INVESTIGATIONS INTO the fatal failure of a suspended access platform on the Avonmouth Bridge near Bristol last Wednesday are focusing on the working methods being used on the bridge.
Four steelworkers were killed after plunging 25m from the temporary platform while carrying out work on Costain's troubled contract to strengthen the bridge.
The accident happened as the men were upgrading the pair of I-section platform support rails which run the length of the bridge. The platform was suspended from the rails by points either side of a 2m section that was being replaced (see diagram).
A specialist access consultant working on the site when the accident occurred suggested to NCE that inadequate tethering allowed the gantry to move suddenly in a freak gust of wind. The supporting trolleys, he said, ran off the severed beam and the four unsecured men were spilled onto the south bank of the river Avon, just before midday.
Wind then carried the 5.2m by 1.5m, 4.5t platform, dangling from just two support trolleys, some 800m across the river where it crashed into scaffolding erected around one of the bridge's back span piers.
The platform was craned to the ground last Saturday to allow the Health & Safety Executive to begin inspections.
Costain and subcontractor Kvaerner Cleveland Bridge, which was carrying out the work, declined to comment on the accident. They said details about equipment in use and working methods could not be released until the HSE had completed its investigations. Detailed inspection is to be carried out at the Health & Safety Laboratory in Sheffield.
Three of those killed worked for Kvaerner and the forth was from Scottish firm Motherwell Bridge. Kvaerner has now been prohibited by the HSE from carrying out work using an identical access platform on site until findings of the investigation have been reached.
But experts in rope access and supplier-operators of aerial platforms this week insisted that the simplicity of the platform, and of the system being used to suspend and operate it, left little room for mechanical or structural failure. Early reports from the site suggested that equipment failure played no part in the tragedy.
The four point suspension system and simple design of the platform, which is enclosed by steel hand rail, and toe and knee boards, meant workers were not required to wear safety harnesses. None of the Avonmouth men was attached to the bridge or cradle when it collapsed.
The bridge carries the M5 motorway across the river Avon. The complexity of the strengthening job has already extended completion from May 1998 to March 2001. Costs have rocketed from the original £50M tender price to £125M (NCE 19/26 August).
The rails were being upgraded prior to installing a permanent inspection gantry on the bridge. The platform being used was a temporary structure but was suspended via similar beam trolleys to those used on the permanent platform. Sections from both rails had been removed for renewal.
It is understood work started at the southern end of the bridge and had been progressing towards its centre. The bridge arcs over its length, with its mid-point higher than its ends, and work was advancing against this gradient. The accident happened about two-thirds of the way along the southerly back span, near the flattest part of the bridge.
The access consultant who spoke to NCE on site believed that workers may have secured the platform to one of the bridge's cross-beams via a diagonal rope to prevent it rolling down the slope. But, he said, if they had not secured the platform in the opposite direction it would have been free to move up the gentle slope under wind loading.