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O'Mahony Pike Architects' Cedar Brook housing scheme at Cherry Orchard, Dublin, required an innovative construction system to combine speed with high quality. On a 4ha site, the housing is 30 per cent shared ownership for people on the city council's list, with the balance for sale at fixed, affordable prices to the private market. It is a high-density scheme of three and four storeys, comprising 376 apartments, town houses and duplexes arranged in 90m-long terraces, providing residents with highstandard, modern homes in an acceptable level of open space.

The Cherry Orchard competition that O'Mahony Pike won called for the development of a new system of building which was fl exible, certifiable, durable, readily available and cost effective, within a tight programme of delivery. 'The system is similar to the vintage 1950s children's building toy Baco, ' explains architect John O'Mahony. The appeal of Baco, for those too young to remember, lay in threading rows of flat panels over a line of evenly spaced vertical metal rods jammed into a base, basically building reinforced external walls to a model house then able to take the loads of its red pitched roof. 'The whole idea was to use prefabrication off site as much as possible, ' says O'Mahony.

'We would have liked to have used precast-concrete ground beams but they were too expensive so the only cast in situ work is in the footings, which were then covered in a concrete raft.' The system works by stacking the locked-together concrete wall panels on top of each other. Vertical high-tensile steel bars were cast into the ground-floor slab, projecting 900mm along the 6m panel grid of the perimeter and internal walls at regular centres (see Working Details, pages 8-9).

Prefabricated full-height reinforced concrete panels were cast in steel batch moulds to standard sizes by Concast. The largest is 9m x 3.6m, with holes running through them from top to bottom at the same centres. Brought to site, the panels were lowered over the vertical steel bars to stand upright. The panels were temporarily laterally restrained using steel props. 'Each precast panel interlocks with adjacent panels through an over-locking sawtooth edge detail, with extra support at corner junctions provided by a steel dowel running down the joint' says O'Mahony.

Once all the interlocking external and internal walls are in place, the support restraints are removed, allowing the 200mm-thick, 50kN concrete walls to stand as a self-supporting structure. A double polyethylene seal to all joints ensures resistance to water penetration. Weep holes every 400mm along horizontal joints ventilate the panels and allow escape for condensation, anticipated only in extreme conditions. High-tensile steel bars introduced into the reinforcement holes through the top of the cavities are then anchored into place using a high-strength cement grout.

Once the grout sets, the structure is ready to carry the 2.4m-wide precast, pre-tensioned, concrete floor slabs that span up to 6m onto internal party walls 200mm thick. The floor slab was finished with screed over a mild-steel reinforcement mesh.

Then the whole process can begin again to take the building up another floor. Services are introduced into the screed and internal wall structures only, in order to keep all party and external walls clear of electrical interventions, allowing the full strength of the concrete panels to be retained.

External panels were cast in the same concrete mix as the rest of the project to provide economies of scale but finished at the factory with a 3mm coating of trowel-on acrylic paint in textured pigment shades of red and white. Highlight timber cladding features were used in selected areas. Large timber windows were installed and glazed once the panels had been located on site, using cherry pickers to access the upper floors. Smaller windows only feature on each end of the four-storey apartment blocks that flank the building, where these sheer walls stiffen the structure.

Eliminating scaffolding through prefabrication gave a massive saving and also dictated the choice of roofing system.

'We wanted to use a pre-made cassette roof but costs could not be stretched that far, so a profiled Nordman metal roofing system was laid over timber trusses, with the load carried through the stacked structural panels of the external walls, ' says O'Mahony.

These durable, well-finished buildings were delivered within a tight construction programme, taking just 14 weeks to put up a three-storey block and 18 weeks for a four-storey block, the use of the precast-concrete construction system reducing the building programme by 30 per cent.

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