By Catherine A. Cardno, Ph.D.
A vegetated spiral staircase under construction in the atrium of a multiuse building in London known as Ampersand offers meeting spaces that appear to float between floors.
Part sculpture and part structure, a spiral staircase currently being constructed for the atrium of a mixed-use building in London combines two unlikely elements to form a signature design: a ribbon of planters and “floating” platforms. © Paul Cocksedge Studio
July 22, 2014—Part sculpture and part structure, the three-story spiral staircase currently being constructed in the atrium of a mixed-use building in London combines two unlikely elements to form a signature design. Planters have been adapted to flow down the spiral stairway, much like a living wall transformed into a spiraling ribbon, and "floating" platforms have been added at each story to provide meeting spaces nestled within the greenery. Designed by the London-based design firm Paul Cocksedge Studio with structural engineering by the London office of the global engineering firm Arup, the spiral staircase has no central support column and instead appears as a three-story-tall sculptural form with no visible means of support.
"The inspiration behind "Living Staircase" came out of a desire to rethink the way we move from point A to point B," said Paul Cocksedge, a cofounder of the firm, with Joana Pinho. Cocksedge wrote in response to written questions posed by Civil Engineering online.
"The traditional spiral staircase is a tightly wrought, narrow structure with a load-bearing central column," Cocksedge said. "Here, instead, the spiral's diameter has been enlarged to reveal entirely new and easily accessible spaces at the center of the staircase—spaces where people can meet, talk, relax, reflect, make tea, read books."
Structurally, the staircase's spiral has a 5 m diameter and is completely independent of the atrium's glass roof, according to Christian Dercks, Dr.-Ing., a senior structural engineer in the London office of Arup. The staircase is constructed of steel—clad with a thin oak overlay—and cantilevers from the edge beam of the four floors that open onto the atrium. A back span located within each floor supports the stairway, Dercks says. By cantilevering from the edge beams, the platforms are given their floating appearance.
The circular spaces located at each landing will offer space nestled within the greenery for meetings and relaxation.© Paul Cocksedge Studio
"People think that the design is driven by load capacity/bearing," Dercks says. But actually the structural design is driven by "dynamic criteria to minimize any vibration effects," he explains.
Footfall-induced vibrations can make people unwilling to use a staircase because they do not feel it's safe, says Dercks. To minimize these vibrations, "the structural system is a hollow section formed of the soffit plate and stair riser, stringer, and stair top plate," Dercks explains. "A hollow section was used for [its] technical aspect, to provide sufficient bending and torsion stiffness" in the staircase, he says. The steel elements that form the hollow box range in thickness from 6 to 16 mm.
To trick the viewer's eye into thinking the stairs have a thinner profile than they do, the box section was tapered in cross section to give as thin and light an appearance as possible at the stairs' exterior edge, Dercks says. The outer edge is 200 mm thick while the inner edge is 300 mm thick, he notes. This was accomplished by angling the soffit, located beneath the stairs, as it extended between the inner stringer plate and outer stringer plate.
The stairway's balustrade will contain a continuous outer planting box, as well as an internal secondary planter so that water can drain from the plants without affecting the stairway or balustrade, says Cocksedge. The sunlight that will fill the atrium from above will make it possible for this ribbon of plants to grow, according to Cocksedge. While the balustrade is not part of the structural system, the massing that it will provide via the plants and water will help dampen vibrations introduced by people using the stairs, Dercks adds.
"The Living Staircase is designed to be a staircase, a relaxation zone, and a sculpture all in one," Cocksedge said. "All these activities are distinctly low-tech, to provide a contrast and a counterpoint at the heart of the very high-tech environment that is Ampersand."
According to the architects, onstruction of the staircase has just begun and completion is expected later this year.