PERI REACHES THEPYLON TOP

Central to the construction of the œ300 million Second Severn Crossing has been the construction by Anglo-French joint venture contractor Laing-GTM of two identical pairs of 137m-high concrete pylons. With wind speeds reaching 180km/h, the supply of labour, plant and equipment to this particular part of the project - now almost completed - looked distinctly daunting.

Contractors Laing-GTM hit upon a solution from Peri, one of the world's largest formwork manufacturers. Peri offered the contractors an automatic climbing system designed to withstand windspeeds of 180km/h. Peri's ACS75 system comprising 15t platforms driven by two hydraulic climbing units proved a more attractive option than slipforming.

The need to keep slipform shuttering constantly moving was incompatible with both the geometrical accuracy of the cable anchorages and the high density of steel reinforcement.

'The various angles of the anchorage guide tubes in the towers are so critical to the job that they have to take precedence. We could not have afforded to forsake that in order to keep construction going so we quickly decided not to slip through the anchorage zone,' said Second Severn Crossing project manager, Norman Haste.

'We also looked at slipping lower sections of the towers but again, with reinforcement generally in excess of 400kg/m3 of concrete and in some places close to 600kg/m3, which is quite dense steel, we homed in on using totally independent climbing forms.'

Each heavily reinforced pylon is 4m wide with vertical upstream and downstream faces to full height. The opposite sides taper from 10.2m at the base to 5.5m at a height of 77m, just below the lowest cable stay anchorage point. Each tower has 80 precast concrete cable stay anchorage units to accommodate the composite steel and concrete bridge deck's 240 pylon cables. Each pair of towers is braced by two 1200t concrete cross beams.

After concreting of the lower beam's two ends had taken place, Peri's formwork retraction and climbing mechanism - pre-assembled on shore - was put to work.

Wrapped round the outside of the towers the two sets of Peri forms climbed independently in 4m shifts. Standard Peri shaft shuttering, handled by crane, was used for each of the pylon's inner walls.

The climbing shuttering, complete with six working platforms and Alimak hoist access floor, hyraulically jacks itself up on a series of climbing rails. These vertical rails are supported and guided on special brackets bolted to anchors cast into the tower wall. Climbing and securing each pylon's formwork before the next concrete pour takes around 30 minutes.

Platform climbing speed is 0.5m/min and is performed through a series of 640mm extensions and retractions of the sytem's hydraulically operated climbing jacks. Regardless of the different loads on each platform, the hydraulic control system automatically monitors the flow to each jack and synchronises the climbing mechanism to keep the platforms level.