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Ray of light

GLA; Potent symbol for local government; 21st century landmark; serious structural engineering. Andrew Mylius gets to grips with the Greater London Authority building.

Rather than mere architectural whimsy, the 'giant fencing mask' to be built on the south bank of the Thames near Tower Bridge is shaped by a drive for greater thermal efficiency. To minimise solar gain the glass-clad Greater London Authority headquarters has been tilted south, towards the sun. The angle of incline means that, like an egg balanced on end, this giant space-pod of a building will present a minimal surface to warming rays.

Engineer for structure, services, facade, fire and acoustics, Ove Arup calculates the GLA building will be 50% more energy efficient than a comparable orthogonal office block, thanks in large part to its raked profile. But the building's asymmetry means it has had to design some highly unusual forces.

Put bluntly, the building wants to fall over. At its most acute, the exterior wall meets the ground at 22degrees, generating a lateral load five times that of wind - the combined wind and over-balancing forces add up to an impressive 450mN. Ten tubular steel primary columns and a cantilevered, reinforced concrete box core provide support for the structure's nine superstructural floors. Columns are anchored to the core via I-section primary floor beams. To hold the entire structure in equilibrium the core has to work hard, says project director David Glover.

The core rises vertically through the centre of the 42m tall building, stopping just short of its top. It is composed of two 'tubes', each made up, in turn, of three sub-boxes. In plan, the whole core structure is arranged in a blockish horseshoe, with a maximum width of 20m and depth of 9m. The tubes are bound together at each floor by a reinforced concrete ring beam.

Glover points out that the core is no larger than the structural core in a straight-sided building would be; it will be cast in normal C45 concrete. Strength is gained by minimising piercing of the core's walls. Openings for lifts and stairs, housed within the core's six boxes, are relatively few; they can be engineered without call for significant extra mass. 'It's the services that mess up a structural wall,' Glover notes. Ducting for heating and ventilation, cabling and plumbing are being housed outside the core in shafts that can be tapped into without compromising the core at all.

Though loading placed on the core decreases over its height, for economy of construction wall thickness is being kept constant. It will be jump- formed in two-storey stages.

Radiating from the core, primary floor beams connect with the building's perimeter columns. As with all other junctions in the GLA building, the structural nodes attaching beams to core will be fabricated. Each will be unique.

The columns themselves, reducing from 600-400mm diameter over the height of the building, are to be constructed in straight sections following its curves from base to top. They carry vertical loads only but, because they follow the curve of the building, a steel ring-beam at each floor helps restrain the columns from being forced outward under compression.

In section, the northern face of the GLA building bulges gently outward as it leaves the ground, before sweeping aerodynamically skyward and backwards from the third floor up. As the building's primary columns arc toward the building's apex, more and more overturning force is transferred along primary floor beams to the core.

From its top, the building descends into a steep, undercut facade on its south side. In profile it is stepped, with floors overhanging those below by as much as 2m. Beams on the south side of the core are principally under tension, pulling against it.

To resist the building's overturning moment, the core is cantilevered from a massive, 3m thick pile cap. It will collect together 20, 2.1m diameter piles, 20-25m deep, with 6m diameter underreams. Though the foundations will be hefty, no special ground works are required to prepare the riverside site, says project engineer Malcolm Turpin. Ground is typical London clay below alluvial gravels.

The building has just one basement level, to contain plant and committee rooms for the Assembly, which will be accommodated within excavations already present. 'Right through the engineering process we've tried to simplify design and construction,' Turpin says.

Simplification has involved major revision of the structure. The design that earlier this year won Arup with architects Foster & Partners the GLA commission in competition against architects Alsop & Stormer, had the core thrusting diagonally through the building with floors cantilevered off it. A vertical core was substituted to optimise structural efficiency, improve circulation within the building - vertical lifts travel faster than diagonal ones - and reduce cost. The GLA building, which is just one part of a 240,000m2 development on the site, is valued at £35M by client and developer CIT Group.

Pursuing simplicity, the Arup-Foster team has also managed to reduce tonnage of steel from 2,700t in the competition design to 1,700t now. The structural columns are lighter than a lattice-like diagrid considered earlier. Weight savings have also been made in the floors which employ a secondary web of standard steel beams to carry loads between the primary beams.

For the floor slabs themselves, in place of conventionally used 150mm trapezoidal steel decking, the engineers are using decking with a 175mm section. This enables larger spans to be achieved with less intermediate support. In a building with a maximum 50m diameter floor plate, it results in significant economy, Glover observes. Above the decking reinforced concrete rafts will be cast.

As a design project, the GLA building has been complex. Architect, engineers and steel industry collaborators have been working from a single computer database, sharing ideas on how the building should look, behave structurally and be constructed. Because it all but defies description as two-dimensional plans, sections and elevations, three-dimensional computer modelling has been central.

The design process will help deliver a highly polished end product, says Glover. But this means tolerances during construction will be very tight. Margins for correction are being designed into the construction process. Casting of the concrete ring beam following erection of the core boxes will offer a first opportunity. Connection of the primary beams from core to columns and construction of the concrete floor plate makes further adjustment possible. And tolerances can be fine-tuned through hanging the facade from the primary columns. Glover, Turpin and team are still working on resolution of the cladding.

Planning permission was finally secured at the end of last month and the first stage of tendering should be under way before the end of this year. Further design changes are envisaged as contractors start feeding information on preferred construction methods into the project. Construction itself, to be managed by Mace, is expected to take close to two years.

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