VANDALS FORCED the closure of the M60 motorway near Manchester last week after attacking a cable stay footbridge with a hacksaw.
They cut some of the 5mm thick strands making up five of the structure's the backspan cables, said Highways Agency Area 10 performance manager Roy Wood. Broken blades were found nearby.
The Mersey Valley footbridge is a cable stay structure with a 74m main span and a 28m back span. It spans the motorway plus two slip roads.
Its deck is supported by 20 cables. These form two groups of fire fanning out from the pylon to each edge of the main span plus two more groups to the back span.
Police closed the M60 between junctions 7 and 8 on 3 January after the damage to the footbridge was reported by a member of the public.
The motorway reopened following a Highways Agency inspection.
Other Highways Agency areas have been alerted of the incident so a nationwide check can be carried out.
Mersey Valley footbridge will remain closed until a detailed inspection can be carried out by bridge designer Halcrow.
'Detailed calculations are under way, ' said Wood.
Each cable comprises 84 high tensile wire strands, giving an ultimate tensile strength of 1,770N/mm 2. The cables provide a safety factor of 2.5, Wood said - the bridge was designed with structural redundancy to allow cables to be removed and replaced one at a time.
Until the structural assessment is complete the bridge will remain closed to remove live load, which Wood calculated to be in excess of 20% of total loading.
Wood said that the 'senseless act of vandalism' was probably the worst in the Agency's history, but that wilful damage was becoming more commonplace.
Wood said that 'metal sleeves round the cables may be used [to prevent repeat attacks], isolating the cables from public access.' Fitting mesh to the parapets to deny vandals access to the cables is another option.
Fitting the cables with acoustic monitors that can set off alarms could also be used to detect future attacks, Wood said.
A similar vandal attack on a bridge in New York was identified by this method in 2000 (NCE 13 July 2000).