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Taking a bow

Structures - A lightweight concrete roof for the Serbian national stadium is still innovative despite a 13 year delay in completing the project. Words and pictures by Adrian Greeman.

The Serbians get as excited as any about sport, and in the southeast corner of Europe that means basketball above all. Their team has a good record and hopes to show off its skills when Serbia hosts the European Championships this summer.

They will be showing off, as well, the gleaming silver-grey box which is the new national stadium for indoor sports. The Belgrade Arena will be finally completed in time for the event, after long delays during the 1990s.

Belgrade University professor of civil engineering Zivota Peri-i is as enthusiastic about its free spanning concrete roof as about basketball. This shallow inverted dish makes use of lightweight externally prestressed beams which give a span of nearly 133m by 103m without any internal supports.

An even bigger version, with a 220m span, may soon be built in Alma Alta in Kazahkstan, for its famous open ice rink.

Peri-i has been waiting more than 10 years to see his project through to the end.

'It is a sorrowful story, ' he says. 'It began in the early 1990s with an international competition for a new hall to host the 1994 World Championship, won by contractors Energoprojekt and Napred working with the University on the design.' But national disintegration intervened and the championship was withdrawn - an unfair punishment the Serbs believe - which delayed the construction.

Nevertheless the roof system devised by Peri-i was built in the two years from 1992 to 1994 when the project got under way.

It comprises a grillework of lightweight composite trusses, three longitudinal and four spanning across, linked together at the nodes.

Each truss is a double chord, concrete above and steel tendons beneath, behaving rather like a bowstring arch.

Each arch rises to 8m above the horizontal at mid-point. Arching action is enabled by pre-tension in the steel tendons and by intermediate support, provided by concrete leg spacers at the nodes. These account for catenary sag in the cables.

The trusses are obviously much lighter in weight than normal internal prestressed beams explains Peri-i. 'You get a huge advantage when you can space the cables away from the concrete because the leverage is huge; the efficiency when the distance is 10 times the size of the concrete section is 10 times.' The concrete spacer legs in the nodes help to transmit the forces from the concrete arches to the tendons, he says. 'It is as if you had 12 columns under the roof.' Of course external stressing has been used before, he agrees, but in the case of the roof the trusses have to interact as a complex structure, particularly at the crossover points.

The calculations were complex: Live loads can be large. Serbia gets its fair share of snow in the winter and the roof has to support up to 100kg/m 2 over an area of 15,000m 2, possibly in high winds. The flexible structure will move a maximum 170mm under snow, he says.

'The advantage is that you can handle the load. There were ideas at the beginning to have miniature snow ploughs or perhaps heaters on a more conventional roof but they were ruled out.' Another option would have been to use steel, he agrees, but Serbia does not have a big enough steel industry and would have needed to import, at some cost. This solution is aimed at those places which use concrete for economic reasons.

Construction requires care.

The roof has to be stressed with the loads balanced on the various cables - ideally 32 jacks should be used together. 'But we only had eight which meant working round in a repeated sequence.' The roof was prefabricated in 43 differently shaped pieces weighing from 60t to 100t.

These were assembled on a framework just 5m above ground - enough to clear the cable sag - and the whole stressed before strand jacking lifted the complete framework on 12 side columns which give the main support.

Once erected the roof supports 500t of lighting and sound equipment while a more conventional concrete structure forms the lower stadium.

A special ventilation system controls humidity in the open roof space to prevent corrosion of the tendons which are also sleeved and protected. And the other potential hazard - fire - will be controlled with a Swedish mist-burst sprinkler system that produces a high pressure spray of very fine droplets. 'Tests show it will stop a fire in one minute, ' says Peri-i.

The arena has variable seating to allow for a variety of indoor sports from boxing to, it is hoped, ice skating, although money for the rink is not yet available.

Meantime the Serbs have their fingers crossed for the basketball team.

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