Transparent acrylic glass cladding on the dramatic cable-net roof of Munich's Olympic stadium, is being stripped off and replaced because nearly three decades of weathering has turned it opaque. Glistening spidersweb roof sections have deteriorated to ugly patchwork quilts.
The operation presages work that will be needed in future to refurbish London's Greenwich Millennium Dome when the fabric covering now being erected there reaches the end of its life.
Since the 1972 Games, many of the roof panels on Munich's stadium and other cable-net structures in the surrounding Olympic park, have weathered to a cloudy whitish finish.
There is also a significant maintenance problem with the main structures. Corrosion protection is peeling away in places from vital cables and connectors which form the tent-like structures of the roofs to the stadium, hall and swimming pool.
Principal work is now concentrated on the stadium where a DM26.5M (£9M) refit by a joint venture of specialist contractors began last August. The JV, including cladding specialist Koch Hightex, has to strip and replace the damaged weather protection and replace all the roof panels in time for a major concert in July.
Preliminary work is also under way on the Olympic hall, swimming pool and ancillary structures. But here the main contract has yet to be let.
Detailed photographic surveys using helicopters preceded the stadium repair contract which will restore the roof to its original appearance.
Basic structures of the roof are tensioned nets of cables slung by cable stays from a series of pylons and tensioned via massive guys to foundation anchorages. The covering consists of polymethyl methacrylate panels (PMMA) varying in size up to about 2m square.
Client Olympia Park Mnchen decided to renew the roof using exactly the same PMMA material as the original and the panels are again being manufactured by German specialist Rhm of Darmstadt. They are framed with aluminium channelling attached to the cables by a combination of metallic clip wedges and bolts secured via bonded rubber mountings.
Other principal contractors in the JV are Rheinhold & Mahla Fassadentechnik, Bihler + Overnieder Dach + Fassade, and Fassanden Waagner Biro.
Replacement of the corrosion protection is being carried out by specialist contractor Vogler from Cologne. Wherever the original coatings are damaged, operatives are stripping the structure back to bright metal using compressed air grit blasters.
Access is achieved from scaffolds built up from the ground or slung from the roof itself. Temporary tents, assembled around the areas being stripped and repainted, protect repairs from the weather. Repair work is concentrated around the complex nodes of the net structure at its peaks and at points where the ground anchored main guys are yoked into the network.
Koch Hightex is the same company which was originally engaged to supply polyvinyl chloride coated polyester fabric for the Greenwich Dome and is now suing client the New Millennium Experience Company for breach of contract.
While the final Teflon-coated glass fibre fabric, eventually chosen for the dome roof, bears no resemblence to Munich's PMMA panels, the supporting cable network is very similar in detail. Design life for the cables and their connections is about 25 years and, as it now seems likely the dome will remain at Greenwich for much longer than that, extensive and expensive roof repairs can be expected in about 2020.
Work on the Munich stadium is highly labour intensive. Dozens of Koch Hightex operatives are at work, scrambling across the top of the steeply sloping roof structure, many seemingly without harnesses of hard hats.
Nets have been slung beneath the work front to catch any that slip or fall through the voids left where the fairly rigid, window-like plates of acrylic have been unfastened from the roof.
A tower crane and large, telescopic mobile are servicing the difficult to access work site - hoisting operatives in a cage then lifting down the deteriorated panels and supplying pallet loads of new replacements.
Each new window panel has to be clipped in place with its perimeter wedge pins. It is then drilled insitu so that the security bolts can be threaded through the acrylic and fastened to intermediate rubber mounting blocks.
The original rubber guttering which seals the approximate 100mm gaps between the aluminium perimeter frames of each window has deteriorated in places to brittle shreds of material. It is being replaced totally with new strips which have to be vulcanised together insitu.