Special brew

Many architects and engineers are familiar with the need to adapt to tight, penned-in sites. Indeed, some may have tackled more extreme situations than this one in Leeds. But try it on for size.

Brewery Wharf is a 9600 m² site, bordered on one side by a busy road and bridge, on another by a river and on the third by another construction site. It is also constrained in the vertical dimension. The development is on water-bearing gravel, but the plans demand a basement car park covering the whole footprint, and planning consent places a rigorous cap on building height. In addition, the client expects an acoustic performance that exceeds that contained in the new Part E of the Building Regulations.

These were the challenges confronting Robinson Design Group after asked by Totty Construction asked it to design 326 apartments in five buildings that lie on the banks of the River Aire. Taking its name from the nearby Tetley’s site, Brewery Wharf was scheduled in three phases over 38 months. The only way to embark on such a large-scale project was to break it down into elements to identify the most cost-efficient construction methods. Simplicity and buildability were emphasised throughout.

Brewery Wharf’s client, Barratt Leeds, automatically specifies the use of concrete over steel in its apartment developments for two main reasons – one, to achieve the best floor-to-ceiling height and avoid cluttering internal spaces with bulkheads and beams, and two, to take advantage of concrete’s good acoustic properties. Therefore, the architects and engineers within Robinson Design Group knew what material they had to work with – what remained to be decided was the specifications that would meet the constraints posed by the site.

The development’s riverside location meant building on open-textured gravel with a great deal of water retention – and water levels could rise at any time. Brian Smith, Robinson’s design director, decided a water-resistant structure using a standard C40 blended concrete mix would do the job, without the need for waterproofing membranes or concrete admixtures. Smith says he is not a great believer in the use of admixtures, primarily because they don’t stop the concrete from shrinking as it cures. David Begg, the project manager at David Ashley, the concrete contractor on the project, agrees. He says: “The problem with admixtures is that they need a plasticiser, which means that the concrete takes longer to reach its strength. We were achieving concrete strengths of 28 N/mm² within three days at Brewery Wharf, which I think is pretty good.”

Reinforcement in the basement slab and walls was specified at 100 mm centres instead of the more typical 200 mm to prevent cracking and to help guarantee a waterproof structure. Finally, the basement slab laid over the pile caps was designed to resist uplift from the hydrostatic forces from below.

For the structure above ground level, Smith’s analysis led him to a 225 mm thick reinforced concrete flat slab. The attraction of this approach was that it enabled a floor-to-floor height of only 2975 mm with a continuous soffit and no unsightly downstands marring the ceilings of the apartments.

One element in the initial design that Smith is particularly pleased with, was the way structural rigidity has been incorporated into the buildings. The blueprint that was given planning permission presented a restricted opportunity to incorporate the shear walls and shafts that are normally used to give lateral stability to a building. Working closely with Totty, Smith responded with a design that increased the width of the columns, but which did not unduly affect the interior layout of the flats.

Smith says: “We did some interesting and useful work in the process of resolving this lateral strength issue. It’s broken new ground for us in terms of medium-rise projects.”

The flexibility of concrete also came into play in the underground car park, which had to be designed with as few columns as possible to allow vehicle access. This was solved by incorporating 850 mm deep beams that take the loads bearing down from the superstructure’s columns and transferring them to columns in the basement. Standing under the transfer beams beneath the tallest block at St James Quay, it’s impressive to think that the weight of nine storeys is being diverted 2.5 m sideways before reaching the foundations.

Another interesting decision, taken after the first phase, was to replace steel construction with concrete for the penthouse apartments. It was felt that concrete was more architecturally compatible with the scheme – besides the attraction of having one less contractor on site.

The efficiency of the design has kept the amount of reinforcing steel needed for the columns to the moderate level of about 3% by weight and 130 kg/m³ for the floor slabs. Not only did this mean less reinforcement being stored on the confined site, but it also made the concrete easier to place, leading to a high-quality result.

Of course, the new occupants of Brewery Wharf may relish all the brashness and excitement of city-centre living, but they don’t want to hear it at home. Barratt set an acoustic standard of 34-35 dB for external walls and windows and an airborne level difference for internal and party walls of 53 dB. Blockwork walls and an acoustic floor screed helped to achieve these targets, but the main factor was the sound barrier provided by the flat slab and concrete frame construction.

The third and final phase of the development, containing 98 apartments, is due for completion by mid-August 2005, bringing the £60 m project to a close. Now that the external works are complete the aesthetics of Brewery Wharf can clearly be seen – a contemporary industrial feel made by the use of a simple palette of materials including metric brick and terracotta cladding.

For the inhabitants, the whole development is a place to get away from it all, right in the heart of one of England’s biggest, buzziest cities.

Client Barratt Leeds
Architect Robinson Architects
Structural engineer Robinson Consulting
Project manager Turner & Townsend
Main contractor Totty Construction
Concrete contractor David Ashley