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They may not be aware of it But visitors to US architect Rafael Vi帽oly鈥檚 spectacular 拢90m Regional Performing Arts Centre in Philadelphia will have to pass beneath a row of massive cast-iron weights dangling 3 m above the ground to enter the building. The weights form part of an innovative solution devised by the building鈥檚 structural engineer, Dewhurst Macfarlane and Partners, to support the building鈥檚 two massive glass facades.

The facades form the end walls of the centre鈥檚 huge barrel-vaulted glass roof, which unites a 2500-seat concert hall and a 650-seat theatre. It will have a semi-circular profile and will be clad entirely in glass, creating a distinctive, 50 m diameter transparent envelope sheltering the arts complex and a pedestrian plaza below.

Vi帽oly wanted the semicircular facades at either end of the vault to be made of glass to maintain the feeling of light and space. But not only were the facades to be transparent, Vi帽oly wanted them to be 鈥渨ithout thickness鈥�. This presented the structural engineer with a major design challenge, and one that had to be resolved urgently to meet the construction programme.

The engineer's problem was that, in addition to supporting the massive static weight of the facades鈥� glass panels, the structure has to resist the huge dynamic loads imposed on it by the wind.

The solution would normally be to construct a rigid steel structure behind the facades, with supports running horizontally and vertically, but the architect's brief prevented this. Instead, Dewhurst Macfarlane is supporting each facade from a series of 22 mm diameter high-tensile steel cables hung from a slender supporting arch.

The arch will be built as a separate structure because the barrel-vaulted roof is expected to move by up to 100 mm under wind or snow loading. If the loads from the roof were transferred to the arch, the arch would deflect. This could shatter the facades, showering glass shards on the concert-goers below.

Although the arch is to be built separately, it needs to be braced laterally from the main barrel-vault roof. The bracing ensures that no loads are transferred from the main roof to the arch, but allows wind loads from the facades to be transferred to the roof.

The facades' supporting steel cables are hung centrally from the arch and are equally spaced across the semicircular opening. A cast-iron weight at the foot of each cable keeps the cables under tension; the heaviest weighs 9 tonnes. The cables' lengths vary so that the weights form a horizontal line suspended across the bottom of the glass wall.

The cables are attached at the top only, where they are connected to the arch. At their foot, they are unsupported, allowing them to move up and down, but they are prevented from swinging by lateral restraints. The glass cladding is then hung from the cables.

When the wind blows, it exerts pressure on the glass cladding. These forces would be resisted by a rigid structure, but the scheme does not have one, so the wind causes the facades鈥� huge surfaces to bow inward or outward by 760 mm.

As each facade bows, it drags the supporting cables with it, lifting the weights. Because the weights move, only the static weight of the facade is transferred to the arch, so the downward load on the arch remains constant. However, as the facade bends, the change in the cables' geometry means that they pull horizontally at the top and bottom, transferring the wind loads laterally via the arch and restraint arms to the barrel-vault roof.

The engineer can control the deflection of each facade by increasing or decreasing the weight on the cable. This allows some delicate adjusting to take place.

The glass cladding is attached to the cables using specially designed stainless steel clamps. The clamps attach to the corner of each of the 1.7 脳 1.2 m toughened glass cladding panels at nodes. These nodes support the glass while allowing it to swivel as the facade moves.

However, even though the middle of each facade moves 760 mm, the area of facade is so large that the movement of each panel relative to its neighbour is actually very small. A flexible, structural silicone sealant around the edge of each of the glass panels stops rain and snow entering the space.

This is the first time such an innovative facade has been built, and Dewhurst Macfarlane is taking no chances. A form of computer modelling 鈥� non-linear finite element analysis 鈥� is being used to model the loads while taking into account the change in geometry as the facades deflect.

A wind tunnel verifies the calculated wind pressures using a 1:50 rigid scale model and an aero-elastic model that deflects as it interacts with the wind. The tests will direct wind from all possible directions to check that the glass walls behave as predicted.

Work on the arts centre is due to start next year, so Dewhurst Macfarlane will not have to wait long to see the fruits of its labour.