The stadium's owner ordered the installation of the propping system after Cardiff City Council refused to issue a safety certificate. This is needed to allow the middle tier of the Laing-built stadium to be used for the concert by Welsh band Manic Street Preachers, which is expected to sell all its 62 000 tickets.
Council officials had queried the ability of the stands to withstand the loading of hundreds of rock fans jumping up and down at the same time. As Building revealed on 3 December, there were concerns that this would cause the cantilevers to vibrate, making the fans panic and scramble for the exits.
The owner, Millennium Stadium plc, said it was confident that the £250 000 propping system, which is being installed through Laing, will satisfy the council and make the stadium safe for the Manic Street Preachers concert.
So far, 52 000 tickets have been sold, but the middle tier needs to be available before the remaining 10 000 can be sold.
The props are made from hollow-section steel tubes and will be winched down from each of the 74 sections of steelwork supporting the fourth level of the stand. However, one project source said not all 74 would be needed for the new year's eve concert because the stage will obscure part of the stand. The props will also not be needed for sporting events, when they will be winched up to the underside of the cantilever.
Meanwhile, a row is brewing between the stadium owner and Laing over who will foot the bill for the propping system.
Stadium manager Bob Evans said the stadium would pay initially. But he admitted that there was "a cost implication", which he said "the stadium would have to recoup".
A statement issued by the Millennium Stadium also said: "Laing has always known that there was an issue of structural stability in relation to the stadium. However, the perception of safety is something that the local authority raised, being concerned about the comfort of the spectators.
"Laing, with the local authority, has recommended that additional work be undertaken to address the perception of safety issues."
But a spokesman for Laing, which has already lost £26m on the £126m project, said: "The bill is being met by Millennium Stadium plc."
He added: "We delivered a stadium that met our contractual obligations. The props are only necessary to meet the council's requirement for a safety certificate."
Sean Jones, vice-president of stadium architect HOK+Lobb, said the problems were "a structural engineering issue … the stadium was always designed as a multipurpose venue – that includes concerts".
A legal expert close to the project said the contract documents were "an absolute mess" and that the situation could be hard to resolve.
Fears about stadia with cantilevered stands being used for non-sporting events were first raised in government guidance in 1997.
The guidance was based on advice by the Football Licensing Authority, which has now sent new guidance to stadium operators. This is understood to advise on the dynamic loading and amount of stiffness required for a seating deck not just to be safe, but to feel safe to spectators.
Commenting on the letter, Evans said: "By installing the props, we are ahead of current stadium guidance."