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Theme: cladding and curtain walling

Facade design had been like a historical pendulum, swinging from total integration in the building to near-complete dislocation. We have now reached a new synthesis where we are beginning to make facade choices based on considerations of the complete building performance In the 18th century, as the craft-based business of building needed to become an industry and turned to science and technology for solutions, the role of the architect as master builder diversified. Structural engineering was born and with it came new opportunities. The development of structural framing, from the late 18th century, allowed the external building envelope to be relieved of its load-bearing function, and turned into a facade. The facade still had to perform, and the demands upon that performance soon grew as the options supplied by industry expanded. The story of how facade engineering has been treated reflects these wider technological and professional swings.

As Reyner Banham wrote: 'It fell to another body of men to assume responsibility for the maintenance of decent environmental conditions: everybody from plumbers to consulting engineers. They represented fianother culturefl, so alien that most architects held it beneath contempt.'

1While the building's structural engineering and internal environmental engineering spawned new professions, building professionals relied on manufacturers of commercial products to carry out the engineering development of the building skin and to create new glazing and cladding systems that could be specified. As this product specialisation grew, it was possible to enhance building envelope performance enormously, while the professionals' understanding of these developments dwindled. The result is that we have inherited a building culture in which building performance generally played second fiddle to building aesthetics for much of the 19th and 20th centuries.

The dislocation was most clear in the deprioritising of building science behind an architectural look. These ranged from 'pure' glass facades to decoration with 'expressed' services and structure, from no-hands structural solutions to the desire to make framed buildings look like load-bearing masonry.

These remain contemporary aesthetic concerns, camouflaging the building science needed to achieve them.

The pendulum has now swung to the point where the demands placed on the skin of a building are so sophisticated that facade engineering has not only been reintegrated into the professions but also takes centre stage in any consideration of total building performance. That is why the engineering of the skin is increasingly required early on in a building's development to demonstrate a project's overall viability.

As a result, we have seen the growth of facade engineering as a new discipline within most of the major engineering consultancies, and as a masters course that is being taught in at least one university.

2In most consultancies, facade engineering is multidisciplinary. At Whitby Bird & Partners for instance, the skill set that makes up facade engineering is drawn from across construction - architecture, structural engineering, building physics and materials science - and this reflects the concern with total building performance.

Facade design has always involved reconciling appearance, structure and the delivery of comfort to the building's occupants. New constraints on facades and the introduction of evaluation tools combine with the range of options available to greatly extend the complexity of facade design. Maintaining the knowledge bank within the specialisation is demanding.

As Stephen Ledbetter has written: 'The concept of half-life applied to an engineer's knowledge is a useful one! How long is it before half of what we have learnt is no longer relevant or useful. The corollary of this is how much, and what should we learn to stay in the game.'

3This half-life of knowledge has already shortened dramatically.

With the continued and accelerating development of cladding products and materials, it is likely to reduce still further.

The pendulum swing back to integration is also wrongly conceived as being anti-glazing. The architect's love affair with transparency and glass can still drive the development of facade structures provided the issues of integrated performance are confronted early on. The tension facade, where the restraint of the facade is carried out by a tensioned cable with no bracing, is being developed to new levels of sophistication.

Although the Kempinski Hotel 4in Munich, with its vertical and horizontal cable system, still stands out for purity, the Electronic Arts headquarters 5in Chertsey, Surrey, shows the principle simply executed by Eiffel UK.

At Canary Wharf 6on Canada Square, London, the vertical tension facade has two major refinements, again executed by Eiffel UK. The atrium facade is four-storeys high and the glazing is propped away from the restraining vertical cables. This propping frees up the linear form previously deemed a requirement, allowing a cone form to be produced within a flat facade. The transition from flat to cone is carried out with the use of partially bent laminated glass panels produced by Cricursa SA in Spain.

The detailed developments in glass engineering to which these buildings testify are producing a greater understanding of a complex material, primarily through the extensive analysis and testing being carried out for new cladding products. This can be illustrated by the work carried out by Fischer Fixings in the development of its underreamed fixing for glass 7, currently being installed at Century House 8. Push-pull and shear tests were carried out, as well as finiteelement simulations of the three-dimensional thermal pre-stress.

DuPont has developed a new interlayer 9, called SentryGlas, for use in laminated glass.

This allows engineers to get more strength for less material. Due to the increased shear interaction of the film material with the glass panes, this interlayer allows the full thickness of the glass in a laminate to be used structurally for short load durations, such as wind loading. This will increase the potential for using this brittle material in columns, as vertical stiffeners and as glass shear panels. Testing of these proposals 10 is under way.

In structural terms, the functionality of the building skin has swung back to a previous position and, on framed buildings, they are loadbearing once more. This was previously the norm in the early 20th century, when the framed building was first developed. Then the facade stone or brickwork carried its own load to ground level.

Winterton House 11 in east London was completed in 1996 and is an early example of this reintegration of facade and frame.

That integration is driven by the structural frame. Since the building was being redeveloped, this frame already existed but needed to take increased floor loading requirements. The solution was to build the additional load-bearing requirement into the new brick facade. Here Hanson's Melford Yellow and Ibstock's Aztec Grey and Blue take not only their own load, but also the increased floor loading, and provide lateral stiffness to the frame as well.

Precast-concrete stone facade panels, previously simply clipped to the building frame, have now been developed to be loadbearing.

They range from self-supporting facades to those that support the frame (which can free up internal space by removing the need for perimeter columns) and also to panels that both support and restrain the frame, freeing up the core as well. These solutions also allow a reduction in the extent of cladding movement joints. A building where this is exploited is 140 Aldersgate 12 , which has stone-faced precast panels by Malling Precast. The development has seven storeys of precast-concrete self-supporting facade. This solution reduces the perimeter loads on the frame and allows a continuous stone facing for the whole of the building's height.

A similar approach has been taken in the 'framed' 40 Grosvenor Place 13 building. But here the precast-concrete facade panels both support the perimeter of the floor slab and restrain the building. No perimeter columns are required and the core is non-structural above ground floor.

27-30 Finsbury Square 14 carries this approach even further with the use of stone columns by Szerelmey. Like 40 Grosvenor Place, 27-30 Finsbury Square is a framed building but it has pre-stressed external Portland Stone columns supporting the floor beams. The stone, Bowers Base Bed from Albion Stone, needs restraint and this is achieved via the external precast-concrete beams, again by Malling Precast, and, ultimately, by the cores.

In these examples, the facade carries out a function that is usually exercised by the building's structure. To be able to use facade engineering in this way requires an integrated design and construction team capable of producing an integrated solution.

The facade represents one of the main variables in a building's energy use. The energy rating method - Carbon Emissions Calculation Method - employed by Part L2 of the Building Regulations has now made the facade criteria dependent on the building form. Previously, simple elemental performances were used. The new method means that the ratio of the building's surface to floor area becomes critical in calculating CO 2emissions. As a result, in deep-plan buildings, where daylight is at a premium, the required facade performance can be reduced in order to maximise light transmittance.

As overall building performance becomes increasingly critical for many developers, a detailed analysis of how facades perform is becoming more relevant. One consequence of this is that decorative finishes to glazing are now beginning to be quantified for their solar performance. These include fritting, ranging from black through to translucent white to acid etching/ grit blasting and the range of printed interlayers being marketed by Cesar Color Inc from the US. Similar solutions are Vanceva Design interlayers from Solutia, and DuPont's SentryGlas Expressions. The work has just begun and, although little data is currently available, what we know about decorative finishes will increase as the demand for accurate predictable results grows.

The number of double facades proposed is also increasing. Such solutions, however, are not always solely performance-driven (although usually performance-justified) as the aesthetic of the 'simple' flush glass facade still has great appeal even if the required performance cannot be achieved in a simple manner. In order to have certainty of performance at a reasonably early stage, detailed analysis is required, usually by computational fluid-dynamic modelling.

Double facades are likely to result in the greater use of unitised curtain walling systems. The UK industry appears to be gearing up for this change, led by system manufacturer Schuco. This has to be applauded as it should bring us in line with the rest of Europe. Unitisation gives consistent manufacturing quality and greatly increases the installation speed of facades. The cost of unitised curtain walling has already reduced significantly, and this method will become more available on lower-budget facades if the trend continues.

What we can be sure of is the need for a continued reduction in building energy consumption, and this will drive a need to be able to measure and quantify the performance of the building skin. Such a requirement should help to accelerate the current trend for the re-integration of the facade into the development of the total building design.

The benefit of re-integrating the design of the facade with that of the structure and environmental systems, is that cladding integration should soon be on the agenda at the concept stage on all major projects. We need to aim, in the words of Reyner Banham, at an architecture in which environmental technology and structure have been ' finally and naturally subsumed into the normal working methods of the architect, and contributed to his freedom of design'.

Will Stevens is associate director in charge of the facade engineering group at Whitby Bird & Partners READER ENQUIRIES Albion Stone 1400 Cesar Color Inc 1401 Cricursa SA 1402 DuPont 1403 Eiffel UK 1404 Fischer Fixings 1405 Hanson 1406 Ibstock 1407 Malling Precast 1408 Schuco 1409 Solutia 1410 Szerelmey 1411

REFERENCES 1Reyner Banham, The Architecture of the Well-tempered Environment , first published in 1969 by The Architectural Press, London. Second edition (The University of Chicago Press, Chicago, 1984) page 11.

2 MSc in Facade Engineering, University of Bath. www. bath. ac. uk 3 Dr Stephen Ledbetter, Facade Engineering: The challenge for structural engineers , The Structural Engineer, Volume 79/No11 5 June 2001 pp13, 14.

4 Hotel Kempinski, Munich Airport. Architect: Helmut Jahn. Engineering: Schlaich, Bergermann & Partner (Tragwerksplanung).

5 Electronic Arts UK headquarters, Surrey. Architect: Foster and Partners. Facade and structural engineer: Whitby Bird & Partners.

6 DS8 Canada Square, Canary Wharf, London. Facade architect: Zeidler Grinnell Partnership Architects. Facade and structural engineer: Whitby Bird & Partners.

7 Fischer Fixing Systems, FZP-G. http: //www. fischer. co. uk 8 Architect: Pearce Group. Engineering: Arup.

9 DuPont SentryGlas® Plus. www. dupont. com 10 Andreas Luible, ICOM EPFL Lausanne and Dr. Wilfried Laufs, Whitby Bird & Partners, London 11 Winterton House, London E1. Architect: Hunt Thompson Associates. Facade and structural engineer: Whitby Bird & Partners.

12 140 Aldersgate street, London. Architect: Sidell Gibson Partnership. Facade and structural engineer: Whitby Bird & Partners. Image: @Smoothe Limited 2002/John Maclean.

www. smoothe. co. uk 13 40 Grosvenor Place, London SW1. Architect: Hellmuth Obata & Kassabaum. Facade and structural engineer: Whitby Bird & Partners.

14 27-30 Finsbury Square, London E2. Architect: Eric Parry Architects. Facade and structural engineer: Whitby Bird & Partners.

15 Reyner Banham, The Architecture of the Well-tempered Environment , first published in 1969 by The Architectural Press, London, Second edition (The University of Chicago Press, Chicago, 1984) page 111.

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