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  • Writer's pictureRaj Singh

Jacking a Century-Old Truss Bridge: Post #2 Conceptual Design

Updated: Apr 30

For a background on this series review Post #1 here.

In 2019, a bearing monitoring program unveiled a significant restriction in the movement of the expansion bearings (Figure 1 and Figure 2) at on Pier 1 of Truss Span 1. This restriction was inducing restraining forces in the truss members. To counteract this, Associated Engineering designed a solution: replacing the rollers and rails assembly with an elastomeric bearing pad (highlighted yellow in Figure 1).

Existing Bearing Upper Assembly Retained and Lower Roller Assembly Replaced
Figure 1: Bearing Rehabilitation at Pier 1 of Truss Span 1

Figure 2: Seized Roller/Rails (highlighted in red)
Figure 2: Seized Roller/Rails (highlighted in red)

The replacement process for the structure would involve a three-step procedure. First, the structure would be jacked up. It would then be supported on blocking throughout the duration of the replacement. Finally, it would be jacked down onto the refurbished bearings. This procedure would be scheduled to occur after the completion of the deck rehabilitation works on the project.

Maintaining Bearing Articulation During Replacement

In conceptualizing the design for the jacking assembly, one key consideration for the Spannovation team was to maintain the intended articulation of the bridge at Pier 1 during blocking. This meant allowing flexural rotations and longitudinal movement while restraining transverse movement at each bearing location. The challenge was to allow the bearing rocker to continue its articulation unhindered, which meant the jacking assembly could not be connected to the structure above the bearing rocker, such as directly to the truss bottom chord. The image below, Figure 3, illustrates the rocker and roller components of the original bearing. The truss's vertical leg can be seen resting on the rocker's top plate.

Figure 3: Rocker & Roller Components of the existing bearings
Figure 3: Rocker & Roller components of the existing bearings

Thinking "Inside" the Box

The contractor expressed a preference for an assembly that wasn't directly connected to the existing structure. This led to an inventive idea: why not use the open cavity in the existing bearing to accommodate a longitudinally oriented jacking beam (Figure 4) - literally inside the box solution. Initial calculations validated this concept but required filling the cavity entirely with high strength steel.

Figure 4: Limited Space in the Bearing Cavity (128mm x  240mm from rivet head to river head)
Figure 4: Limited Space in the Bearing Cavity (128mm x 240mm from rivet head to river head)

Jacking & Blocking Assembly

The jacking beam is bolted to four blocking corbels prior to jacking operations with horizontal bolts. The four blocking stools are anchored to the concrete below. The vertical connection bolts between the corbels and stools are left loose during jacking to allow up and down movement. Upon lifting to a desired height and installation of shims to suit, the bolts are torqued to connect the corbels to the stools. Jacks are retracted to transfer the bridge on the blocking assembly. This configuration (Figure 6) allows the rocker of the existing bearings to function unhindered while maintaining longitudinal movement. The result is a bridge supported on a jacking beam, connected to portal frame supports anchored to the concrete pier.

Figure 5: Jacking & Blocking Assembly Concept
Figure 5: Jacking & Blocking Assembly Concept

Conceptual Calcs

Through hand calculations, we estimated the maximum load per bearing during jacking to be 175 tonnes. Jacks were sized with a safety factor of 1.5. Considering two jacks per bearing location, the expected load per jack was 130 tonnes. The eccentric loading from longitudinal placement tolerances and thermal span movement resulted in an estimated load of about 150 tonnes.

The contractor suggested using 200-ton jacks for the blocking system configuration to allow adequate room if the detailed design required larger jacks. These load estimates were employed to scale the assembly components during the conceptual phase. The jacking beam was envisioned as five 25mm thick plates sandwiched together.

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