This roof extension to an existing industrial office building at Garrett Street, EC1 consists of a series of steel portal frames carrying a profiled metal roof deck. This sits on steel transfer beams on stub columns from the existing roof structure. There is a timber floor structure spanning between these beams.
The challenge for this project was to create a simple structure that would be largely exposed with the front and end elevations fully glazed and the rear elevation partially glazed. Portal frames made from 300x100 rectangular hollow steel sections provide lateral stability across the width of the building. Hollow sections can be difficult to connect on site. It was not possible to bring a whole frame to site in one piece, so the columns were neatly welded in the fabrication shop with 1.3m of roof beam already attached and an inner weld backing spigot section in place. Then the remaining roof beam section was slotted onto the spigot section and a site weld made. The location of this weld was designed to be at the point of least moment to ensure the minimum stress on the site weld.
Stability along the length of the building was more challenging as the only opportunity for bracing is in the rear elevation. The need for bracing in the front elevation was avoided by using the plate action of the roof plate together with the frame action of the portals to deal with the torsion arising from the eccentricity of the bracing in the long direction.