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With its cascading green plants, a proposed apartment development on top of an old bingo hall promises to be a modern-day hanging gardens 鈥� albeit in Clapham, south London, rather than Babylon. What passers-by are unlikely to realise is that the apartments are hanging too.

This unusual structural solution will 鈥� subject to planning 鈥� enable a long abandoned bingo hall to become a commercially viable redevelopment. Originally a cinema built in 1937, this grade II-listed former bingo hall has a particularly fine auditorium with a highly decorative ceiling. Because this could not be altered or subdivided, it has put off potential purchasers for the past seven years.

Now developer Grange Estates plans to make the project financially viable by selling new apartments and using the money to restore the auditorium and allow it to be used by cash-poor voluntary and religious groups. Forty-nine apartments will be created inside and at the rear of the building, but the really smart move is the proposal to suspend 18 apartments above the building without disturbing the auditorium ceiling. The project team hasn鈥檛 done anything quite like it before, and Andrew Murray, director of engineer Adams Kara Taylor, calls it an engineering conundrum, not only in design but also in construction.

The suspended apartments will be arranged over three storeys and will hang from a row of five arches spanning the building. These are supported on columns within the building but outside the all-important auditorium.

Niall Cairns, associate with Assael Architecture, explains why arches were chosen: 鈥淲e examined a number of structural solutions 鈥� we looked at normal flat trusses and parabolic curve trusses 鈥� but with the 1937 architecture the simple arches worked best.鈥�

The design team has not decided what material to use for the arches, but at the moment steel is the favourite, according to Cairns. 鈥淚nitially we thought we would have to go overseas for the arches, but we found that we can source from companies in Yorkshire. This is almost bridge building. It is technology that can be made here at a reasonable cost.鈥�

The structural engineer is deliberating whether to go for a tube or teardrop shape for the arches. The teardrop profile would be created by joining a small and large tube together with two plates. Murray says this could offer several advantages: 鈥淭he arch is subject to significant bending forces. The teardrop shape will reduce the tonnage of steel required because it is more efficient in bending and additionally reduces its visual impact.鈥�

The apartments will rest on a suspended steel truss attached to the arches, with apartment balconies and walkways attached directly to the arches. The existing pitched roof will have its apex sliced off but will not cause disruption to the ceiling below as there will be a gap of

100 mm between the roof and the suspended apartments. The project team has not yet decided whether it will use volumetric modular or panelised technology to build the actual apartments. 鈥淢odular would be fantastic because this is such a difficult site to construct on, on all fronts,鈥� says Murray.

Even eagle-eyed passers-by won鈥檛 see the gap between existing building and new apartments, because the roof will be re-covered to incorporate the underside of the apartments. Nonetheless, apartment buyers may wonder whether there is any risk of the apartments swaying in a high wind. None at all, says Murray: 鈥淭he new columns are designed to take the vertical loads, with the arches and existing building structure taking the horizontal. These loads are quite small in comparison to the vertical and the arches are pretty substantial.鈥�

The project team still has a long way to go in designing and detailing the scheme, and, of course, planning approval has yet to be won. Negotiations with the planners are continuing, and the project team hopes the scheme will be on site early next year.

Project team

client Grange Estates
architect Assael Architecture
structural engineer Adams Kara Taylor
historic buildings consultant The Richard Coleman Consultancy