top of page
  • Writer's pictureSri Kota

Jacking a Century-Old Truss Bridge: Post #4 On-Site Jack-Up Operations

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


A significant milestone was reached in the St. Andrews Lock and Dam Bridge project on January 31, 2023. On this day, a carefully executed jacking operation lifted Truss Span 1 at Pier 1 off its bearings. This operation marked a critical point in the project as it involved raising the trusses and transferring the weight of the bridge onto temporary blocking. This process created the necessary space for the refurbishment of the permanent bearings. The operation had to be completed during an 8-hour overnight closure of the bridge, from 10pm to 6am.

Figure 1: Assembly of Jacks and Temporary Blocking at the Three Truss Plane Bearings
Figure 1: Assembly of Jacks and Temporary Blocking at the Three Truss Plane Bearings

Before starting the operation, we conducted a thorough inspection of the installed temporary works. This inspection included checks on the jacking beams, blocking stools, the positioning of the jacks, and the longitudinal and transverse restraints. The team ensured that all potential restraints to vertical movement were removed and that the jacks were positioned equidistant from the bearing centerline to minimize any eccentric loading.


Figure 2: Jacking System Schematic
Figure 2: Jacking System Schematic

Jacking system pressure gauges and needle valves
Jacking system pressure gauges and needle valves

The jacking system consisted of six jacks connected to a hydraulic pump through a single manifold with dial gauges. To monitor the bridge's elevation during jacking, a displacement dial gauge was used as the primary method, and a tape measure served as a secondary method. A dial gauge was placed at each bearing along with a worker stationed with a tape measure to call out both the readings. This dual-monitoring system ensured accurate readings throughout the operation. Despite the six jacks being connected to a single manifold, the jacking process involved differential jacking at each bearing location by locking off the jacks at the other two bearings. The limit of this differential displacement was determined through our finite element analysis. This ensured that no component or connection would be overstressed and could safely resist demands from this differential movement. This gave the contractor more control over the jacking process, as they could determine the load and displacement at each location at every step.


Worker stationed at Truss C location to take tape measure and dial gauge readings.
Worker stationed at Truss C location to take tape measure and dial gauge readings.

Once traffic was stopped on the bridge at 10pm, the jacking operation began by testing the system and performing small test lifts at each location. The jacks were pressurized to approximately 2000psi and began lifting the truss sequentially—starting with Truss A, then Truss B, and finally Truss C and backwards. This sequence was to be repeated until the entire bridge was raised by 12mm. At each step, it was crucial to not exceed a maximum elevation differential of 2.5mm between any two adjacent bearing locations.


After an hour of initial testing and streamlining the call process with the workers, the operation advanced relatively quickly. However, an issue arose when a lock collar for one of the jacks at Truss B snagged on the blocking stool web plate once the bridge was raised about 8mm. The team decided to lower the bridge back onto the bearings, remove the jack, and trim the interfering stool web plate to resolve this. The bridge was lowered down sequentially again ensuring the limit of 2.5mm was not exceeded. Once the issue was addressed, which took a couple of hours, the jacking-up process recommenced and the bridge was successfully raised by 12mm at all three bearings.



The above video shows workers inspecting the jack for the stool web plate interference, the jacking system manifold, pressure gauges and needle valves.


After the successful lift, packer plates were installed to engage the longitudinal restraint, and the nuts for the transverse restraint were tightened. This setup ensured the bridge was safely restrained in both directions for the intended bearings replacement while still allowing traffic to cross safely.


The bridge was ready for traffic flow by about 2am, four hours ahead of the scheduled traffic closure window. This operation demonstrated the value of thorough planning, close attention to detail, and the team's capacity for on-the-spot problem solving.

Comments


bottom of page