Balustrades of SentryGlas® offer clear, frameless retail shopping views
Around 2,000 free-standing panels of laminated glass, containing DuPont™ SentryGlas® structural interlayer, are used to form balustrades at the new Westfield London shopping center, Europe’s largest urban shopping mall, which opened for business on 30 October 2008. 20 percent thinner than other glass laminate constructions of equivalent performance, the glass balustrades, manufactured by UK architectural glass supplier Kite Glass, provide a safe and secure shopping experience for all visitors to Westfield, while affording them unobstructed views of the more than 280 shops located there.
Europe’s largest urban shopping mall
The Westfield London shopping center, a £1.7 billion development designed, constructed and operated by the Westfield Group, boasts 150,000 square meters (1.6 million square feet) of space over two floors – the equivalent of 30 soccer pitches. In addition to its retail units, Westfield London will contain a cinema (due to open in late 2009), nearly 50 restaurants, leisure space and a library – and is expected to attract over 20 million visitors per year.
Once inside the vast shopping complex, visitors will discover an atmosphere of spaciousness and calm, created in part by the influences of nature – water, light and earthy materials are recurrent themes in its architecture – and also by the integration of wide and expansive malls for a stress-free shopping experience.
It was as a reflection of the architectural vision for Westfield London, that CMF Ltd. of Feltham, Middlesex (UK), conceived the transparent glass balustrades and handrails for the first floor of the centre, its staircases and sections of the external causeway leading to Westfield London.
The glass panels for the balustrades were supplied by Kite Glass of Weybridge, Surrey, a leading supplier and installer of toughened and laminated architectural glass.
"The balustrades at Westfield London consist of around 2,000 free-standing, laminated glass panels (three quarters of which are flat, the rest curved), each measuring 1.1 by 1.3 meters and fixed by a cantilever support hidden in the flooring of the mall. There are no clips or connections between the panels apart from the handrail which is point-fixed to each of the panels as an additional safety measure for visitors, but provides no structural support to the balustrades,” explains Julian Clayton, senior design manager at Westfield. “Our decision to use laminated glass with SentryGlas® came out of discussions with CMF and Kite Glass, who advised us that its use would solve many important challenges regarding building control requirements, compliance with the fixing system and aesthetic appeal."
The structural benefits of SentryGlas® interlayers
Cantilever glass panels, when used as shopping mall barriers and thus exposed to crowd loading, are subject to the highest category of British Standards and Building Regulations with regard to their load-bearing capacity. In such instances, according to standards BS 6180 and BS 6399-1, they are required to deflect less than 20 millimetres when subjected to a linear load of 3 kN/m.
"CMF required a thinner, stronger and preferably more cost-effective solution than standard PVB laminates," says Leroy Reed, CEO of Kite Glass. "We were able to demonstrate, using results obtained from previous balustrade tests conducted at Kite Glass, that the enhanced structural performance of laminated glass panels, with SentryGlas® as an interlayer, would allow us to create a balustrade with a thinner glass system, an extended performance range in terms of its strength and deflection, and improved post-glass breakage performance."
The glass construction used for the balustrade panels consists of two 12 mm (0.5 inches) tempered glass sheets, with a 1.52 mm (60 mil) layer of SentryGlas® interlayer – a 20 percent thinner construction than the PVB laminate solution required to satisfy building requirements (involving two glass sheets of 15 mm).
“Not only are the panels with a PVB interlayer heavier and more expensive to produce, but the reduced depth of the laminated glass panels with SentryGlas® also means that they fit perfectly within the structural track fixing without sacrificing strength or performance,” adds Leroy Reed. Testing of the laminated glass panels at Kite Glass premises with SentryGlas® interlayer, intended for the Westfield development, was successfully taken to 1.5 times design load (4.5 kN/m – a test usually applied to steel barriers).
Impressive post-breakage performance
A further benefit of laminated glass panels with SentryGlas® interlayers are their excellent post-breakage safety: Following a sufficiently heavy impact the glass will break, yet the dangerous fragments of glass will remain adhered to the interlayer. The risk of injury to passers-by is thereby significantly reduced and, at the same time, the role of the glass balustrade as a barrier is maintained.
“During transportation and installation of the panels at Westfield London, one or two of them cracked as a result of inappropriate handling, yet remained rigid and with the glass intact – providing us, and the site’s risk manager, with an impressive, first-hand demonstration of the interlayer’s post breakage performance,” confirmed Julian Clayton. “Overall, the end product has been first class. The glass balustrades and handrails, with their clean lines and simple design, conform perfectly with the light, spacious feel of Westfield London. At the same time they provide the many thousands of visitors to our centre with an unobstructed view of the exciting retail displays above and below them as well as a safe and secure shopping environment.”
SentryGlas® was created by DuPont to provide a superior solution in the use of glass in architecture and construction. Its superior strength and lasting clarity, even after years of service, has opened up a world of possibilities for both architects and structural engineers.
Björn Sanden, development programs manager at DuPont, explains: "SentryGlas® represents an economically advantageous solution: because of the mechanical properties of the interlayers, it is possible to reduce the weight of the glass and consequently of the supporting structure, as in the case of the Westfield London shopping centre building. As well as their high stiffness, SentryGlas® interlayers retain their original high transparency after years of usage and exposure to atmospheric conditions. Indeed, when used for external applications, as is the case for some of the balustrades at Westfield London, the resistance of laminated glass made with SentryGlas® to moisture and the effects of weather comes into its own in terms of its edge quality and clarity."