HAMMERSMITH BRIDGE in London will be closed to traffic for at least three months while damaged caused by last week's terrorist bomb blast is assessed and repaired, it was revealed exclusively to NCE this week.
The bomb exploded at 5am on Thursday morning and blasted a 600mm by 350mm hole through the web at the end of a transverse wrought iron beam. It was placed on the southern side of the bridge, at the connection between the first transverse beam and the main downstream longitudinal beam.
'They couldn't have put it in a better place, ' said Anvar Alizadeh, London Borough of Hammersmith & Fulham structural engineer. 'I don't think they wanted to bring the bridge down, but they did a good job of causing as much damage as possible.'
Engineers were only given full access to the site by police forensic investigators on Saturday. A brief emergency inspection on Thursday confirmed that the structure was not in danger of collapse.
However, it has now been revealed that the blast cut through a critical palm plate connection to the first hanger on the southern side of the bridge.
'The bomb has made one of the supports redundant, ' said Alizadeh. 'There is probably enough support for the dead load, but we don't intend to allow any live loads on to the bridge until we have replaced the support.'
Investigations into the structural stability of the bridge will continue over the next few weeks and include ultrasonic testing on the damaged beam to determine how best to carry out the repairs.
The closure is set to cause a repeat of traffic congestion caused by recent strengthening and resurfacing. Restriction started in February 1997 and the closure was only lifted at the start of this year.
Police believe the explosion was caused by a Semtex charge.
It punched a hole straight through the web of the transverse beam and ripped off a section of the riveted flange. The flange was also damaged, causing localised buckling of the web of the longitudinal beam. The explosion wrecked two redundant tie rods left over from an earlier strengthening.
Alizadeh also believes the blast may have shifted the longitudinal beam towards the river.
'We will need to get inside the abutment and look at damage to the pins, ' he said. 'Depending on the extent of the damage, we will cut off the damaged sections and replace them. The worst possible scenario is that we will have to replace the whole transverse beam, which will mean taking up the deck.'
But the priority for engineers is to find out exactly what damage has been caused by the blast.
Inspection specialist Sandberg will carry out ultrasonic tests to assess blast damage. This is likely to take some weeks to complete.
Roger Khanna, director of highways at Hammersmith, said the blast was confined by the side of the abutment, the ground and the timber baulks below the deck. 'The blast rolled across the bottom of the deck and blew out the plywood footway panels on the other side, ' he explained.
'This means there may be microstructural damage, such as microcracks in the wrought iron transverse beam right across the deck, ' he said. 'We will need to build up contours of damage.'
The damage is complicated by the laminar microstructure of wrought iron. Intense shock loading can cause it to delaminate, with no apparent damage on the surface. This also makes ultrasonic tests difficult.
'It's early days yet, but we anticipate having to take cores and doing an extensive series of material tests, ' said Khanna The bomb was also sited just 1m away from a gas main, which was shielded by the longitudinal beam.
This remained intact, although the lagging and casing of the gas pipe were ripped off.
'We were relatively lucky, ' said Khanna, 'Wrought iron does not fragment and form shrapnel in the same way as cast iron - cast iron shrapnel might have punctured the gas pipe.'
Hyder Consulting, designer of the strengthening work which was carried out on the bridge and completed last year, will be brought in to design the repairs.
It already has a finite element model for the bridge which is expected to speed design work.