Uncovering major defects
At Mott MacDonald, we were engaged to undertake the inspection, monitoring, testing, detailed structural assessment and feasibility design for the strengthening and refurbishment of this important heritage bridge. We identified a number of sub-standard components. These will be addressed as part of the major Hammersith Bridge Restoration Project launched earlier this year. The major refurbishment works will include strengthening and remediation of the chains, hangers, stiffening girders, decking system and articulation.
However, we also uncovered a number of major critical defects. Given the risk to public safety, these discoveries have resulted in significant restrictions to the bridge use and mitigation procedures being put in place in recent years. The critical defects must first be remediated to stabilise the bridge before the rest of the refurbishment works can begin.
How the suspension works
At each end of the bridge, the suspension chains are deviated through saddles before descending through tunnels where they are locked off and secured. The deviation saddles are supported on the bridge’s large cast iron pedestals via steel roller bearings. The rollers have deteriorated over time and completely seized up. This seizing has generated large restrained horizontal forces in the pedestals, which in turn has caused significant overstress. Major fractures have appeared through the cast iron support structures.
Stabilising the bridge has presented a significant challenge in providing an efficient design while maintaining public safety, minimising disruption and reducing the impact on historic features.
Strengthening the bridge for future generations
Our team at Mott MacDonald has developed a suite of solutions to solve the unique problems presented by this bridge. We will strengthen the saddles and pedestals before the seized rollers are replaced with new elastomeric bearings. These modern bearings will help the load on the bridge to shift slightly, preventing future fractures as the bridge is subjected to the stresses of daily use.
Due to the challenging constraints, the works require a complex sequence of jacking operations, novel construction processes, bespoke parts and materials. The internal voids of the pedestals have already been infilled with steel fibre reinforced self-compacting concrete, and works are progressing to install temporary and permanent steelwork in advance of jacking.
We hope the structural refurbishment and rehabilitation of Hammersmith Bridge will demonstrate how cutting edge engineering can preserve important historical transport infrastructure and secure it for future generations.