It's almost finished. The vast bulk of the 660 steel-grey panels making up the facade of the GLA headquarters are in place, but the unusual shape of the building has brought unique challenges for the cladding specialists.
"It's the building's shape that makes the cladding such a challenge," says Gerry Sinnott.

As cladding co-ordinator for construction manager Mace, Sinnott is responsible for ensuring that the cladding on the £45m headquarters building for London's mayor and governing body, the Greater London Authority, is installed without a hitch. "It's not directly comparable to any other cladding scheme," says Sinnott, who has been grappling with its unique challenges.

Situated on the south bank of the Thames, close to Tower Bridge, architect Foster and Partners' curvaceous response to this sensitive site is an elliptical-shaped building that leans backwards, away from the river and the City's financial heart beyond. Its smooth-curved north facade rakes upward in a gentle arc from the river to the building's domed roof, 10 floors up. By contrast, the building's southern elevation, with its overhanging floor plates, has a much more perpendicular appearance.

Framed in the window of his Thames-side site office, Sinnott can see the tower crane lifting cladding panels into position for installation. The bulk of the building's curved facade from the second floor upwards has already been enclosed in the steel-grey panels, which give it the appearance of wearing a suit of grey scales. Only the northern elevation, where a diagonal grid of tubular steelwork is nearing completion, and the facade of the building's top floor – known as London's living room – have yet to be clad.

A team of riggers is hard at work on the building's northern elevation, securing sections of secondary steelwork to complete the diagonal grid structure that will form what Sinnott calls "the lens". This structure will support a frameless glazing system constructed from horizontal bands of interlocking triangular glazed panels which will create a window for the public onto the building's atrium and main council chamber.

The lens, described as a Christmas tree on account of its saw-tooth outline, is one of the larger packages for which Seele, the German steel and glass specialist contractor, is responsible. The building is "definitely unique", says Thomas Geissler, the company's managing director. As with all components on this elliptical building, dimensional accuracy was critical. Every major section of the tubular steel diagrid, along with some of the secondary steelwork sections, were trial-assembled in Seele's workshop to ensure a perfect fit before being shipped to the UK.

One innovation Seele has introduced to this project is the use of the primary steelwork to heat the building's atrium. Geissler says he was astounded when Foster asked for brackets to be welded onto the lens' steelwork sections, to support the radiators needed to prevent cold downdrafts. Seele proposed the much neater solution of turning the horizontal primary steelwork of the diagrid itself into a series of giant tubular radiators. This was done by designing the steelwork so that hot water could pass through its 300 mm diameter core. Steel T-sections projecting from the diagrid hold the glazing panels in place.

This week sees work begin on installing the lens glazing. To create a smooth appearance, the glazing will be one of the first installations in the UK where the outer pane of glass is not held in place with a mechanical retaining clip. Instead, the gas-filled, double-glazed units are made using an inner pane of laminated glass and an outer pane of toughened glass. Rather than a mechanical fixing, the outer pane is bonded to the inner pane using structural silicone. "We had some lively discussions with building control about this method," laughs Geissler. He admits that the added safety factor of using a larger area of sealant than was structurally necessary may have helped his company's case.

While Seele has been busy erecting the steelwork for the lens, Swiss cladding specialist Schmidlin has been hard at work manufacturing, assembling and installing the 660 cladding panels that make up the rest of the facade. The design of the units has evolved: Foster's original concept of a transparent building was constrained by the need to minimise solar gain – the glazed area has been limited to a 1.2 m high window on each of the main administration floors.

Each cladding panel measures roughly 3 m high by 1.5 m wide and is 300 mm deep. They have a triple-skin construction comprising an inner sealed double-glazed unit and a ventilated cavity on the outside. The single-glazed external skin can be hinged outwards in three sections for cleaning and maintenance. Adjustable louvres between the facade's two skins allow the building's occupants to control glare. A manually operated fresh-air vent is also incorporated into each panel at sill height, which, when opened, will trigger the building management system to open a second vent at the top of the panel, so that the unit will allow ventilation in much the same manner as a traditional sash window.

Although they look roughly rectangular, each Schmidlin unit is unique in size and shape (see box "How the building's geometry was defined").

"It's a standard construction, but put together in a unique way because the geometry is different for each," explains Sinnott. To create the curving elevations, the architect's design demanded each of these cladding panels be set at a slight angle so that every mullion had to be manufactured to a different angle and to precise dimensions. This was made easier by Schmidlin's highly automated manufacturing system which enabled the company to take the three-dimensional co-ordinates that define the location of each of the panel's four corners and input the data directly into its computerised manufacturing process.

Again, to ensure there were no problems, Schmidlin trial-assembled each unit with its neighbouring units. On site, the panels are installed by craning them up the outside of the building and pulling them into position, so that cupped sockets on the unit's base fit over two locating balls fixed to the floor plate. Two additional locating balls hold the top of the panel in place. However, rather than use a cupped socket, a sleeved restraint is used to allow the floor slats to deflect without affecting the cladding.

The cladding package is running slightly behind programme. Originally, work on the facade was due to be completed by the end of September; November now looks more likely. But given the complexity of the cladding design, it is remarkable that the delay is so short.

With the glazing installation now under way on the lens and the bulk of the building now clad, Sinnott should be starting to relax. But he is not. Only if the lens contractor's calculations are precise and only if the cladding units have been accurately made and correctly installed will the two separate cladding packages fit snugly together. It will be another month before Sinnott knows if he has been successful.

How the building’s geometry was defined

Foster and Partners designed the building's shape to create the minimum area of cladding for the maximum internal volume – 25% less cladding is needed for this building than for an equivalent cubic building. "We designed the building from the outside," says Foster and Partners project director Richard Hyams. The design has been revised from the architect's original concept, dubbed the fencing mask, in response to sophisticated computer modelling by consulting engineer Arup. The consultant produced a "thermal map" to show how the heat from the sun would travel over the building's surface throughout the course of a year. To keep the building's cooling requirements to a minimum and so reduce the building's energy load, the architect refined the building's form to produce a more moulded shape that leans over, so as to limit the extent of facade exposed to the sun. The southern elevation, which has the greatest potential for solar gain, has been stepped so that the floor above projects outwards to shade the floor below. On the northern elevation, however, free from the constraints of solar gain, the architects have indulged in a much shallower curve. Although the building looks complex, the geometry that defines the shape of the floors is actually very simple. Imagine the building as a cone constructed of 10 horizontal slices, each one smaller than the one below it. Each of these slices represents a floor of the new building. What Foster has done is to shove each of the cone's slices south slightly so that each floor overhangs the one below. In plan, the original concept was for the facade to wrap around the floor plates as a curved unit complete with curved glazing. But glazing a curve is expensive and the client, CIT, was not about to rewrite the budget. Instead of curved walls, the architect opted for a faceted system of cladding panels: triangular glass panels for the lens on the northern elevation and trapezoidal panels for the rest of the facade. "It was a great move to find out that you could go for flat panels," says Hyams. With the form finalised, the trick was then to accurately define the building's complex curving geometry so that information could be transferred between different design teams. The only way the designers could define a component’s location on site was by using a three-dimensional spatial co-ordinate system. The architect's geometry group produced a nine-step geometric plan to define the building's curvilinear form. These geometric rules allowed the position to be established in three dimensions for each corner of each of the cladding panels as a series of x, y, z co-ordinates. This data could be contained on a spreadsheet, enabling the easy sharing of information. Foster used the spreadsheets to check the co-ordinates of its computer model. The cladding was then tendered as a series of packages under a two-stage contract. Seele was appointed to develop the design on a series of packages, including the lens facade, the roof, the glazing to London's living room, and the "skirt" that rings the building's ground floor. Swiss cladding contractor Schmidlin was awarded the main facade package to clad the bulk of the floors where the administrative offices will be housed. When it came to sending out tenders, the architect played it very cleverly: only the drawings and the set of rules were issued to the contractors bidding for the cladding packages. It was left to the specialists to work out the size of each cladding panel and their precise location using the nine-step plan. Foster and Partners was then able to use the spreadsheets returned with the contractors’ tender documents to double-check its own calculations.

Built Environment 2001

Foster and Partners’ Greater London Authority building is one of six top construction projects that will be featured at Built Environment 2001, a new convention and exhibition for the industry to be staged in October. Building will continue to track the GLA building’s progress, focusing on the technical challenges faced by the project team – and you will have the chance to talk to them about their experiences at Built Environment 2001. Among the other schemes featured at the show are Edward Cullinan’s Weald and Downland timber gridshell, which is being featured in Building’s sister magazine RIBA Journal, phase two of the Millennium Village in Greenwich, which is being profiled in Building Homes, and Mile End Park, which is being covered in Building Services Journal. Built Environment 2001, which is a joint venture between Building’s publisher The Builder Group and United Business Media, will also include workshops, seminars, a conference and an exhibition. It will be held at Britain’s most exciting new exhibition arena, ExCeL, in London Docklands, on 16-18 October. For more information about the show visit

Technical Special: Cladding