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  • Writer's pictureRaj Singh

Jacking a Century-Old Truss Bridge: Post #5 Jacking Down Operation

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

In this new post in our series on jacking a century-old truss bridge, we will be discussing the operations for jacking down the Truss Span 1 at Pier 1 onto the permanent bearings. These operations were executed on February 9th, 2023, and supervised by Spannovation Consulting Limited.

As a recap, the bridge was jacked up on January 31, 2023 and transferred onto blocking assemblies at each of the three bearings creating a 12mm space for replacing the jammed roller mechanism with elastomeric pads (See Figure 1). A gap between the upper rocker assembly base plate and the elastomeric pad can been seen in the Figure. This replacement was carried out over approximately one a week period while the bridge was open to live traffic and supported at Pier 1 on blocking assemblies and held by lateral restraints.

Figure 1. Replaced Elastomeric Pad Underneath the Bearing Rocker Assembly
Figure 1. Replaced Elastomeric Pad Underneath the Bearing Rocker Assembly

Preparation for Jacking Down

Before the operations began, a review of the temporary restraints was conducted. Adjustments were made to ensure no hindrance to free vertical movement. Initially, the bridge supported on the blocking assembly was transferred to six jacks connected through a single manifold with dial gauges to a hydraulic pump, See previous post on more details related to the jacking system.

As was the case during jack-up, to record the elevation change of the bridge during jack down, a displacement dial gauge was used as the primary method, and a tape measure was used as a secondary method (Dial gauge can be seen in Figure 1). Workers were stations at each bearing to call our the readings.

The Jacking Down Operation

At 7pm, the bridge was closed to traffic and the first task was to transfer the load onto the hydraulics and relieve the collars. The jacks were pressurized until the collars could be retracted, transferring the load to the hydraulics.

The operations followed a systematic approach to lower the bridge at various points, ensuring the structure stayed within its stress limitations. The lowering process was conducted in sequence at three bearing locations. This was done to maintain a maximum elevation difference of 2.5mm between adjacent points, a value verified in the jacking analysis for the safe accommodation by the truss structure.

Final Bearing Elevations

The final elevations were recorded after transfer of the bridge onto refurbished bearings. This resulted in the following elevations, measured in "mm" relative to the original datum: Truss A: -1.0, Truss B: -4.0, Truss C: -2.0. The variation was attributed to the manufacturing tolerances of the new elastomeric bearing pads and possibly some variable compressive shortening under differing permanent load reactions at the three bearings.

Post Replacement Assessment

Following this the owner's engineer mandated an evaluation of the restraining demands due to differential vertical movement to ensure the final bridge's integrity and safety. Spannovation, working on behalf of the General Contractor, was assigned to demonstrate this through a detailed analysis and structural assessment which will be discussed in the final post next week.

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