London calling: The headquarters for the Greater London Authority uses glass in its sophisticated and environmentally responsive external skin. (Glass Futures).

Foster and Partners' headquarters for the Greater London Authority (GLA) is London's newest and most significant civic building of recent times. Like County Hall (the institution that housed the Authority's political predecessor the Great London Council), it occupies a riverside site, but in raffish Southwark, near Tower Bridge, as opposed to genteel Westminster. Intended as a paradigm of democracy, accessibility and sustainability, it houses an assembly chamber for the 25 elected members of the London Assembly, together with offices for the Mayor and some 500 GLA staff.

The building's orientation and form are designed to reduce energy use. Its bulbous shape is derived from a geometrically modified sphere, a form with the greatest volume to the least surface area. The glazed facade of the assembly chamber faces north over the river to minimize the amount of direct sunlight falling on it and so reducing solar gain. On the south side, the building steps back as it rises, so that the overhanging floor-plates provide natural shading for the offices beneath. The architects estimate that this form in combination with measures such as natural ventilation in perimeter offices (by opening vents positioned below windows), chilled beam cooling and a highly responsive system of environmental control will cut the annual energy consumption of the building's mechanical systems to around a quarter of that of a typical high specification air-conditioned office building.

Both building form and external skin respond to the sun path; self-shading where required; more solid to prevent heat gain and loss; more transparent where sun angles prevent direct solar gain. A shading strategy was developed in response to detailed analysis of the frequency and intensity of sunlight on the facade. The building is enclosed in a sleek glass skin cladding incorporating opaque, translucent and clear panels with integrated shading devices according to orientation. The cladding system extends around the office facades and abuts the assembly chamber to the north where it becomes a double-glazed traingulated skin, reflecting the volume of the chamber within.

Office cladding is designed as a vented, triple-glazed flush facade. The inner skin is made up of insulated spandrel elements and a low emissivity double-glazed unit 1200mm in height set 900mm above the finished floor level. Shading blinds sit in the cavity between the two skins to provide both solar shading and glare control. Air movement in the cavity cools the blinds and increases the efficiency of the system. The vented cavity also provides an air inlet for natural ventilation. An automatic top opening vent allows hot air to escape at high level when the lower vent is opened.

Each of the building's 3844 glass panes is unique. A computer model was used to determine glass sizes and panel locations. The task was made more complex because the facades are all twisted planes as the circles that define them are not concentric. An internal partition grid of 1.5m generates mullion positions. Mullions were shifted from the vertical in order to overcome the inherent twist and create a flat glass trapezoidal panel.

Spreadsheets were used to record each panel location, enabling the glass supplier to quote accurately for the glass, reducing potentially costly assumptions. It also allowed the facade fabricator Schmidlin to device precise production and laser-cutting schedules. Panels were temporarily erected in the factory before being shipped to site so that Schmidlin could survey the as-built facade and eliminate any fabrication tolerances. Fixing brackets could also be positioned in relation to actual panel size. Each of the panels was individually numbered and fitted together on-site like a giant jigsaw. The level of detailed design was such that each panel had its own lifting position in order to ensure that the bottom edge of the trapezoidal units remained horizontal. Head and base brackets fix the panels to the structural slab. Designed and built within a mere 30 months. the building was completed both on time and on budget.

RELATED ARTICLE: Architect

Foster and Partners, London

Structural, services and

acoustic engineer

Arup

Cost consultant

Davis Langdon & Everest

Facade consultant

Schmidlin

Automatic doors

Blasi

Photographs

Nigel Young