Case Study

PERMANENT ASSET VARIOGUARD® SYSTEM

AVONMOUTH BRIDGE

ASSET VARIOGUARD® has been used as a permanent central reserve vehicle restraint system on the Avonmouth Bridge. The ASSET VARIOGUARD® barrier helped reduce installation time considerably and provided a more cost-effective whole life solution than other barriers.

The specification and installation of a new central reserve barrier for the bridge presented potential problems for the Highways Agency and its appointed consulting engineer Hyder Consulting.

As a consequence of incorporating an extra lane in each direction on the one-mile-long bridge, the central reserve barrier needed to be offset from the crown of the old bridge, which corresponded to the centre line of the bridge. This meant that there was a difference in levels between the lanes on either side of the central reserve barrier.

Initially a traditional double-sided box beam barrier was considered, but this method of construction would have left the top of the post exposed adjacent to the lower carriageway.

The Highways Agency, whilst looking at alternative options, invited ASSET International to present a solution at one of its regular Value Engineering meetings, using the ASSET VARIOGUARD® vehicle restraint system to overcome the problems with Open Box Beam safety fence.

During meetings with Hyder Consulting, Costain (main contractor) and Barrier Services (ASSET VARIOGUARD® installer), potential issues with design and installation were highlighted. With input and advice from all the parties concerned, solutions were agreed.

When used as a temporary barrier, ASSET VARIOGUARD® only needs anchoring at each end no matter how long the length. On the Avonmouth Bridge, however, due to the narrow width of the central reserve, the deflection with this system at 70 mph was too great.

To limit the ASSET VARIOGUARD® deflection for this permanent installation, engineering calculations showed that by anchoring the ASSET VARIOGUARD® every 20m, the deflection would be reduced enough so that the system could be accommodated within the 1980mm central reserve. After careful planning and discussion with the Highways Agency, a crash test was carried out using steel fixing to anchor the barrier to a simulated concrete bridge deck.

A dynamic displacement of only 560mm was recorded, using a 1.5 tonne vehicle hitting the barrier at a 20 degree angle at 70 mph (BS EN 1317:1998 TB32 test). This required only one M30 x 150mm stainless steel fixing every 20m, compared with four M20 bolts every 2.4m if the double-sided beam barrier had been used. Another design challenge which had to be met was to devise a way for the barrier to cope with the huge 900mm expansion joint on the bridge. A special ASSET VARIOGUARD® unit was subsequently developed by ASSET for installing across the expansion joint on the bridge. A rubbing rail was also manufactured specifically to cope with this expansion.