Efforts to reopen the severely damaged A4 Hammersmith flyover have been hampered by the complexity of the structure, NCE has learned.
Transport for London (TfL) has delayed for at least another week the hoped-for reopening because engineers still have work to do inside the bridge to assess the full strength of the structure.
This follows the revelation that it had a “serious structural defect” in the form of snapped prestressed reinforcing tendons, which hold the bridge together around the piers.
The four-lane structure in West London has been shut since 23 December when TfL revealed its concerns. TfL surface transport chief operating officer Garrett Emerson said until engineers know the full extent of the corrosion on the pre-stressed steel tendons, TfL could not give a firm date on when the bridge will reopen. It will remain shut until at least Friday, even to light traffic.
The 622.7m long precast segmental, prestressed concrete structure is held together by post-tensioned steel tendons that run along the length of the bridge. The 16-span, 18.6m wide structure comprises 3m long segments and varies in depth between 2m at mid-span and 2.9m at the piers. Each of the 96 tendons carry a force of up to 48t.
More tendons snapping than previously thought
Testing by TfL’s maintenance contractor Amey from September 2011 onwards revealed the number of tendons snapping was higher than previously thought.
As 80% of the load carried by the bridge is self-weight, TfL ruled out introducing restricted traffic measures and instead decided to shut the bridge until the extent of the damage could be ascertained.
“We discovered there was a steel duct full of water at the pier head and as a result some of the cables were completely severed”
Amey consulting managing director Andy Milner
Immediately prior to the decision to shut the bridge, engineers had decided to break out two sections of the tendons close to the pier head from the surrounding grout, which revealed they were in a far worse condition than previously thought.
“We discovered there was a steel duct full of water at the pier head and as a result some of the cables were completely severed,” explained Amey consulting managing director Andy Milner, this week. This problem is believed to be occurring at each of the 15 pier heads between the 16 spans.
The tendons squeeze the structure together and with the discovery of a number that had snapped engineers concluded that each individual tendon would have to be completely assessed so they can be sure of the true strength of the bridge.
80-strong team of engineers
University of Cambridge head of the structures group Chris Burgoyne told NCE that inspection must be thorough because the rate of corrosion can vary greatly between various parts of the bridge. “If you have loss in strength then you don’t know how much of the pre-tension is working,” he said.
The investigation is requiring a team of 80 engineers from TfL, Amey, expert contractors and leading structural engineers, who started their work over the Christmas period. Acoustic monitoring and x-rays have been used to assess the tendons.
This can help the team avoid breaking out all of the tendons to assess them, which Milner said his team was reluctant to do because the surrounding grout partly holds the structure together (see structure box).
In addition to the water induced corrosion, salt that had been sprayed on to the road − required because of a failed internal heating system − had caused corrosion problems that contributed to the tendons’ degradation.
As well as the investigations TfL has gone through iterations of its repair plans.
It had considered reopening the bridge using short term temporary props, after which additional post-tensioning tendons were going to be added to the structure for longer term strengthening.
Tension cables plan
However, TfL director of roads Dana Skelley told NCE that strengthening would now come from adding new tension cables to the structure straight away.
The bridge is on the Olympic Route Network connecting key participants between Heathrow and central London and TfL is confident the bridge will be fully operational by then.
“[The post-tensioning] should be in place before the Olympics,” added Emerson. “It’s far from ideal [to be designing and constructing major works in such as short space of time] and it’s a major challenge.”
Traffic is currently running beneath the bridge and Emerson said that engineers did not believe there was a threat of collapse.
Emerson also dismissed fears of commentators on NCE’s online stories that similar structures in London could be under threat from the same problem − for example, the 1960s, 2.5km long elevated Westway carrying the A40 out of north west London.
“[The Hammersmith flyover] is a rare and unusual structure,” he said.
Warnings from history
TfL director of roads Dana Skelley told NCE that the worst case scenario TfL is facing − although engineers do not believe it is likely − is the collapse of the flyover under its own weight similar to the disastrous collapse of the Ynys-y-Gwas bridge in West Glamorgan in 1985.
That was an 18.3m long segmental, post-tensioned I-beam bridge − a similar strengthening technique to that used on the Hammersmith flyover − that collapsed without warning under its own weight.
The cause of failure was the corrosion of the longitudinal prestressing tendons, as with Hammersmith.
A report by West Glamorgan County Council showed that in almost all of the joints between bridge segments, cardboard tubes had been used as spaces instead of metal tubes to provide continuity of the ducts.
This had failed to provide adequate waterproofing for the reinforcement and exacerbated corrosion.
Engineers had inspected the bridge regularly in the six years prior to collapse but there were no reports of any warning signs of corrosion of the prestressing cable.
History of the structure
Hammersmith flyover was the first UK structure to use precast segmental prestressed concrete, and was designed by Maunsell & Partners (now Aecom).
Construction began in 1959 and completed three years later at a cost of about £1.2M.
Construction began at the west end in 1959 and proceeded eastwards. Erection of precast units and prestressing were carried out during off-peak hours, but all other operations continued in live traffic. On site working space was limited to just an 8.5m wide strip.
The flyover is supported on a single central row of columns. It has 16 spans — 11 at 42.7m long, two of 36.6m, two of 28.8m and one of 22.6m — giving a suspended length of 622.7m. The approach ramps at either end increase the total length to 862.9m.
There is only one expansion joint, 64m west of the flyover’s halfway point, which can accommodate movement of up to 360mm.
The deck is 18.6m wide overall, with a 7.3m wide carriageway and a service footway on either side of the 1.5m wide median strip.
Threat of collapse?
TfL had been aware of the corrosion since it took over the bridge from the Highways Agency in 2000.
However, it played down the suggestion in the media that the current problems could have been foreseen or that decision to shut the flyover was an overreaction.
“We have to be sure [the bridge is safe],” said TfL surface transport chief operating officer Garrett Emerson, adding that he was aware of the seriousness of the consequences. “If the bridge fails, it fails in a big way,” he said.
Emerson added that it was difficult to see the extent of the problem because warning signs are difficult to detect externally.
“It was not until we broke out the tendons that we were proved right [about the extent of deterioration].”