Building the Revenue.

The construction of the new Inland Revenue complex exploits techniques of prefabrication, yet this does not compromise the building's essential tectonic quality.

The new Inland Revenue building proved an immense constructional challenge, in which management, programming and off-site manufacture played a vital part in construction design decisions. The abandonment of the original scheme meant that vital time was lost, but the pressure to complete the project in the shortest possible time, remained. The situation was-further exacerbated by the presence of the redundant piles, which also hindered progress on site.

The Hopkins scheme consists of an arrangement of repeated blocks which became, in effect, a production line where specialist work groups completed tasks and moved to the next unit. But with construction time so critical, the architects were forced to amend their original competition proposal - of precast floor slabs with structural topping between concrete beams supported on in-situ loadbearing brick piers and a row of central columns - to an entirely prefabricated clear span scheme. The revised construction - developed with Ove Arup & Partners - although simple in principle, presented a huge challenge to achieve the necessary levels of accuracy and consistency of finish. In addition, as part of the energy strategy, the ceiling slab was required to provide thermal mass, since the raised floor containing the building and office equipment services masked the structural floor.

As so often in the Hopkins office, the practice looked to earlier schemes and developed techniques to meet the new situation. At Bracken House, in the City of London, precast concrete beams were exposed as part of the ceiling to reduce the floor depth, and in the new opera house at Glyndebourne precast concrete panels were used to form the soffit to the balconies and support the seating above. Here, an as-cast finish was used for a series of shallow curved floor plates 3.2 m wide which spanned the full 13.6 m width of the building. By tying these together and also to the in-situ cores containing the lavatories and lifts, a solid floor element was formed. The individual slab elements, although adopting an arched configuration, were designed as folded plates restrained from splaying at each end with a steel tie. The thickness of the units varied from 280 mm at the edges, to as little as 125 mm, on the 1.6 m office layout module. A deep shadow line was formed between units to allow for potential variations in deflection, although little difference has occurred in practice. Eight full span steel moulds were used for the long units because only the robustness of the steel could guarantee continued accuracy. Twenty seven other moulds in both timber and steel formed the short span units in the core areas and extended the long units as a cantilever to support the overhanging top floor. In total 887 units were cast of which 305 were 13.6 m span units. Although the use of white concrete for the ceiling units was considered originally, the extra cost and the difficulty of achieving a colour match with the in-situ cores meant that grey RHPC with Trent Valley aggregate was used. A white translucent wash to the ceiling and the core units, rather than paint, increased the reflectivity of the concrete and achieved the desired colour match without masking the nature of the material.

Supporting the floor units are solid, loadbearing brick piers. All the brickwork on the site was 'pre-built' by teams of bricklayers using traditional techniques. The piers were then put into store and placed in their finished position many weeks later. To allow for transportation, the brickwork was laid on bases, concrete for the ground floor and steel plates elsewhere, which had a lifting rod attached, running the full height of the unit. The floor plates, which were set on shims to produce the accuracy required, were fixed to the top of the piers. The spaces between the capping pieces and the floor plates are grouted through holes in the seating units on site. Tight tolerances were set for the work using templates, and 1032 piers were produced and transported to site just before being finally positioned.

Although many aspects of this building, such as the tented recreation area, the lifting roofs to the stair towers and the overall energy performance, will attract more attention, the construction of the main office structure may contain the most valuable insight to future building trends. Trent Concrete Structures Limited, with two subcontractors, won the order for the superstructure package in December 1992 and finished erecting the brick piers. and precast elements in December 1993. No major features of the competition scheme had been compromised and several improvements were made possible by the use of this form of construction.