Pre-cast concretePre-cast concrete,

Pre-cast concrete
Pre-cast concrete, like steel, has the advantage over in situ concrete that structural members can be prefabricated away from the site well in advance of site possession, thus greatly reducing construc- tion time. This is important when construction must be planned to minimize disruptions to the season’s fixtures. Such is often the case with sports stadia in the UK, which must to be constructed in phases during the few months between one season and the next. This practice is not yet common in the USA where stadia tend to be constructed in one oper- ation, but even here stadia owners are beginning to experience the difficulties Europe has had for decades, of clubs not wanting to move from the sta- dium while necessary redevelopment or upgrading work takes place.
Pre-stressed and post-tensioned concrete
Pre-cast concrete is widely used for the tread and riser units which form the seating platform. These stepping units are often pre-stressed so that they can be thin and light, but their jointing can cause problems. Choice of materials and detailing of junctions is as important here as for roof construc- tion, particularly if the spaces below are to be as usable rooms. Waterproofing especially will be very important.
When using pre-cast framing, thought should also be given to the future uses of the stadium. If stadia are used for pop music concerts the spectators can set up rhythms in time to the music which may affect the structure, and some stadia in Germany have had to be temporarily propped for concert use to com- pensate for this phenomenon.
Both pre-stressing and post-tensioning are useful techniques in stadium construction – pre-stressing for reasons of lightness, and post-tensioning for offering a reduction (or even absence) of move- ment joints through the structure. This is an impor- tant advantage because many stadia are 100m or more in extent, and normally such a length of con- crete structure should be built as two independent units with a clear expansion gap between. In a post- tensioned structure the thermal movement still occurs but is greatly reduced by the enormous tension placed on the reinforcing rods. The entire
structure is held together like a string of beads pulled together by a thread – the threads in this case being steel rods stressed to a hundred tons or more.
5.3.4 Steel
Steel is cheaper in some parts of the world than concrete and it allows prefabrication off-site, which can be a great advantage for the reasons given above.
It is of course lighter than concrete, both physically and aesthetically. This offers functional advantages, such as cheaper footings on bad soil, and the pos- sibility of slender, graceful structures. Steel is an obvious choice for roof structures, two examples of excellence being the Olympic Stadium in Rome and the Prater Stadium in Vienna, both of which were given new steel roofs over the existing stadia.
Fire regulations will probably require steel mem- bers below the roof to be fire-proofed by encasing, spraying with mineral fibre or vermiculite cement, or by thin-film intumescent coating (which detracts least from the appearance of steel profiles). This could cause steel to lose its cost advantage over concrete. But fire safety regulations are changing as the emphasis shifts towards ‘fire-engineered’ solu- tions, and unprotected steel seems likely to become more widely acceptable provided certain additional measures are taken. These include:
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Ensuring that people can escape from the stadium within a defined time, as discussed in Section 14.6.2, and reach safety well before structural fail- ure commences. This should not be a major prob- lem because the main danger with fire in stadia is not that of structural collapse but of smoke suf- focation. The Valley Parade stadium in Bradford, where 56 people died and many were badly injured in a fire in 1985, had an old pitched roof which effectively contained the smoke and flames. Installing a sprinkler system. New UK fire regula- tions do allow unprotected steelwork if sprinklers are installed, a concession that may in some cases tip the economic balance from concrete to steel in the UK. On the other hand the effects of a sprinkler downpour on an already panicking crowd have yet to be seen.