Before Bierrum developed its patented access system it built large hyperbolic cooling towers from an internal scaffolding annulus. Contracts manager Colin Nodder says there were many practical drawbacks with this method.
Apart from the cost it was impossible to keep the formwork really rigid. And progress was slow, while dismantling the scaffolding afterwards was far from straightforward.
By the mid-1960s, after 25 years of struggling with miles of steel tubing, Bierrum had had enough. In 1965 it had a prototype of a radical alternative on trial on an existing tower, and in 1966 the contractor used the new system for the first time on the Rugeley B power station contract.
Such was its success that Bierrum adopted it as the standard method on all subsequent contracts. By 1977 it was being used to demolish a 92m high cooling tower, and many other applications have followed.
Although the chain blocks which provided the lifting power on the early rigs have long since been replaced, first by air winches and latterly by integrated hydraulics, the principles are still the same. Timber shutters are always used, for their greater flexibility, Nodder says.
And the shutters are independent of the access system so no concrete loads are transferred, he adds.
At the heart of the system are the two climbing leg assemblies which support each 4m to 6m long rig. Four support brackets are used for each leg, and each leg slides within a shorter channel section.
The cycle begins the morning after a pour when the concrete in the previous pour is at least 40 hours old. Shutters are stripped, steel fixed, and shutters re-erected using conventional fixings cast into the pour.
At that point all vertical loads from the access system are still being taken by the lowest pairs of brackets, which, like the two pairs immediately above them, are bolted to RMD screw anchors cast into the concrete wall. Pull-out cantilever loads are taken by the second-lowest pair of brackets.
After the lowest brackets are unbolted, the hydraulic rams come into action. These are connected between the bottom of the legs and the top of the guide channels. As they close, the legs are lifted some 1.5m higher and locked into their new position by automatic spring catches in the three highest pairs of brackets.
Reversing the rams moves the channel guides up to their new position. The uppermost bracket, which is supported by concrete at least 40 hours old, is still not carrying any significant loads, so the system has a significant factor of safety.
At Rugby there is no need for the unique shape-shifting capability of the Bierrum rigs, which can cope with the graceful curves of hyperbolic cooling towers. This is achieved by pin-jointing the walkway supports and inserting adjustable diagonal props beneath them to keep walkways level whatever the slope of the wall. As towers taper, so adjustable cantilevered extensions at each end of the rigs accommodate the changes in diameter.