Future of Bridges | Hammersmith Bridge reopening presents enormous engineering challenge

With the river lapping below Hammersmith Bridge and autumn leaves beginning to fall on the banks either side of the crossing, it is easy to forget that the Victorian structure has been front and centre of an ongoing political wrangle.

This crossing of the Thames between Barnes on its south bank and Hammersmith to the north was closed to motor vehicles in April 2019 due to concerns about the integrity of the structure, which is owned by Hammersmith & Fulham Council. It was also closed to pedestrians and cyclists in August 2020.

Partial reopening

Now, pedestrians and cyclists are once again crossing the 134 year old structure after consultant Mott MacDonald deemed a partial reopening safe in July. River traffic beneath the bridge has also resumed, having been halted last summer because of fears that the structure could suffer a catastrophic collapse. 

Funding arguments between the government, the local council and Transport for London (TfL) have held up efforts to fully reopen the bridge. The Department for Transport’s (DfT’s) Hammersmith Bridge Taskforce has committed the government to stump up a third of the repair bill; with cash-strapped TfL and Hammersmith & Fulham Council responsible for finding the rest of the £141M to £163M needed to fully restore the structure. 

Returning Hammersmith Bridge to its former glory will be a huge feat of engineering, says the taskforce’s project manager Dana Skelly. 

Hammersmith Bridge was designed by Sir Joseph Bazalgette and opened in 1887. It was constructed on the foundations of an earlier suspension bridge. The existing structure comprises a timber deck bolted to cross girders, supported by longitudinal stiffening girders. 

The footway is a timber deck supported by wrought iron cantilever beams that connect to the main cross girders and the stiffening girders. The deck structure is suspended from 172 wrought iron hangers connected to parallel suspension chains. 

The chains are connected to saddles, which are designed to slide on bearings at the top of the wrought iron towers. At each end, the chains pass over abutment saddles supported on cast iron pedestals and housed in ornamental cast iron casings before passing down sub-surface tunnels to the anchorages.

Skelly and DfT engineering director David Coles outline the challenges of repairing the bridge, including replacing the hangers along with the seized bearings on all four pedestals and on both towers. 

The bridge deck must also be refurbished, the pedestals require strengthening and the pedestal casings must be overhauled. 

Returning Hammersmith Bridge to its former glory will be a huge feat of engineering

Coles says that in total “17 major defective elements need addressing before the bridge can be fully opened”, with almost every part of the structure in need attention. The exact list of defects is a closely guarded secret, with the council refusing to release engineering evaluations on the grounds of national security. 

What we do know is that the main problem is that steel roller bearings between each of the four saddles and pedestals at each end of the bridge have seized up. 

As a result there are unbalanced thermally induced cable forces on either side of the saddles, which have caused the pedestals to crack.

Concerns that the bridge could collapse were raised after cracks were found in three of the four pedestals that support the suspension cable saddles at the abutments. 

The discovery that one of these cracks had widened led to the full closure of the bridge in August 2020. 

A report compiled by consultant Aecom for the DfT taskforce reveals that the crack in question was within the north east pedestal and was approximately 160mm to 240mm long. Further investigations by Mott MacDonald for the council found 17 cracks within three of the four pedestals; seven within the south west pedestal, six in the north east pedestal and four in the north west pedestal. 

The cause of the cracks was related to the rollers between the pedestal tops and the underside of the chain saddles being seized

The Aecom report, commissioned by the DfT after the bridge closed in August 2020, concludes that the “cause of the cracks was related to the rollers between the pedestal tops and the underside of the chain saddles being seized”. 

It adds: “In the original design of the bridge, the roller bearings accommodate temperature changes by movement along the top of the pedestals. The pedestals were designed to withstand a force normal to the top surface applied via the rollers from the saddle. However, as the roller bearings have seized, this results in a restraining force being applied to the top of the pedestal, leading to high shear and bending stresses in the pedestal castings for which they were not designed.”

Mott MacDonald’s short term stabilisation plan has recently been approved by Hammersmith & Fulham Council. The initial plan is a short term strengthening programme to get the bridge open. It centres on replacing the seized roller bearings on the abutment pedestals.

Seized bearing replacement

Work to remove the architectural casings around the pedestals is now underway. Once they are removed, contractors will fit temporary frames to provide restraint while the seized bearings are replaced. The deviation saddles over which the chains pass at the pedestals will then be jacked up from the pedestals to facilitate replacement of the roller bearings with laminated elastomeric bearings. Mott MacDonald’s stabilisation plan is earmarked to cost £6M and is due to be completed in under a year.

Strengthening the Victorian structure’s pedestals is an equally tricky job, especially considering its Grade II status. The pedestals were cast to form hollow cellular boxes. Each comprises three longitudinal webs with linking end plates and two intermediate diaphragms. The webs have full-height stiffeners. The upper plate has a machined top surface for the rollers and the bottom plate is fixed to foundation stones via 12 vertical bolts. 

The pedestals are made from grey cast iron using sand moulds. There are elongated openings in each of the faces except the upper plate, largely to facilitate the casting process and permit the sand mould to be removed after casting. 

Aecom’s report adds that “these would have been large complex castings for the time and it is evident that defects and discontinuities were formed in the material during the casting process.” 

When stabilisation work is complete, longer term – more expensive – repairs will become the priority. While the council is unable to release a complete list of work that needs to be carried out yet, it is keen to stress that permanent repairs must not be put on the back burner after the initial stabilisation.

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