The modular aluminium platform at the centre of the accident was manufactured by US firm Beeche Systems, supplied by UK firm Technitube and installed on the Avonmouth Bridge by Kvaerner early in 1996. It was one of two being used on site when the accident occurred.
Beeche Systems founder Gregory Beeche said it was not uncommon to tether platforms with four steel ropes attached diagonally to the bridge's cross beams.
He explained that the system, known as storm lashing, uses the ropes tensioned against one another to prevent longitudinal and lateral movement, and also reduced torsion and wind-induced oscillation. Anchors can be used as back up against failure in suspending members.
The system allows the platform to be moved by taking in one pair of ropes and letting out the other. Ropes should be repositioned one at a time as the platform is manoeuvred. But as it is used in tandem with the platform's primary suspension system, and with three ropes fixed at all times, safety should not be impaired, Beeche said.
Beeche described the method as 'robust', a view echoed by UK firm Alps and the Suspended Access Manufacturers Association. Nonetheless, experts expressed unanimous surprise that no additional restraining mechanisms appear to have been used.
'Without exception, when you park a platform you should secure it against wind load, let alone gravity,' said Beeche.
Platforms are commonly fitted with alternative safety devices, including ratchets or clamps to lock the beam trolleys in place or clamps that can be fitted onto a rail either side of the trolleys.
Nobody consulted by NCE could explain how, even with just one set of diagonal ropes, the platform could have run free across more than two spans of the bridge after the collapse. Beeche also pointed out that engineering analysis, including the effects of climatic conditions, was carried out as standard procedure when platforms were supplied. In the US, he said, a safety factor of 1:8 is built into a platform and operating system and should be re-analysed if reconfigured.
He claimed that Technitube would have carried out rigorous analysis against historical records when it won the order to supply and maintain the platforms.
A spokesman for Technitube said it had supplied risk analysis and method statements for using the platforms when they were installed. He stressed that method statements would have been revised if operation or the configuration of a platform changed.