Designed by architect Foster and Partners, the £45m structure on the south bank of the Thames shuns the perpendicular – it leans backwards, away from the river, like a lopsided egg, supported on a steel structure that rakes outwards at an angle of up to 22°.
The building's unusual shape means that it is without a regular structural grid. Each elliptical floor plate is different in size and shape, the curved walls lean either inwards or outwards, even the roof is domed. The only way the designers could define a component's location
on site was by using a three-dimensional spatial co-ordinate system developed by Warner Land Surveys.
"Achieving extremely tight structural tolerances is the key to the building's success," says Warner. If one of the structural nodes – the points at which the floor structure connects to the primary structural columns ringing the building's perimeter – is out of position by just 75 mm, then, because of the building's lean, the sideways load on the column will increase by up to 15%, which would require the structure to be modified. To prevent this happening, Warner says the nodes have to be positioned within a tolerance of just "10 mm in any direction".
To do this, Warner's team has used 3D setting-out and measuring skills and surveying equipment more commonly seen in process engineering and aircraft manufacture. In fact, the company's role on this project is so extensive that it has spawned a new construction discipline – project measurement.
The company's extended surveying role includes work for client and project manager CIT – it is responsible for ensuring that the building's components are positioned accurately. It also includes a checking role for many of the specialist contractors. For instance, Warner was employed by steelwork fabricator and installer Wescol Glosford to check that each component had been fabricated within the tolerances specified by structural engineer Arup before delivery to site.
Warner started his business 22 years ago and bought his electronic measuring system in 1993 after seeing it in action in Japanese shipyards. Sitting in his site office, hair tousled and unshaven (he was on the winning side in a pub quiz the night before), he explains: "The hull of a supertanker is assembled from a series of sections. Because the survey equipment is accurate to within a fraction of a millimetre in three dimensions, it was used to survey each hull section to ensure that, when assembled, the sections would fit together without a problem."
The system's accuracy has ensured that Warner's services have been in demand, mostly from petrochemical and shipbuilding clients, all over the world. Four years ago, he worked with Foster and GLA construction manager Mace on the British Museum's Great Court scheme, where his work included dimensional control for the construction of the court's curved glass roof.
The firm's role with the same architect and construction manager on the GLA is even more complicated.
On site, Warner is keen to show the part his team has played in the building's construction; he marches around the site, seemingly oblivious to the driving rain. His first stop is at the edge of one of the ellipsoidal floor plates, which he uses as an excuse to light up a Benson & Hedges. "Dimensional control was critical here," he explains between puffs, pointing at the curved steel permanent shutter that forms the edge of the floors "to ensure the building's cladding will fit precisely". Then he's off, striding up the temporary staircase to the upper floors, where the bare steel structure is still exposed before being finished in fire-resistant paint. Here, Warner points out stamp-sized holograms adorning one of the nodes on the primary structural columns.
The holograms are important because Warner's Sokkia measurement system uses wavelengths in an infrared beam that are reflected back from them to measure distances and position the column. Unlike a mirror, however, the holograms can reflect the infrared beam from any angle.
But before the measuring unit can calculate the position of a component, the position of the equipment itself must be established. Warner does this by bouncing the infrared beam from a series of reference targets, the locations of which are known. Once the equipment's position in space has been established, software within the unit will give a readout of the component-mounted hologram's actual position.
The columns are positioned by locating their bases first and then the tops are manoeuvred until they are in the correct position. Next, each column is tied back to the reinforced concrete core by steel beams. When all the columns are in position on each floor, Warner surveys the node positions to check all are still within tolerance, recording their positions in the measuring unit.
Back in the office, Warner explains that the data is downloaded onto a modified AutoCad 3D modelling package. Warner's package compares the actual position of each component with its position on a 3D model of the building to produce a printout that shows the measured position, the model position and any deviation. Any discrepancy is brought to the attention of the design team.
But before Warner's team even got to site, it was ensuring that the structural components were dimensionally correct. "We had to live in Wescol Glosford's fabrication shop in Halifax," explains Warner. The most critical part of the steelwork fabrication is making sure that the faceted tubular steel columns and connection nodes, which form the building's primary structure, are assembled correctly first time.
The rake of the circular columns meant that they had to be assembled from a series of 600 mm diameter tubular steel sections, sliced at an angle so that their end section formed an ellipse. "We had to translate complex maths into 'cut a bit off there mate'," Warner says.
The steelwork contractor was not the only specialist to receive Warner's attention. Warner has been heavily involved in setting out the access ramp that spirals up the building's full-height atrium; it has set-out the diagonal grid that will support the clear-glazed lens forming the building's riverside facade using similar fabrication checks to the main structure. And because the shape of the building means it cannot be set out using conventional equipment, it is even involved in positioning pipework, ductwork and blockwork within the building. The company is also monitoring the movement of the structure as the heavy floor plates are cast. Its final task will be to produce a series of as-fitted drawings once construction is complete.
According to Warner, its measuring skills have been much appreciated on the complex project: "The steelwork fitted together without a snag." However, the biggest beneficiary of Warner's involvement is CIT: "Ours is a new role – an insurance policy for the client."
