access. Traditional options for launching included:
A steel launch nose, heavy and costly to fabricate.
A temporary bent in the ravine, difficult and risky to construct in steep, remote terrain.
Either solution would have added cost, time, and safety concerns. The challenge was to find a simpler, more economical alternative.
The Solution
The construction engineer’s assessment revealed that the girders could cantilever most of the 82m span without assistance. Only the final few meters required additional capacity in the bottom flange over the pier supports. By adding approximately 6 tonnes of steel to strengthen the bottom flange for this temporary load case, the girders could be launched safely without a nose or bent. This decision had two major benefits:
It was more cost effective to add a modest amount of steel to the permanent girders than to fabricate a launch nose or construct temporary supports.
The added steel increased the safety margin for the bridge in its permanent condition, creating long-term value.
The re-detailing of the girder bottom flange and the associated design calculations were submitted in time to incorporate the modifications into fabrication. The contractor paid for the additional steel, creating a win-win outcome: reduced erection cost and time, and a stronger permanent bridge.
During erection, all four girders (approximately 500 tonnes) were launched simultaneously. A counterweight system of concrete blocks stabilized the tail, while careful staged analysis in Larsa 4D modeled camber, wind, and roller reactions at 15 critical stages. Final positioning was completed by lifting the tip 1.5m with a 275t crane at the abutment before jacking the bridge down onto its permanent bearings.
The Result
The Sombrio Bridge was launched across an 81m cantilever without a launch nose or temporary pier. The sequence began with all four girders — about 500 tonnes of steel — launched simultaneously and stabilized by counterweights at the tails. As the system tipped past the pier, the cantilevered tip was carefully lowered onto the supports under static control.
The counterweight system was key to the success of the operation. Instead of a launch nose, stacks of precast deck panels were placed at the girder tails to provide stability during the tipping process. By gradually removing panels as the cantilever advanced past the pier, the construction crew controlled the load transfer onto the pier bearings. Thereafter, panels were added back as the cantilever grew, providing the necessary safety against overturning of the girder system during launching.
In the final stage, a 275t crane positioned behind the south abutment lifted the girder tip about 1.5m above the permanent bearings, allowing temporary supports to be removed. The bridge was then jacked down step by step onto its permanent bearings. A launch nose would have avoided the need for this final lift, but the chosen method proved more economical and efficient overall.
This case is an excellent example of the construction engineer evaluating alternatives and developing a more effective erection methodology than what was assumed by the design engineer. The outcome was safer, faster, and more economical — showing the true value of construction engineering in bridging the gap between design intent and field execution.
📖 Download the full Bridge Design and Engineering Magazine article here.
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