The X2 warehouse at Heathrow is the first two-storey facility of its kind in the UK
In the UK, warehouses are usually sprawling sites beside motorways, rarely occupying more than one level and never accessible by more than one ramp. This is not the case in Asia, which has many multi-storey urban warehouses - the Kerry Cargo Centre in Hong Kong is 20 storeys high. Such buildings inspired X2, a two-storey warehouse served by two access ramps at Hatton Cross, Heathrow, designed by EPR Architects and taken through the construction process by Cornish Architects.
X2 contains eight discrete 6m-high units, with four at ground level and four on the first floor, and two floors of office accommodation at the front. Six units are heated and cooled by a ground-source heat pump. Each level has access to a service yard, where heavy goods vehicles (HGVs) can dock and be loaded and unloaded. This is made possible at the upper deck by a pair of massive concrete ramps.
The project was initiated and is owned by Brixton, a specialist warehousing firm. Before X2 went ahead, Brixton, EPR Architects and Cornish Architects visited Asia, primarily to analyse different ramp-to-accommodation configurations.
‘Prior to development, we consulted many occupiers and letting agents. They saw the logic of the approach, but were cautious about the ramp access,’ says Peter Dawson, investment director at Brixton. ‘In theory, a single ramp can serve as both the up and down route, which is cheaper, but there were concerns about it getting blocked. At Heathrow, we made things a bit more generous to reassure people.’
‘The calculations and construction methods involved in the ramp make it more of a civil engineering project than anything else,’ adds Matthew Archer, associate director at Cornish Architects.
Two intertwining, steeply curved ramps divide up-traffic from down-traffic, but in the event of a blockage, a traffic management system enables the other ramp to be used for movement both up and down. Consulting engineer Waterman Civils used path-mapping software AutoTrack to calculate the curves and clearance heights - 5,400mm under the ramp and 5,110mm under the podium deck - required by the lorries, tangibly expressed in the plans. Vehicle-movement modelling was based on the UK’s largest permissible HGV (16.5m-long with a 4.7m-high trailer).
‘The bends do look tight,’ says Archer, ‘but the drivers will experience much tighter ones on their journey here.’ To further convince prospective tenants, Brixton has made a video showing huge articulated lorries, which require a 24m turning circle, manoeuvring the ramps with ease.
‘There is a clear distinction between the steel frame of the warehouse and the concrete of the ramps and service-yard podium. Designing the junction between them to remain watertight was the biggest challenge,’ says Archer.
The ramps and podium are the defining elements of the project. The ramps comprise reinforced concrete retaining walls that support a reinforced concrete road-deck, which rises to a series of reinforced concrete columns and beams that in turn support the higher level road deck. The floor-loading capabilities are 30kN/m² and 15N/m² (the British standard for warehousing) for the ground and first-floor warehouses respectively.
The high cost of land this near Heathrow Airport was the reason for designing a two-storey warehouse, and this proximity also caused design and construction problems. The project was limited to two storeys because flight paths are just metres away from the building (X2 is just 70mm below the maximum building height in places) and the Piccadilly Underground line runs in a shallow concrete box-tunnel along the southern boundary of the site.
X2 was only 50 per cent more expensive to build than a single-storey building per square metre. ‘A conventional warehouse of this size [22,541m² gross lettable area] would require 5ha at 45 per cent site coverage,’ says Dawson. ‘This project uses 2.6ha at nearly 86 per cent coverage, which is a massive saving.’
The project has yet to be leased but Brixton is considering a second two-storey warehouse. This HGV-friendly, multi-storey typology could be the future of warehousing in the UK, with this innovation marking the halfway point between our current single-storey stock and fully-automated storage towers.