Mirror, mirror on the wall

Highly reflective cladding looks impressive but can tell a sorry tale. At Birmingham New St, tackling glare was integral to the design

Rafael Viñoly must be rueing the immutability of the law of reflection, responsible for the ‘death ray’ effect that plagued his Fenchurch Street tower in the City of London this summer. Luckily for Birmingham, London-based Alejandro Zaera-Polo Architects (formerly FOA) has integrated this fundamental physical law into its design for the reclad of New St Station, whose reflective stainless steel facade is taking shape in the city. 

AZPA project architect Charles Valla finds it hard to describe the nature of this reflective facade appliqué alone, because the total design approach for the reinvention of this 1960s concrete modernist station is much more than the sum of its parts. New St station is in some ways the de facto city centre, whose tracks and cuttings effectively divide it north/south. FOA, which won a competition to redesign the station in 2008, acknowledged this and proposed a multi-directional matrix of connectivity over the tracks – celebrated in a new 23m high central domed atrium space. 

Valla argues that New St’s complex and undulating stainless steel cloak, far from skin-deep ornamentalism, is actually as modernist in principle as the windowless concrete box it covers. The firm was keen to use the formal language of the station to play on internal and external views and express the building’s function in a way that was previously hidden. ‘There the act of reflecting is not an architectural gimmick but a tool to define the nature and context of the station itself,’ Valla emphasises. ‘We wanted the facade to reflect specific views and capture activity inside and around the station – crowds, trains, platforms, sky. The curving mirror facade is a twist on the modernist dogma of form follows function.’  

Two way stretch

But as the design team became aware, reflection works both ways. If the new facade, fixed to the walls of the existing station via a secondary steel structure, designed by engineer AKTII, reflects the sky or a train to a pedestrian walking by, the effect is reciprocated, so the pedestrian is visible to the train, or the sun can dazzle the passer-by. This was quickly picked up by Network Rail’s signals committee, concerned that the reflective facade could have implications for the safety of rail traffic pulling into and out of the station. But with the winner already announced to great fanfare and planning permission in place, there was little will from any­one, political or otherwise, to go back to the drawing board and propose a different cladding. Since the signals committee had the legal power to halt the scheme if it had safety doubts, the design team was charged with proving that reflections would pose no hazard to drivers or the general public.

Valla explains that this meant the firm returning to the sections it had created around the building to create the form and reflect train, sky or the public realm at key points along the facade’s perimeter, which were then connected, dot-to-dot style, to generate the elevations. Valla concedes that as the design was engineered, it changed from a free-flow to a more rationalised form, but the principle remained – on the east and west sides there was more emphasis on reflecting sky and tracks, while on the north and south it was about the public realm. On the west side, which had an access ramp to the existing car park and a footbridge over the tracks,the facade was intended to extend nearly 36m beyond the original wall. This aspect has since been put on hold. 

Valla says AZPA’s model was interrogated using Grasshopper, a plug-in for Rhino software that allowed the firm to carry out light ray studies. ‘The kind of problems that beset Viñoly’s tower didn’t really affect us as that facade was regularly concave while ours is ­irregularly convex, dissipating heat and glare, rather than concentrating it,’ Valla notes. Even so, Network Rail needed irrefutable evidence that luminosity wouldn’t be excessive at any point during the year, so Arup’s specialist lighting team was brought in.

The north facade of Birmingham New St station receiving its mirrored stainless steel appliqué on a secondary steel structure attached directly to the existing concrete structure.
The north facade of Birmingham New St station receiving its mirrored stainless steel appliqué on a secondary steel structure attached directly to the existing concrete structure.

Moment’s reflection

‘We identified three key factors,’ says Arup ­associate John Waite, who with colleague Richard Morris carried out the studies on the chroic facade. ‘Not just “disability glare” for train drivers, but also the possibility that reflection might affect the ability to judge the status of the 26 signals governing entry and exit, as well as possible confusion from the reflection of multiple signal effects on the facade itself.’ Using bespoke ‘Radiance’ software developed at the USA’s Berkeley University – recognised as the most accurate ray tracing software out there – Arup first analysed a 10m-long mock-up of the facade. After reporting on its illuminance values it extended the research to the whole skin design. Waite says that accuracy was critical to building the 3D model, both to inform the facade design and to take account of future track realignment. There was also the problem of modelling the effect of reflection on different train engine cab window profiles. For this the firm created a ‘virtual’ window that embodied the attributes of every train, from a high-speed ­Pendolino to a shunting engine.

Waite says disability glare is time critical – 500+ candela is only a problem if it is endured for more than three seconds at a time, so Arup reassured the signals committee on the safety of the design using short movies that simulated the approach into the station. Using a methodology that employed both annual solstices and equinoxes as control dates for when problems would be most pronounced, any illuminance ‘pinch points’ on the facade were identified by the software and sent back to AZPA, which modified the model that was then re-­interrogated with the new parameters. 

With Radiance ray-tracing every point on the facade, and any localised change having a corresponding effect across it, each simulation model took about three days to generate. ‘With trains pulling in from the east and west, the most significant changes made to these facades dealt with low early morning and late afternoon sun,’ says Waite. It’s been two years since they started the study and he’s confident that the analysis has produced a geometry that has mitigated the worst effects of solar glare for the drivers; adding that the model is so ­accurate that Network Rail is discussing buying it for its own use. This all augurs very well for the project satisfying rail safety demands, but Waite adds a caveat. ‘Our professional remit was to deliver a solution for rail traffic, but reflective effects on neighbouring buildings are unavoidable for a project on this scale.’ In this regard, it seems, AZPA’s iconic urban project may yet prove transformatory.