From dark unwelcoming platforms to bright, wide open spaces, Birmingham New Street station has undergone a huge transformation.
Built to replace a war damaged cast iron structure, the 1960s incarnation of Birmingham New Street station was a dark and complicated building to navigate. Low floor to ceiling heights and oppressive closed in concrete frames made the busiest station outside London an unwelcoming place to visit.
It was also woefully constrained in terms of capacity, built for only 60,000 of the 170,000 passengers which now pass through it every day.
“On a busy day we can have up to 250,000 people using the station and at certain times the only way the station can deal with that is to close it,” says Network Rail project director Chris Montgomery. “They had to enact that around half a dozen times a year - not a great first impression to the city.”
However, through a five-year and £750M project carried out by funder Birmingham City Council, operator Network Rail and delivery partner Mace, the station has undergone a dramatic change. The structure has been subjected to a huge series of modifications to transform it into a light and airy space, with the demolition of floor and roof slabs to create a new open atrium concourse area. Platforms have been de-cluttered, 36 new escalators have been installed, wayfinding has been vastly improved and the capacity of the concourse areas where passengers circulate has been increased fivefold. If this wasn’t enough, all the construction work was carried out while the station continued to operate a normal timetable.
Throughout the project, AKTII, the structural engineers for the new atrium roof, worked closely with Atkins, the engineers working on the existing building, to produce a global stability analysis of the whole structure. Nine separate concrete sway frames on a three-by-three grid, with movement joints between them, make up the existing building. The stability of the building is through portal action. A careful eye had to be kept on the lateral stability of the building in its new state and throughout the construction period due to the huge structural modifications it was undergoing.
“We changed the approach to the global stability model,” explains AKTII design director Steve Toon. “We thought: how would the original designers have designed it? We reversed the clock back to understand how they would have detailed the junctions to make the building stand up.”
The steelwork to support the atrium roof was installed before the concrete roof slab was demolished. This kept the building weathertight and allowed the trusses to be installed over the live railway before the ethylene tetrafluoroethylene ETFE infill panels were installed.
Four 1.5m to 2.5m deep steel “wishbone” trusses support the three domed areas of the roof. They span 40m across the large new concourse area, underneath which was once dark and covered by low concrete slabs. Due to space restrictions on site, the new trusses had to be lifted into place in sections, supported by temporary trestles and then bolted or welded together once in position.
New Street redevelopment: Existing building challenges
The 1960s multi-storey carpark above the railway was due to be left in place, however when it was examined, the concrete was found to be in extremely poor condition due to chloride attack from de-icing salts used on the roads.
The structure had to be demolished and rebuilt, adding another layer of complexity to
Demolition also proved tricky. The original programme was around 12 months, but asbestos found on site made this challenging.
The asbestos elongated the programme, so time savings elsewhere had to be found.
“Our original methodology was to cut and lift but we had to look at different sequences, working on multiple work faces and ended up using the ‘mega muncher’ , an excavator with an oversized set of demolition jaws on it,” says Mace project director Martyn Woodhouse.
“There were certain working restrictions as it made it top heavy with quite a small boom arm. It would munch out the main beams of the slabs more quickly than the previous plan and we managed to take the original programme from 12 months to six months.”
To avoid putting horizontal loading into the existing concrete frame, the team designed the roof trusses not as arches, which would have put a thrust into the supporting structure, but as curved beams ensuring that very little lateral load was transferred into the existing structure.
“They have un-restrained fixings at one end which allow horizontal movement in most instances under differing loading conditions,” says Toon.
These curved beams are supported by a 1.5m-deep box girder which effectively creates a ring around the atrium void underneath the trusses which spans between columns on the existing concrete frame. The connection of this ring to the existing frame is in itself an intricate bit of engineering.
To accommodate movement, the roof steelwork is supported on a total of 14 elastomeric bearings.
“The truss literally does breathe with thermal loads. Under snow load it will spread, and then under wind loading it actually gets sucked up slightly,” says Toon. “Only two of these bearings are locked in position (in plan) in both directions, all others allow movement in either one or two directions.”
Tight engineering tolerances were imposed on site for the installation of the bearings, the shoes in which the bearings sit and for the specially designed plinths which connect them to the existing structure.
During a value engineering exercise the design of the trusses was modified to allow for the guttering on the roof. The trusses which appear triangular prism shaped, are actually only a V-shape and lack the usual link between the two top chords of the trusses so that the deep trough of the gutter can be accommodated. To prevent the top chords from buckling sideways without the connecting bars to restrain them, the diagonal members in the truss have been beefed up, acting as cantilevers beyond a horizontal member connecting the trusses about half way down their length.
“This change added a little bit of structure but it made a big saving on the overall position of the guttering detailing,” explains Toon.
From below, and now that the trusses above the main concourse has been clad in a tensioned fabric, this clever guttering detail has been hidden from view.
Changing the perception of the station had to be an inside and an outside job. Architecturally, the existing station façade was drab, tired and ugly. Now, mirrored panels curve and facet around the exterior of the building, reflecting the sky and passenger areas below.
The cladding skin itself is made up of 3mm thick, 1.5m wide stainless steel, rain-screen panels. Originally the design by architects Foreign Office Architects, called for the panels to have double curvature.
However after rationalising the model’s geometry, AKTII was able to change most of the panels to have only single curvature. This allowed the sheets to be transported from the factory in Portugal in flat sheets, pre-cut and bent into shape only when they were installed on the new supporting steel frame - saving the project a considerable amount of money.
This new steelwork which would bolt onto the existing concrete frames to support the new façade was also a complex challenge.
Until work commenced on site the existing frame could not be surveyed and therefore designs for the connection brackets could only be finalised during construction.
Specialist contractor Martifer undertook the cladding works in conjunction with Atkins and AKT II.
One of the challenges was that the new steel sub-frame had to respect the existing movement joints in the station building and the localised fixing capacities of the existing structure.
This was the tricky part. Without comprehensive existing building drawings, each location had to be scanned for rebar and the connection designed specifically for that point.
“Martifer would have to cut out the existing cladding and then done a detailed survey of the existing reinforcement,” says Toon. “It was an iterative exercise of the fixing details in order to avoid bolt/rebar clashes. Each fixing type underwent a set of on-site proof tests to verify the fixing design and substrate capacity.”
All the hard work has paid off and after a last push effort by the teams on site to remove plant, tools and hoarding, the station opened on Sunday 20 September to rave reviews. It incorporates a new shopping plaza and has now become a destination in its own right with the capacity to deal with the thousands of passengers who use the station every day.