4 Pancras Square

A mammoth Vierendeel truss wraps around the building at first-floor level. This truss was fabricated by Severfield and brought on to site in 18 sections – the largest being 17m long and the heaviest 72 tonnes – and craned into place.

Despite the weighty statement of the truss, there is still movement in section. The ground floor is drawn in, and above this two sets of four floors shift to provide blocks of south or north facing balconies, with those on the southern entrance elevation running across the entire width and those on the plainer north elevation cut out of six bays. The top-floor office space is set back, leaving the final section of the exoskeleton framing the sky.

Internally the structure is a concrete post-tensioned flat slab supported by internal concrete columns, yet at the edges this post-tensioned floor is supported off the external weathering steel frame at intermittent support points. The driver for the internal floor typology was mainly in response to the environmental design strategy, which is a displacement ventilation system.

With support from the truss at their edges, the post-tensioned concrete floor plates are free to be simple and vast, able to be read back to a simple core, housing stairs and lifts with little obstruction from internal columns. This also allows for a huge amount of light, and the windows promote close-ups of the exoskeleton’s construction and attachment as well as the brise soleil. The only departure from this otherwise spartan provision is a landscaped green roof.

It is the first office building in the UK to achieve BREAAM 2014 ‘Outstanding’.