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A hybrid solution for the new Conran Shop

Two features of the existing building influenced the choice of a hybrid concrete solution for the structural frame of the new Conran Shop built behind the retained facade of the Burton Stables in Marylebone High Street, London, explains Tom Schollar

Tom Schollar of structural engineer F J Samuely and Partners was the partner in charge of the Conran Shop project

The two-storey terracotta facade of the Burton Stables is an attractive landmark of the northern end of Marylebone High Street in central London. This facade and the adjacent 6m strip of the structure have now been cleaned and refurbished, while the sheds and workshops behind have been demolished for the construction of the new four-storey Conran Shop.

The design-team members did not start with any preconceived ideas of what the new structure should be. However, two features of the existing building and the site directed them to a hybrid concrete solution. Firstly, the triangular shape of the site called for a column layout that could provide sensible column spacing at the rear facade which co-ordinated with the arch spacing on the Marylebone High Street frontage. Secondly, the proposed floor-to- floor height was limited by the need to match the existing floor levels on the Marylebone High Street frontage and also to comply with strict planning limits on building height. These constraints determined the design team to use concrete and so make a virtue out of necessity in the design of the floors.

Initial ideas focused on a simple flat-slab structure. This was a logical enough solution but one which would have resulted in the column grid being greater in one direction than the other (so the longer span would determine structural depth), and it would probably require a flat false ceiling. A more interesting design was then explored whereby the soffit was exposed and all mechanical and electrical services were located within the raised floor void.

It was decided to leave the structure exposed as an architectural feature with a vaulted ceiling. This provided greater actual headroom in addition to the perception of increased headroom due to its shape. The use of limestone aggregates from the Baladon quarry provides a light concrete finish which further enhances the advantageous spread of light from suspended liner uplighters. The finish of the vaulted ceiling is fair-faced with robust detailing that expresses the nature of the material.

Sitting the flat slab on top of precast beams would have reduced the available headroom, while using halving joints in the ends of the floor slabs would have been difficult within the required small structural depth. Furthermore, precasting the columns would have meant using a standard billet detail to attach the beams to the columns - not ideal when the billet underside is to be located close to shoppers' heads.

The solution was to use precast floor slabs with in-situ beams and columns. This had a number of advantages. It took out tolerances and gave uniform invisible joints. Only one mould was required. A level soffit with minimum depth was achieved. However, the design team had to accept that the appearance of the in-situ work would differ from that of the precast concrete. In the event, the quality of work by both the precast manufacturers and the in-situ contractor meant that the difference was minimal.

Design is an interactive, not a linear, process. As such a number of related issues affected the proposed hybrid solution. These included recognising that the largest width of precast slab that could be sensibly placed on a lorry was 2.4m wide, and that the slabs required a minimum thickness for fire resistance but the thinner the slab the more economical and easy it was to transport and erect. A further design consideration was that as the vaults ran lengthways, the depth of the ribs could be the same as the depth of the in-situ beams. Normally main beams are deeper than secondary ribs, but if the main beams span in the same direction then the rib depth can be the same. Above all, exposure of the structure demanded a high-quality uniform finish.

Close co-operation between the engineers and with the precast manufacturer, Malling Precast, resulted in a number of design modifications. Holes for sprinkler pipes and smoke detectors, together with suspension points for light fittings and goods displays, were all detailed to be cast-in. Some 128 units were cast using a steel mould. Where the new structure met the existing building, truncated units were cast which used the stop end from the same steel mould.

The main contract was won by Wallis and the subcontractor for the in- situ concrete was Mitchelsons. The exposed floor slabs which are used at the first and second floors were constructed within three weeks.

Following construction of the columns, the shuttering for the in-situ beams was set up on falsework. After considerable debate on the material best suited for the beam shutters, it was decided that the required finish could be achieved using ply shutters. The precast units were then erected with some additional propping to limit the load on individual props. After fixing the reinforcement and checking the shutters, the in-situ beams were poured.

The inherent accuracy of the precast concrete units cast in one steel mould allowed the floors in effect to set themselves out. By placing the units and casting the heads of the in-situ columns in the right place, it was simply a question of measuring the distance from one precast unit to the next.

The precast concrete slabs are of very high quality. Any contrast with the in-situ columns was minimised by the contractor working to a tight specification based on pre-tendered examples of the precast slab units. Indeed, the quality of the finish of both the precast and in-situ concrete is so high that many might believe they were looking at a form of plaster rather than structural concrete.

The Reinforced Concrete Council has examined a wide range of hybrid solutions in its report 'Hybrid Concrete Construction' ref 97.337, £13.50

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