Ghosts of Pattullo: Temporary Works in Bridge Construction – TP3 (Part B)
- Jhordy Rodriguez
- 3 days ago
- 3 min read
This Part B article underscores the complexity of temporary works in bridge construction, where settlement, seismic performance, and constructability must all be actively managed under real-world constraints.
Designing for Settlement Mitigation
The floodplain soils beneath TP3 were soft, compressible, and highly variable. Deep foundations were not permitted due to archaeological sensitivity, and measurable short-term settlement was anticipated over the tower’s six-month service life. While the end span superstructure could tolerate limited settlement including differential at the TP3 location, the allowable limit established by the designer was just 20 millimetres. Anything beyond that threshold would require correction.
To manage this risk, TP3 incorporated an integrated jacking system at the cap-beam level beneath each edge girder. Jacking points were positioned on either side of the bearings, allowing the superstructure to be incrementally lifted and re-levelled if settlements exceeded acceptable limits. Geotechnical assessments defined anticipated short-term total and differential settlements over the lifespan of TP3, and the jacking system provided a controlled means to actively manage these movements. Elevations at the underside of the girders were monitored throughout construction, ensuring geometry and load distribution remained within tolerance.
An Intentional Seismic Strategy
Despite its temporary nature, TP3 was required to remain functional under a 100-year return-period seismic event. However, it was never intended to act as the primary seismic load-resisting element for the end span.
Early studies showed that forcing global seismic demands from the superstructure into TP3 would result in excessive foundation loads. Given the shallow footing constraints and poor soil conditions, the already substantial spread footings—measuring approximately 10 metres by 11 metres and 2 metres deep—would have become impractically large. Reinforcement demands were significant, and the bottom of each pipe column was infilled with concrete to establish a flexural connection with the footing, but even this had limits.
Rather than resisting seismic loads directly, the strategy deliberately redirected them into the permanent structure. Temporary transverse bracing was installed within the superstructure, tying the three girder lines together and forming a rigid transverse diaphragm connected to permanent piers S2 and S3. Under seismic loading, these permanent piers provided the primary resistance. Once the end span deck was fully cast and structural continuity established, the temporary bracing became redundant.
The bearings at TP3 were detailed such that their transverse restraint would effectively fuse under extreme seismic demand. This isolated the tower from global load effects, allowing TP3 to respond only to its own inertia while the permanent structure carried the seismic forces for the bridge as a whole.
All About Constructability
Beyond its structural role, TP3 was designed with construction sequencing firmly in mind. It provided intermediate support during steel erection, enabled the installation of stability bracing between girders, and served as an intermediate working platform that allowed safe access at height along the girder lines using lifelines.
The tower also facilitated installation and later removal of the temporary superstructure bracing. These construction requirements were not secondary considerations; they were embedded directly into the tower’s design.
Even TP3’s own diagonal bracing reflected this philosophy. Each brace was detailed with a single-bolt pivot connection at one end, allowing it to be rotated into position during installation and reversed during removal. This approach minimized pre-assembly, reduced crane time, accelerated installation and dismantling, and limited worker exposure during elevated operations.
Six Months, Then Gone
Once the back span was connected and the stay cables were stressed, the structural system changed almost instantly. Load paths shifted into the cable-stayed system, and TP3’s role came to an end.
Dismantling followed the logic of erection—only in reverse. Bracing was released, the cap beam removed, and the pipe columns lifted out after severing their connection to the footings. Soon the concrete spread footings would cut into smaller pieces and removed entirely. The floodplain will be restored, leaving no physical evidence that a 35-metre-tall tower had ever stood there.
The end span now behaves exactly as it was always intended to: cable-supported and fully integrated with the cable-stayed back span. But for six critical months, it stood only because TP3 did.
Legacy Line
The temporary TP3 tower was never meant to stay. But without it, the bridge could not have come into existence under this construction sequence.
Comments