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Demolition of a post-tensioned footbridge in Leeds city within a strict time period with constrained crane heights and a limited collapse radius.
Client: Balfour Beatty
Demolition Contractor: S Evans & Sons
Overview
The demolition of the Spence Lane footbridge in Leeds was the first of three footbridges over the Armley Gyratory to be removed and replaced with more accessible footways.
Built in the early 1970’s, the structure was a four-span footbridge with the largest of the spans stretching 22.6 metres and weighing around 50 tonnes.
Project Scope
Andun Engineering Consultants (AEC) provided the demolition sequence, including the justification of the structure during the demolition for the removal of the footbridge.
This included the submission of the Approval in Principle (AIP) to the Highways agency.
AEC created a 3D visual model of the demolition sequence allowing the demolition team to effectively communicate the sequence to both technical and non-technical stakeholders.
Access to the existing archive drawings meant that AEC were able to determine the construction process and that the post-tensioned tendons were grouted.
These assumptions were verified through extensive site investigations, including a Post Tensioned Special Inspections (PTSI) report.
Analysis was carried out to review the different stresses during the structure’s service life, and compared to those to be experienced during the removal.
Where stresses differed from these two cases, they were justified numerically to ensure the beam did not fail during the process of removal.
Project Constraints
The footbridge location was over a highway and close to a railway.
The removal of the footbridge required the partial weekend closure of the road.
This created a time-pressure constraint, which led to using one large crane as there would not be sufficient time to set up two smaller cranes in the time available.
The position of the footbridge also required a crane with a shorter boom to reduce the collapse radius. Limiting the collapse radius impacted the demolition methodology, as the beams would be lifted at different locations to their original support points.
Something that should be carefully considered and risk assessed for post-tensioned structures.
Another key constraint was that the bridge was also post-tensioned, meaning a specialist post-tensioned analysis was required.
Demolition Methodology
To mitigate the risk of changes in stresses in the structure, the demolition sequence was similar to the reverse of the construction sequence.
Each span was cut and lifted away by crane onto the low loader, one by one in sequence.
Once the spans were lifted out, the in-situ cantilevered columns and mushroom heads were demolished mechanically using large excavators operating on-road protection.
Project Completion
The bridge was successfully demolished and a new footbridge has now been installed.