The sculptural atrium roof of Birmingham New Street station called for an ingenious waterproofing solution
A soaring 24m high atrium, roughly the size of a football pitch and covered in inflated ETFE bubbles, forms the centrepiece of the £750 million refurbishment of Birmingham New Street station.
The arcing roof structure, designed by London-based architect AZPML, is formed from a series of sweeping wishbone lattice steel arches, each clad in white tensile PVC fabric and supporting the teardrop-shaped cushions of ETFE.
The roof of the existing 1960s building, the Pavilions shopping centre, had to be demolished to create a hole for the new atrium. The reinforced concrete structure, divided into nine discrete sections with movement joints in between, posed a serious challenge for the scheme’s designers, headed up by lead consultant Atkins.
‘The challenge was to keep the building stable with a large chunk taken out of the middle and the addition of a stainless steel facade around the outside of the building,’ Stephen Ashton, engineering director at Atkins, told RIBAJ. ‘The new steel frame for the atrium had to span a moving structure and these changes in loading altered the way the building moves.’
Atkins utilised 3D global stability analysis software to make sense of the constraints, based on initial designs by AZPML and structural engineer AKT II.
The atrium roof structure channels only vertical loads into the building, the steel arches stand on special bearings on the tops of the existing columns, designed to take horizontal movement of up to 75mm. The columns were strengthened with concrete ‘jackets’ to increase their vertical capability.
An ingenious waterproofing solution was required for the areas of the roof that surround the atrium, developed by roofing contractor Briggs Amasco in collaboration with SIG Design and Technology, which assisted with the supply of roof waterproofing systems across the project.
A new 600mm concrete upstand separates the atrium roof from the main roof. Outside it, the existing flat roof deck had a series of mastic asphalt falls retrofitted onto the surface, designed to channel rainwater through various outlets.
To design a new ‘truck deck’ around the atrium, with new sloping falls and outlets, it was necessary to first determine the amount of existing asphalt and the datum of the concrete slab below. Scans through the substrate were carried out using ground-penetrating radar, normally used to scan geological conditions, to avoid the need to core potentially thousands of holes through the existing waterproofing.
Tony Lawther, operations director at BriggsAmasco, told RIBAJ: ‘This data was correlated with GPS location markers across the surface to give us a digital map of relative thicknesses across the roof. It allowed us to design the falls with the minimum economic amount of new product.’
The quick-setting mastic asphalt Flexiscreed, supplied by SIG, enabled the contractor to remove the existing waterproofing layer and install the new layer straight away. ‘It was vital to allow us to keep the fully operational building dry, we had 140,000 commuters a day passing through the concourse directly below. A cement screed would have prevented access for about three weeks,’ says Lawther.
Flexiscreed is manufactured from selected bitumens, limestone filler and specially graded aggregates. It is designed to provide drainage falls as well as a stable base for the specified roof waterproofing system in modern fast track construction projects.
SIG also supplied around 10,000m2 of temporary waterproofing to cover targeted areas of the station concourse floor during the atrium roof installation. The Armourplan PVC single-ply membrane, typically used in roofing applications, was considered resilient enough to support site traffic as well as prevent water from penetrating through the concourse to the station platforms below.