Stairway to heaven

Shortlisted for a 2018 Structural Award, the Tiffany Gallery Glass Staircase, one of New York’s latest design icons, it has attracted much attention among the global engineering community. Nominated for The Small Projects (of under £1 million) category, one of the chief structural consultants behind the project – Tim Macfarlane, director at Glass Light and Special Structures (GL&SS) – offers his take on this project.

Eva Jiricna, the architect for the New Tiffany Gallery in New York, is internationally known for her glass staircases, and the Tiffany Gallery Staircase is another in a series that goes back to her first collaboration with me in 1989.

With Eva every new project is an opportunity to explore a fresh solution based on the particular circumstances of the brief. In the case of the Tiffany Gallery the staircase was a central element encountered on entering the gallery.

Spiral and straight forms were considered until the natural flow of a gently curved ascent emerged as the best solution; the rear wall of the staircase providing a screen to the lift beyond. The desire to minimise the visual impact of the leading edge of the flight led to the idea of cantilevering the tread support from a vertical fin, which then had the threefold role of providing a screen, a balustrade and a structural support to the stair.

The glass cantilever beam supporting the treads was initially considered as a separate element from the fin and, as such, needed to be bolted to it creating a moment connection. This resulted in increased fin and riser dimensions and unsightly bolts. In an effort to simplify the clumsy bolted connection the idea of fabricating a T section fin arose.

Cutting glass to this shape, and then toughening it, is not common practice so although theoretically possible there was a need to verify the concept at an early stage. A mock-up of three treads and fins was commissioned and load test were carried out to verify the load bearing capacity of the T section fins. Loads up to three times the design load were applied to the mock-up, which passed the test successfully.

The glass balustrade on the leading edge of the staircase was designed to be cantilevered from the glass riser tread assembly. The initial connection of the balustrade to the risers resulted in a connector which was visually too massive. After careful consideration a detail was developed using an embedded puck within the laminated assembly, which was visually minimal.

The realisation of a staircase in glass demands the highest level of collaboration between the architect and structural engineer as every element and connection is visible. So, a harmonised response is vital and every inch of technical know-how needs to be employed by the engineer to arrive at a solution that matches the architect’s vision.

We are delighted to have been recognised on this year’s Structural Awards shortlist and look forward to the announcement of the winners in November 2018.