Ghosts of Pattullo: Temporary Works in Bridge Construction – TP3 (Part A)

The Tower That Wasn’t Meant to Stay

For six months, a 35-metre (115 ft) steel tower stood in the Fraser River floodplain, supporting the 84-metre (276 ft) end span of the new Pattullo Bridge—now known as the stal̕əw̓asəm (Riverview) Bridge—before a single stay cable was engaged at the end span. When the cables were finally stressed and the end span began to behave as a cable-stayed structure, the tower disappeared. No trace remains. But without it, the end span could not have come into existence. This project highlights the critical role of temporary works in bridge construction, demonstrating how custom-designed enabling structures support critical stages of construction before being removed.

Before the Bridge Could Balance, Something Else Had to

In cable-stayed bridge construction, balance is everything. Typically, a central tower supports back-to-back cantilevers that advance symmetrically, with each new segment counterweighting the last. The new Pattullo Bridge, however, did not entirely follow that script.

Its 84-metre end span—part of the 578-metre (1,896 ft) main crossing—had to be constructed well in advance of the balanced-cantilever backspan. Only after the backspan was completed through balanced cantilevering and physically connected to the end span could the stay cables be stressed. At that point, the end span would begin to act as a counterweight, enabling the progressive cantilevering of the remaining main span.

Until then, the end span had to stand on its own.

Image 1. stal̕əw̓asəm (Riverview) Bridge showing the main, back, and end spans. The main and back spans were erected in balanced cantilever, after which the end span was engaged to counterbalance the remaining progressive cantilevering of the main span.

Image 1 captures this unusual condition clearly. The end span is already fully erected, while the backspan and main span are still advancing through balanced cantilever construction. For several months, the structure behaved not as a cable-stayed bridge, but as an independent two-span girder bridge.

Once the concrete deck was placed, this condition became even more critical. The completed end span needed to remain stable under dead and transient load effects, supported only by permanent bents S2 and S3, with a temporary third “bent” introduced between them called Temporary Tower TP3 (TP3).

A Temporary Bent with Permanent Responsibilities

Although temporary by definition, TP3 was required to perform at a level more typical of permanent infrastructure. It rose approximately 35 metres above grade on two steel pipe columns, each 2.5 metres (8 ft) in diameter. These columns were fabricated using salvaged cut-offs from permanent foundation piles and spaced to match the bridge’s 26-metre (85 ft) spacing between edge girders.

Image 2. End span shown supported by bents S2, TP3, and S3, with structural steel erected and deck concreting in progress. At this stage, the cable-stayed back span remains structurally disconnected, and the end span functions as a two-span continuous girder bridge.

As shown in Image 2, the tower aligned directly beneath the three-girder superstructure, supporting both edge girders and, during erection stages, the central girder as well. The steel cap beam at the top of TP3 served as the primary load-distribution element, transferring forces from the superstructure into the two pipe columns below.

TP3 was far more than a simple vertical support prop. It became a multi-role support system, carrying the staged erection of the three I-girder steel superstructure, supporting worker access platforms, providing lateral restraint during steel erection  and supporting construction vehicles on the end span deck. At the same time, it had to accommodate longitudinal thermal movements of the superstructure while providing lateral restraint against wind and the capability to withstand a 1 in 100-year earthquake. Image 3 shows TP3 supporting the West edge girder and center girder during steel erection.

Image 3. End span during steel erection between Bent S2 and TP3. The scale of the temporary tower relative to the permanent bent is clearly visible, along with TP3’s primary components: twin pipe columns, transverse cap beam, inverted-V bracing, and worker access platforms at the top.

The Strength of Heavy Civil

At peak demand, TP3 was designed to carry total vertical loads approaching 25 meganewtons (MN), driven primarily by dead load and construction effects, with additional demands from wind and seismic actions. These forces were distributed through the transverse steel cap beam into the two pipe columns, which resisted both axial load and bending induced by eccentric and lateral effects.

The girders soffits were temporarily attached to custom-fabricated wedge plates, machined to match the bridge’s longitudinal grade and girder cambers. Guided pot bearings were installed at the interface between the steel superstructure and the tower. These bearings supported the high vertical reactions and restrained transverse movement, while allowing longitudinal sliding.

Image 4. Top of the TP3 tower showing the temporary centre-girder support with jacking stool. On the far side, the edge girder bears on a guided pot bearing and wedge plate. Worker access platforms and the transverse cap beam—formed by twin W-sections connected with intermediate cross-frames—are also visible.

Image 4 illustrates this arrangement clearly. On the near side, a temporary center-girder support is visible, required only during the steel erection stages. On the far side, the pot bearing beneath the edge girder is visible, supporting the end span during its temporary phase as an independent girder bridge. Coming next week: Part B explores how TP3 was engineered to manage settlement, seismic demands, and eventual removal—performing like a permanent structure, then disappearing without a trace.