# Transcript

NEIL OLIVER
Edinburgh reaches over the Firth of Forth with two great bridges. While the famously robust Victorian rail bridge is the more photographed, the slender sweep of the 1960s road bridge behind it carries hundreds of thousands more people across the water. But there could be a ticking timebomb in the design of this suspension bridge and others like it around our coast.
Alice Roberts discovers the Forth Road Bridge’s hidden weakness.
ALICE ROBERTS
The most important parts of any suspension bridge are the cables that suspend it. And here they are. These ones are sixty-one centimetres in diameter and they stretch all the way from one end of the bridge to the other, and the roadway down there is literally suspended from them. It's a brilliantly simple idea but it may have a fatal flaw, and these cables are causing serious concern.
(Radio traffic broadcast) Inside Queen Street there’s queuing traffic heading towards the Forth Road Bridge, drivers should expect major delays.
ALICE ROBERTS
The cables that hold up the roadway are gradually corroding. Twenty-four million vehicles a year use this bridge. The nearest alternative crossing means a forty mile diversion. If you took away this bridge tomorrow, it’d be an economic disaster for the entire region.
Forth Road Bridge Master, Alastair Andrew, is trying to keep the bridge going.
ALICE ROBERTS
The traffic across the bridge is pretty relentless.
ALASTAIR ANDREW
This is one of our quietest days, believe it or not.
ALICE ROBERTS
Really?
ALASTAIR ANDREW
Yes.
ALICE ROBERTS
So, Alastair, what is the problem with these cables?
ALASTAIR ANDREW
Well, the problem is that water has found its way into the cable and has allowed rust to take hold inside the cable.
ALICE ROBERTS
But surely you waterproof them, they’re outside all day?
ALASTAIR ANDREW
Of course. There are several layers of protection and that’s why we’re so surprised because that waterproofing hasn’t worked.
ALICE ROBERTS
If you’ve got corrosion inside the cable, that’s weakening it from the inside?
ALASTAIR ANDREW
It is weakening the cable but the cable is perfectly safe at the moment. But, if we cannot stop that corrosion, the predictions are that we will have to stop heavy goods vehicles using the bridge in 2014 and, ultimately, close the bridge by 2019.
ALICE ROBERTS
So you’re in a bit of a race against time then to stop this corrosion?
ALASTAIR ANDREW
It's very much a race against time now.
ALICE ROBERTS
To see why the cables are so vulnerable, I'm going into the very innards of the bridge with Keith Perryman who was Inspector here for nine years.
ALICE ROBERTS
So Keith, there’s thousands of tons of roadway suspended up there?
KEITH PERRYMAN
Yeah, and in order to suspend that steelwork, we need two cables across four towers, anchored at both sides of the estuary into good, firm, solid rock, which is where the cable comes down into the anchor chamber over here.
ALICE ROBERTS
Following the cable inside reveals what it's really made of.
KEITH PERRYMAN
This is where we see the cables for what they really are and that’s loads and loads and loads and loads of wires.
ALICE ROBERTS
There must be thousands?
KEITH PERRYMAN
There’s in excess of eleven and half thousand wires.
ALICE ROBERTS
Wow! And each of these wires is actually quite slender. I mean it's about five millimetres in diameter.
KEITH PERRYMAN
Yeah, just under five millimetre in diameter.
ALICE ROBERTS
This is what’s holding the bridge up?
KEITH PERRYMAN
Yes, without the wires, no cable, without a cable, no bridge - this is it.
ALICE ROBERTS
A corroded or rusty wire is weak. Any one of the 11,500 wires in each cable is liable to snap at any time anywhere along the whole length of the bridge. The corrosion of the wires is a slow death sentence.
ALICE ROBERTS
Well, down here underneath the roadway, you get a really good idea of what it is that’s suspended from those cables; the sheer weight of all this steel and the roadways themselves and this traffic thundering past.
Wow! Every time a lorry goes by, the entire thing shakes, and all of this is suspended in mid air, sixty metre above the waters of the Forth.
So is there a solution to the corroding cables holding all of this up? Without knowing how fast it's weakening, the bridge team’s working blind. Somehow, Bridgemaster Alastair Andrew needs to get to the heart of the problem.
ALICE ROBERTS
But, Alastair, how can you know what’s going on inside those cables because presumably you can’t open them up?
KEITH PERRYMAN
No, absolutely, we can’t open up the whole length of the cable. The only way to do this is to actually listen to the cable. And what we have here are microphones which are attached to the cable.
ALICE ROBERTS
Which are there?
KEITH PERRYMAN
That’s the microphones there and they’re listening for any wire breaks that may occur inside the cable.
ALICE ROBERTS
So that’s like a stethoscope listening out to the health of the cables?
KEITH PERRYMAN
Exactly, the difference being that we have fifteen microphones placed over the entire length of each cable and we’re listening all the time.
ALICE ROBERTS
Continuously, twenty-four hours?
KEITH PERRYMAN
Absolutely.
ALICE ROBERTS
The microphones began their round-the-clock vigil in August 2006, and straightaway the computers began to pick up strange sounds hidden in the background noise from the traffic.
(Clicking sound)
These innocuous sounding clicks are actual wires snapping. So far the bridge has lost around ten percent of its strength. At that rate, the bridge has only got thirteen years left.
But there might be a way to save the bridge. The plan is to inject dry air into the cables to stop the corrosion. It’ll be the first time in the world it's been attempted, and there’s a big dilemma. By the time they find out if the dry air’s worked, it’ll be too late to build a new bridge. There’s a lot at stake.
KEITH PERRYMAN
Given that we’re handling twenty-four million vehicles a year, it's quite impractical to consider that traffic diverting on a mile detour. So it will have a major impact if a new bridge is not provided before we have to consider closing this one.
ALICE ROBERTS
And people around the world face the same prospect. Cable corrosion is affecting bridges in the US and Europe. But closer to home, it's even more severe, the old Severn Bridge is urgently having to tackle the same problem, and others may follow. We may soon have to rebuild bridges across the coastal barriers we thought we'd conquered.