WEMBLEY STADIUM'S retractable roof will take three-quarters of an hour longer to close than originally planned because of fears over its safety when moving at design speed, NCEI has discovered this week.
Originally designed to take 15 minutes to close, it actually takes 56 minutes and 30 seconds for the retractable roof to move 100m to its final position on the steel supporting struts.
NCEI understands that concerns have been raised about the ability of the main northsouth trusses (see diagram) to cope with dynamic loading generated if the roof suddenly stopped while travelling at its design speed of 0.11m/s.
The roof is being investigated as part of the testing and commissioning process before the stadium opens next year. Investigators have recommended that the speed of movement of the roof is slowed to 0.03m/s and only carried out when the stadium is empty.
The retractable element of the stadium's roof was designed to ensure the health of Wembley's 'hallowed turf'. The new stadium has a steeply raked bowl ranged close to the pitch. The retractable roof is needed to allow air and sunlight into the ground so the grass will grow.
Client Wembley National Stadium confirmed that engineers carrying out a risk assessment of the structure ordered the roof-opening speed to be slowed. 'This won't be a problem on FA Cup Final day as we will decide in the morning whether or not the roof will be open, ' said a spokesman.
However, he refused to comment on whether the slow roof would cause problems at other times, simply stating that: 'We were aware of the change to moving times and this formed part of our recent negotiations with Multiplex.' This refers to the settlement deal struck between the two organisations.
A Multiplex spokesman said: 'Technically the roof can close in 15 minutes.' But he refused to explain why investigating engineers had stipulated that it be closed four times more slowly.
At its design speed of 0.11m/s the roof would have moved at one 6.6% of walking speed. At its new rate of 0.03m/s, it will move at just 2% of walking speed.
The moving sections of the roof comprise a central cantilevered section and three sections at both the north and south ends of the stadium, supported by two trusses. It is feared that these trusses could fail if the roof stopped abruptly.
Dutch steel contractor Hollandia, which installed the moving roof, was unavailable for comment.
Structural engineer on the stadium was Motts Stadium Consortium, consisting of Mott MacDonald, Connell Wagner, Sinclair Knight Merz and Weidlinger Associates. The consortia is facing a legal battle with Multiplex over design changes to the stadium.
Stadium guru Stephen Morley, the consortium's structural design director until 2003, was unable to comment directly on Wembley.
However, he said: 'On any moving roof, if the interfaces between the elements involved are dealt with properly, it shouldn't be necessary to slow the speed.'