Ghosts of Pattullo: Temporary Works in Bridge Construction: End Span Superstructure Bracing

This article highlights the critical role of temporary works in bridge construction, where custom-designed, short-lived systems provide essential stability and load paths before the permanent structure is ready to do so.

The Bracing That Was Never Meant to Stay

Before the end span of the new Pattullo Bridge—now known as the stal̕əw̓asəm (Riverview) Bridge—could behave as a cable-stayed structure, it had to survive a very different phase of its life.

During steel erection and before the concrete deck was fully cast, the end span existed in an intermediate state: fully supported vertically, but without the transverse stiffness required to behave as a unified system. The permanent seismic load paths—through the deck slab, end diaphragms, and into the permanent piers via the bearings—were not yet available. And yet, the structure still needed to remain stable under seismic demand.

This created a fundamental question of construction engineering: how do you provide lateral stability when the permanent structure isn’t ready to do its job?

Temporary Works in Bridge Construction: Creating Stability Before the Permanent Structure Can

At this stage, the end span functioned as a two-span continuous girder bridge supported on permanent piers S2 and S3 and the temporary tower TP3. While TP3 provided critical vertical support, relying on it to resist global seismic forces would have pushed demands into foundations already constrained by soft floodplain soils and shallow spread footings—an issue explored in the previous article on TP3 (Part B).

The solution was not to make TP3 stronger. Instead, the strategy was to deliberately change where the forces went.

Temporary superstructure bracing was introduced to create a short-lived but intentional seismic load path. These braces tied the three-girder system together, forming a virtual transverse diaphragm that could distribute lateral seismic demands into the permanent piers much like the permanent deck, bypassing TP3 almost entirely.

This was not redundancy. It was choreography.

Temporary superstructure bracing installed beneath the end span, tying the three-girder system together to provide transverse stability during steel erection and deck construction, before permanent load paths were available.

Designing a System Meant to Disappear

The temporary bracing system was developed in close coordination with the bridge designer, LAP and their sub-consultant MATERIA INGENIERIA, who carried out staged deck-casting analyses. This analyses determined the demands the temporary bracing would be required to resist at each deck casting stage—demands that varied significantly along the length of the end span as geometry, stiffness, and dead weight evolved.

Rather than applying a single conservative bracing size throughout, the team adopted an iterative design approach. Initial bracing demands from the staged analysis were used to size the members, after which the resulting bracing stiffness was fed back into the structural model. Because changes in bracing stiffness directly influenced superstructure behaviour and force distribution, each adjustment altered the demands themselves. Approximately three rounds of iteration were required before the bracing stiffness and force demands converged, allowing the system to be optimized for both performance and constructability.

Plan view of the end span showing temporary bracing layout and zoning. Three bracing types were used along the span, optimized through iterative analysis to match local demand and geometry.

The final configuration employed three different bracing sizes, each selected to suit the local demand and geometry. The bracing members were designed as tension-only elements using high-strength DSI threaded bars in three sizes—32 mm, 57 mm, and 66 mm—reflecting this optimized demand distribution. Couplers welded to steel plates connected the bars into the existing superstructure, engaging permanent girder–floor beam splice locations at one end and a new direct connection to the floor beam bottom flange at the other. These interfaces were deliberately selected to align with permanent connection geometry and load paths, minimizing the need for additional temporary steelwork.

Finite element analysis of the temporary bracing connection to permanent girder–floor beam components, performed using IDEA StatiCa, verifying acceptable stress utilization in plates, bolts, and welds under construction-stage demands.

Connection-level finite element analysis was carried out using IDEA StatiCa to verify that the temporary bracing forces could be safely introduced into the permanent components without overstressing plates, bolts, or welds under construction-stage demands. This ensured that the system could be fully removed once redundant, leaving minimal residual trace and no adverse impact on the long-term performance of the permanent structure.

Geometry Was the Hidden Load Case

What made this system particularly challenging was not just the force demand, but the additional restraining demands imposed by geometry.

Illustration of relative vertical displacements and rotations between center and edge girders at successive floor beams, highlighting the geometric incompatibility that introduced significant second-order demands into the temporary bracing.

The end span girders were cambered, and erected in stages. The temporary bracing connecting at the bottom flange level of floor beams diagonally across bays, meaning that the two ends of each brace were neither level nor aligned. As the deck erection progressed and the structure deflected under concrete self-weight, the brace end points displaced and rotated relative to one another.

These relative movements introduced significant second-order effects that could not be ignored—and if conservatively captured these secondary forces that could easily overwhelm the member and connection design to beyond practical means.

To address this, the team developed a rigorous methodology that explicitly incorporated the concurrent deflections and rotations at each bracing location, extracted from the staged analysis at the point of maximum primary demand. These geometric deviations were then used to quantify realistic second-order effects, which were combined with the primary forces to ensure the bracing bars, couplers, splice plates, and bolts were sized for the true combined demand—rather than a conservative, idealized approximations.

It was this careful accounting of second-order behaviour that allowed the system to remain efficient, buildable, and predictable in the field.

Holding On—Only Until It Was Time to Let Go

Once the concrete deck was fully cast, the structural behaviour of the end span changed. The deck slab and floor system provided the transverse stiffness needed to distribute seismic forces as originally intended in the permanent design.

At that moment, the temporary bracing became redundant.

Removal was planned from the start. Braces were released in a controlled sequence, couplers unthreaded, and temporary splice plates removed. Permanent bolts were reinstated where required, restoring the bridge to its intended final configuration.

Nothing about the finished bridge reveals this phase. No visible trace remains of the system that once tied it together.

The bracing did its job by knowing exactly when to stop. It held the bridge together when the structure could not yet do so on its own—and once it could, it quietly let go.