Case Study: Gilbert River Bridge – Capturing Camber in Incremental Launching Analysis
- Raj Singh
- Sep 16
- 3 min read
As part of our Incremental Launching Blog Series, the Gilbert River Bridge in Quebec highlights why accurately capturing camber in analysis is critical to the success of steel girder launching. Built in 2013, the 326-meter-long crossing carries Highway 73 over the Gilbert River on a new four-lane alignment. The bridge superstructure consists of eight steel plate girders with a composite concrete deck, weighing around 4,000 tonnes. To span two central 90m spans and two 73m side spans, the contractor elected to use incremental launching in two sequences of four girders each.
Why Camber Matters
Camber — the intentional upward curvature built into steel girders — is designed so that once the bridge carries its self-weight of steel, the cast-in-place concrete deck, and superimposed dead loads such as barriers and wearing surfaces, the final profile closely aligns with the intended roadway geometry within the construction tolerances.
During launching, however, only the bare steel self-weight is acting. Much of the cambered shape therefore remains unbalanced, making the erection-stage geometry very different from the in-service geometry.
If camber is not properly captured:
Girder soffits may unexpectedly clear, sit lightly, or bear harder on temporary roller supports than anticipated.
Support reactions may redistribute unevenly, overstressing cross-frames or local stiffeners.
Cantilever deflections and tip clearances may deviate significantly from predictions based on an uncambered model.
For these reasons, launch analysis that ignores camber risks producing unrealistic force and geometry predictions.
Approaches to Modeling Camber
Geometric Input: One option is to directly model the cambered shape of the girders in the finite element geometry. This approach is physically accurate and allows for straightforward visualization of the bridge’s intended profile during launching. However, it can be difficult to update across multiple staged models, and splice alignment may become more complicated as the cambered shape interacts with supports at each stage.
Support Adjustments: Another method is to keep the finite element model uncambered and instead apply vertical offsets to the temporary roller supports at each stage. This effectively simulates the presence of camber while maintaining a simpler base geometry. The advantage is that it is flexible and easier to apply across numerous stages of launching. The drawback is that it requires precise calculation of offsets for every stage and can make it harder to see the true cambered shape in the model.
Load Cases: A third approach is to use equivalent load cases, such as imposed “temperature loads” or artificial strains, to replicate camber curvature during the staged analysis. This avoids the need for complex geometric input or repetitive support adjustments. The method is efficient, especially for multi-stage models, but it is abstract and relies heavily on the analyst’s experience to calibrate correctly. Ultimately, the choice of method depends on analyst preference, project requirements, and the balance between clarity, complexity, and efficiency.
How It Was Done at Gilbert
For Gilbert, the following approach was followed to model camber:
The uncambered girder geometry was modeled in the FE system.
Support elevations were adjusted at each stage to reflect camber and the slope of the launch nose.
Compression-only supports and non-linear static analysis ensured that only engaged temporary roller supports carried load, allowing others to lift off realistically when camber dictated.
Staged construction analysis was performed in a single file, with the bridge geometry updated at every tenth of a span — a requirement of the project specifications.
In total, dozens of incremental stages were analyzed, each incorporating unique support elevations that translated the design camber into erection-stage reality. This allowed the analysis to reflect actual contact conditions as the launch advanced span by span.
This approach struck a balance between rigor and efficiency: rather than building a highly complex cambered model, camber effects were captured through controlled support adjustments. The outputs — support reactions, tip deflections, guide forces — were then compiled into a detailed erection manual for use by the launching crew.
Explicitly including camber in the staged analysis resulted in reliable predictions of geometry and forces that could be verified on site. Crews compared real-time measurements of support reactions and girder tip positions against the manual at each stage, ensuring the 4,000-tonne superstructure was advanced safely and accurately into place.
The Gilbert River Bridge illustrates a key lesson: the success of incremental launching of steel girders depends significantly on capturing geometric effects like camber to accurately determine demands on both permanent and temporary components. Careful representation of camber in the analysis ensured the bridge met its geometric and structural requirements throughout erection.
📖 Download the full Bridge Design and Engineering Magazine article here.