Transport secretary Philip Hammond this week ordered a risk review of road and rail bridges, as engineers raced to prevent the collapse of a fire damaged M1 bridge.
Review of businesses under bridges
The review will look at how many other raised structures are at risk of fire from businesses underneath them.
The M1 bridge in north London was so badly damaged by the blaze last Friday that it was still partially closed as NCE went to press on Tuesday.
Last week’s fire broke out in a scrapyard beneath the 1964-built concrete bridge in Mill Hill, north London between M1 junction 2 and Scratchwood Services.
The concrete bridge suffered serious spalling that left its steel reinforcements exposed. Detailed assessments will not be made until temporary propping is completed and the motorway is reopened. At present the depth of spalling and damage to the steel is unknown, said prop supplier and installer Mabey Hire Services structural support and bridging director Chris Carter.
Props installed below deck
Mabey Hire Services, subcontractor Jackson Civil Engineering and consultant Atkins were working around the clock to install props below the bridge deck as NCE went to press. Atkins is part of the Connect Plus consortium, along with Balfour Beatty, Skanska and Egis. Connect Plus maintains this section of the M1 as part of the M25 DBFO contract.
Around 200t of Mabey steel props were being installed beneath the 36 pre-stressed longitudinal concrete beams forming the 45m wide bridge.
The propping structure is a series of spreader beams on the ground, on top of which sit Mabey Superprops at intervals of 1.5m, in rows of five.
Each Superprop has a loadbearing capacity of 240t. On top of these, large header beams lie horizontally and at right angles to the bridge’s concrete beams, supporting smaller, galvanised Mabey Mass 50 props. These are spaced regularly in line with the concrete beams, each of which is supported by five Mass 50s. Each Mass 50 has a 250mm² cross-section and can support loads of up to 500kN.
The propping system is roughly 5m tall and is designed to comply with full Highways Agency loading specification and British Standard BS 5400 − the standard to which all trunk roads and motorways and their bridges are designed. The standard applies to all bridges designed to take standard high loads plus exceptional industrial vehicles.
This means the reopened lanes are capable of taking all normal traffic loads − but while the full width of the road is not open a precautionary temporary 40mph (64.4km/h) speed limit is in place in both directions.
The Highways Agency said its priority was to install the props as quickly as possible to allow it to re-open the full three lane dual carriageway and hard shoulders in both directions, before undertaking detailed investigations of the damage and confirming what remedial work is needed.
As NCE went to press two northbound lanes were still shut as was one southbound lane and the southbound hard shoulder.
Damage assessment “could take time”
Consultant Structural Studies & Design director Jonathan Wood said this is the best approach, because the damage assessment could take some time.
“Once you’ve got control of traffic flow you can do the rather more difficult process of a full appraisal,” he said.
“Highway management should be the top priority.”
Lanes were being re-opened gradually as the propping system was completed below them. The northbound hard shoulder was reopened at 6:40pm on Saturday, followed by lane 1 on Sunday at 1:30pm. Southbound lanes 2 and 3 were re-opened on Monday at 4:05pm.
“Highway management should be the top priority”
Structural Studies & Design director Jonathan Wood
Northbound lanes 2 and 3 were next in line for propping and re-opening, while the southbound hard shoulder and lane 1 − believed to be the worst damaged part of the bridge − were to be addressed last.
Roads minister Mike Penning said that the fire was thought to have exposed the bridge to temperatures around 1,000ºC. Above 300ºC, steel begins to lose strength and concrete weakens substantially, said Sandberg Consulting Engineers partner Mike Eden.
Cracks and spalling assessment
Engineers are likely to assess the fire damage by taking concrete cores for assessment at the microscopic level to determine the maximum depth of spalling and cracks. Steel samples are also likely be taken and tested for tensile strength to determine how far the reinforcement has weakened, he said.
A Highways Agency spokesman said no decisions had been made on long term rehabilitation of the bridge because detailed assessments have yet to be made.
But Penning told the BBC: “Some of the [southbound] inside lane and hard shoulder are beyond repair and are going to have to be replaced.
“I’ve never seen concrete heat and expand as badly as this one. It won’t be back to normal for some considerable time.”