20 March 2013 · By Felix Mara
This year’s cladding and curtain walling overview focuses on frameless curtain walling, drawing on the expertise of structural and facade engineer Eckersley O’Callaghan, and looking at the technology of Apple Glass Cubes and Gorilla Glass
According to a survey conducted by curtain walling manufacturer Sapa, solar control and thermal efficiency are architects’ top priorities in facade design, with acoustic performance coming fourth after appearance and integrated automatic ventilation and photovoltaics in bottom place. This suggests architects are less preoccupied with appearances than critics say, and arguably more concerned with passive environmental design than gismos. Of course it’s all good stuff, and few would seriously say facades’ appearances don’t matter, although many believe they can and should be byproducts of performance design.
These findings coincide with changes in English building regulations, codes, standards and government policy, referred to in Laura Mark’s Overview of Insulation design and specification in last month’s AJ Specification and also by AJ building regulations columnist Geoff Wilkinson (AJ 24.02.13). As Wilkinson observes, new requirements for Energy Performance Certificates in accordance with The European Performance of Buildings Directive have been introduced, with public buildings affected from 9 January 2013 and others as of 9 July 2013. But an announcement on new changes to Part L is still pending.
Cladding and curtain walling design has also been affected by the repeal of fire safety provisions in Local Acts in January 2013. Buildings with excess height and volume no longer need extra protection such as sprinklers stipulated by local codes such as Section 20 of The London Building Acts. Existing Section 20 buildings will no longer be required to meet these technical requirements, as conditions imposed under previously approved schemes are no longer enforceable.
As pointed out in Laura Mark’s CPD on the design and specification of curtain walling, sponsored by Comar (AJS 10.2012, also on our website), it can be divided into four main categories: ladder, stick, unitised and structurallly galzed. Our main focus this month is structural glazing.
James O’Callaghan, director at structural and facade engineer Eckersley O’Callaghan, remarks on significant developments in frameless glazing in the last 10 years, driven by evolving fabrication equipment and connection technology. To use a topical phrase, what follows is mainly ‘from the horse’s mouth’. Seeking to refine and limit the type and number of connections in this type of facade, Eckersley O’Callaghan encouraged fabricators to develop equipment able to handle much larger glass panels. ‘As a result, certain fabricators can now toughen and laminate glass up to a maximum of 18m x 3.6m in China and slightly less in Europe,’ says O’Callaghan. ‘These epic panels can cover much larger surfaces of the building without joints and connections which compromise transparency.’ For example, the Apple 5th Avenue Retail Store Glass Cube in Manhattan, originally built in 2006 with 90 glass panels was re-engineered and rebuilt in 2011 with only 15, exploiting developments in fabrication equipment.
Apple 5th Avenue Retail Store Glass Cube Mk 2 in Manhattan, by Bohlin Cywinski Jackson and Eckersley O’Callaghan
‘Curved glass can be achieved in several ways depending on the radius required and the application of the glass,’ says O’Callaghan. ‘In most buildings, glass must be laminated for safety, bringing the complication of ensuring that the layers are suitably curved to “nest” well in the lamination oven.’ Where strengthening isn’t required, glass can be slumped over a mould, enabling interesting double curvature surfaces, subject to the limitations of furnaces, which can be up to approximately 6m x 3m. Mechanised curving of toughened panels is more applicable to buildings. These are curved as they are toughened and are typically limited to 5m x 2.7m, with typical minimum radii exceeding 2m. 12.8m x 2.8m is available in China. A more recent trend is ‘cold bent glass, relying on a stiff interlayer to hold the glass in a form rather than using heat to form the curve. Layers of glass are bent to the curve required and held in this form while laminated. The advantage is that the quality of the glass surface is not compromised by the heating required for tempering and it is therefore clearer, with fewer disturbing distortions. This technique is limited by a relationship between the required radius and the glass layers thicknesses: the tighter the curve, the thinner the layers needed to accommodate the bending involved and radii exceeding 4m are most practical. The other limitation is autoclave sizes. Their lengths can now exceed 15m.
15m x 4.5m diameter Autoclave used for laminating glass
Connections have been extensively researched and tested. Eckersley O’Callaghan along with construction specialist Seele has developed an alternative to the more traditional bolted connections used in spider fittings and similar technology. Laminated metal connections inserted in the body of the glass layers help to avoid bolt fixings and puncturing of reflective glass surfaces. Dupont Sentry Glass interlayers (AJS 07.12.12) are well suited to this type of connection because of their strength and stability.
Eckersley O’Callaghan worked with architect Gabellini Sheppard on the 8m-high storefront of the Village, a retail development at London’s Westfield White City. This has a sinuous sawtooth plan geometry and uses a simple connection detail that avoids drilled holes in the annealed glass fins, which were polished after lamination, producing precise edges which enhance the storefront’s jewel like quality. The simplicity of the connection allows each fin to be angled differently relative to the face glass panels without custom machined parts. More recently, Eckersley O’Callaghan worked with Carpenter/Lowings Architecture & Design to develop the Glass Lens, a curved glass entrance feature for Robin Partington Architects’ Park House mixed use development in London (AJ 20.12.12). Custom-designed clamp fixings are bonded to the inner face of its laminated glass, which has a printed interlayer creating a doublesided pattern and is chemically strengthened to avoid visual distortion, leaving a smooth exterior. Like the shopfront at the Village, this installation provides for large live load deflections in the structure above, to avoid glass damage. O’Callaghan suggests a checklist for frameless glazing in building envelope design.
O’Callaghan’s advice to avoid automatically choosing off-the-shelf systems and design your own may come as a surprise, challenging the custom of throwing expensive proprietary products at ‘prestige’ commissions and also questioning the view that architects shouldn’t dabble in technologies best left to specialists.
There is a cynical view that contractors increase tender prices where they think overzealous architects may be a liability. However, Sapa project consultant David Brierley reaffirms the wisdom that architects must be technically savvy, willing to roll their sleeves up and produce detailed annotated curtain walling tender documents. If these practices are less common now, this reflects pressure on architects’ fee levels, growing concerns about litigation and, arguably, weaknesses in training and practice. Although incorrect detailed drawings and specifications carry a risk element, despite the widespread practice of contractor design, architects should not timidly produce documentation which tells tenderers little about design qualities and performance standards, as they will only increase their prices to cover their increased risks.
There is a cynical view that contractors increase tender prices where they think overzealous architects may be a liability
It can be interesting to compare architects’ and fabricators’ drawings. IBI Nightingale’s drawing of a junction between the curtain walling and rainscreen cladding for its Whitegate Drive Primary Care Centre in Blackpool differs from fabricator Quality Glass’s detail because it is more specific about trades outside the curtain walling fabricator’s remit, for example gypsum board. It shows basic information about the curtain wall, but omits double-glazed unit gasketry, presumably referred to in the specification. It shows continuity of insulation with the double-glazed unit cavity and the principle of a rainscreen supported by SFS framing, vapour control and a breather membrane lapped by a metal angle trim. Quality Glass’s drawing identifies fabricated components and shows this angle more positively overlapping the insulating block that is framed into the curtain wall. One role of this type of architect’s detail can be to clarify the scope of each sub-contractor’s work and also whether the structural engineer or a sub-contractor will be responsible for secondary structure, information that must be clear and consistent with other documentation. Given the high risks of failure, delay caused by uncertainty and imaginative contractors’ claim departments, it’s not surprising that Brierley emphasises the urgency for architects and sub-contractors to thrash out a design for an envelope that works as one.
Curtain walling performance is only as good as the sealant at its interfaces. Foster + Partners Walbrook development in the City of London uses Sika’s Universal Membran EPDM synthetic rubber sealing system to provide an airtight, watertight seal around the curtain wall and windows. It is solvent free, durable, UV and ozone resistant, can be bonded to most substrates and controls moisture egress, minimising interstitial condensation risks. Manufactured in 25m x 0.6mm rolls, it is available in widths from 100mm to 1,400mm. After cleaning the surfaces to which it was to be bonded, Sikabond TF Plus elastic adhesive was applied to the surface and the Sika Membran was put in place.
Glass manufacturer Corning has developed a product called Gorilla Glass. Unlike conventional soda lime glass, it is formed by continuously fusing molten glass layers flowing vertically through air rather than on traditional tin float lines, allowing it to be uniquely thin, with significantly fewer impurities in contact with its surface during production. It’s the surface flaws in glass that propagate and cause failure, so limiting these increases its strength. When fused and cut into sheets, it is chemically strengthened through the exchange of ions on its surface in a heated bath of salts. Because ions in the bath that exchange with the glass’s surface ions are ‘fatter’, its surface compression is very high, up to 700Mpa. It therefore has a significantly higher tensile strength than steel. The tensile strength of a 1mm sheet of Gorilla Glass roughly equals that of 4mm fully tempered glass or 15mm annealed glass.
Eckersley O’Callaghan has been researching architectural applications of Gorilla Glass, initially developed for mobile phones and displays. ‘It may have applications for security glazing, which currently involves cumbersome thick glass panels and plastic laminates,’ says O’Callaghan. Gorilla Glass’s low weight and high mechanical resistance may also revolutionise overhead glazing and its combination of slenderness and strength can provide unique flexibility potentially applicable to highly warped building skins or in transparent tensile surfaces to rival fabric or ETFE foil.
SageGlass Saint-Gobain’s Cool-Lite Extreme installed at 62 Buckingham Gate in London, designed by Pelli Clarke Pelli Architects with Swanke Hayden Connell for Land Securities and due to open later this year, uses a static triple silver coating on face 2 of its double-glazed units, providing high light transmission with good solar control. Cool-Lite Xtreme 60/28 has the highest selectivity available from Saint-Gobain Glass, with a high degree of neutrality in transmission and exterior reflection. It is available in annealed and ‘to be toughened’ versions.
SageGlass Saint-Gobain Cool-Lite Extreme at 62 Buckingham Gate, London, designed by Pelli Clarke Pelli with Swanke Hayden Connell
Another dynamic glass facade product, SCHOTT GlassX, aims to combine the optical properties of glass with the thermal behaviour of concrete. GlassX crystal, presented at BAU 2013, integrates four system components within one unit able to be installed like conventional insulating glass: transparent thermal insulation, protection from overheating, conversion of energy and thermal energy storage. It contains a salt hydrate hermetically shrink-wrapped within a polycarbonate container, which acts as a phase-change material that absorbs excess heat then releases it during colder evening hours. When subjected to intensive solar radiation, it melts and stores energy, later released through crystallization. In the summer, a prismatic panel between the panes reflect sunrays at angles exceeding 40°, whereas in the winter, radiation can pass through the sun protection at full intensity. Internally, modules are enclosed by single panes of 6mm safety glass and are customisable.
SCHOTT GlassX containing phase change material
Architects’ cladding tender documentation shares certain principles with curtain walling, discussed above. Eric Parry Architects worked with stonework contractor Szerelmey on the St James Gateway mixed-use development for Stanhope in London’s Mayfair. Although the architect’s drawings show how different trades come together in a coordinated generic design, paying attention to interfaces and principles of waterproofing, insulation, airtightness, deflection, movement and installation, Szerelmey’s drawings focus on componentry, setting-out and details of fixings and support rails for the stone and faience cladding, with particular emphasis on provision for its adjustment to meet targets for precision and tolerance. Integration of fixings, waterproofing and insulation are particularly critical in this type of subcontractor’s detail. Buildability relies on architects anticipating how these will be achieved and this involves thinking threedimensionally. For example, the sub-contractor’s work will be more straightforward and less prone to failure if there are continuous step-free backing surfaces to apply membranes to. Sometimes buildability must be sacrificed to architectural quality, but this is likely to have cost and programme implications.
There are various proprietary ventilated facade systems on the market. Alsecco’s Airtec stone comprises composite panels with an 8-10mm top layer of natural stone, bonded to 19mm lightweight concrete. This is significantly lighter than solid stone equivalents, so panels are easier to work with and install and can be up to 3.75m². Panels are secret-fixed to aluminium rails using clips. Helping hand brackets have isolation pads to prevent cold bridging, with horizontal rails attached to T-sections. Clips are attached to the back of the panels with ceramic fixing points bonded to the backing and the reverse face of the stone. Fixings are aligned and attached using the correct torque before panels are clipped on to the horizontal rails.
Ruukki’s Energy Panel, recently installed at Itella’s new logistics centre at Pennala, Finland is a proprietary sandwich panel system intended for commercial, industrial and logistics buildings, distinguished by its high standards of airtightness, with leakage rate of 1m³/h.m² at 50Pa. Part L requires 10m³/h.m². This is achieved by the system’s joint design, energy gasket interface detailing and quality control standards. It is available with wool, PU and PIR core materials.
Corian has entered the arena as a viable external cladding material. Alison Brooks Architects’ Northampton Park Extension, a residential project in North London has a Corian rainscreen, chosen for its longevity, versatility, fire-rating, low maintenance, light weight, workability, repairability and, subject to colour, UV resistance. The Blackberry Ice Corian was templated, cut to individual panel sizes on site and installed using an adhesive specifically formulated for aluminium rails, dressing the watertight structure. Zinc cladding was considered but was rejected because it would have been vulnerable at ground level.
Alison Brooks Architects’ Corian-clad Northampton Park extension
‘The sheets are absolutely flat, and able to be cut to very precise tolerances with mitred edges where several planes meet,’ director Alison Brooks explains. ‘The through colour is stable and therefore offers minimal maintenance and Corian’s smooth finish means the faceted sides of the building reflect light in varying shades from silver-white to black, depending on the weather.’
Faience, i.e. glazed ceramic, is also enjoying increasing popularity, though not for the first time: it was a favourite of the Victorians. Art historian Tamsin Pickeral explains this was partly down to its durability and versatility. Also, glazed finishes repel pollution and it is lightweight and relatively easy to work, inexpensive and transportable. ‘Faience offers an endless range of finishes, colours and effects and it has a luxuriant, tactile quality,’ says Pickeral. ‘Faience is far less consistent than stone however and once it has been fired there can be quite a range of shrinkage which can make it difficult to work with, which may add to its artisan appeal.’ Contemporary examples of faience include Dixon Jones’ Quadrant 3 in London’s Piccadilly (AJ 24.11.2011) and Eric Parry’s 50 New Bond Street (AJ 3.12.2009) and St James’s Gateway (p. 32), where artist Richard Deacon has designed a cornice comprising 39 uniquely coloured sculptural faience pieces working alongside Parry with faience producer Shaws of Darwen and faience and stone contractor Szerelmey.
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