Architecturally Exposed Structural Steel (AESS) sits firmly on the boundary between architecture and engineering.
The material requires the highest level of technical knowledge from the architect and the highest level of design acumen from the engineer.
The steel needs to be designed to be structurally sufficient but, given that it is exposed to view, is also a significant part of the architectural display. Design, detailing and finish requirements are typically of a far higher standard than the concealed alternative.
In skilled hands, it is the ideal material for exposed features such as trusses, canopies, unique support systems, storefront facades and much more.
The new glass-and-steel entry pavilion at the mixed-use Brookfield Place (formerly World Financial Centre) in New York is an elegant example of AESS. Featuring a pair of 53-foot tall funnel-shaped steel columns that support the structure, it was designed by Pelli Clarke Pelli Architects with Thornton Tomasetti serving as the structural engineer.
The complex geometry and changes to New York City Building Code created several project challenges. Newly enacted and more stringent seismic requirements had an impact on the fabrication of the structural steel.
The steel used is also covered by intumescent paint which increases visibility of imperfections. The tight tolerances also required particular quality control and an advanced system of dimensional control during erection.
Designed to accommodate an estimated 100,000 pedestrians daily, the pavilion is linked to the underground passageway from the World Trade Centre and the Fulton Street transportation hubs. Because the pavilion sits atop a new underground passageway, existing train tunnel relieving platform and a former pedestrian bridge pile cap, Thornton Tomasetti’s engineers had to focus on just two points of contact for the columns underground. This was integral to the design and placement of the steel columns.
At One 57, one of Manhattan’s tallest residential buildings, AESS has been used to create undulating canopies which extend organically from the glass curtain wall.
Typically, a canopy of this size would require cables or rods to anchor the structures to the side of the building. Instead, a 15-foot backspan extends into the building, which allows for an unsupported cantilever. These backspans are not hidden, and instead are backlit and polished, providing symmetry to the aesthetic of both the interior and exterior.
On a completely different scale is the Music City Centre in Nashville. The $415-million project boasts a grand entryway fabricated using arched barrel steel joists, wave roof and many structural elements that remain exposed in its finished state.
A collaboration between architects Bell/Clark and TVS design with Ross Bryan Associates as structural engineers, the challenge involved selecting the best expansion joint solution that would accommodate the building’s exterior features that resembles the rolling curves of an acoustic guitar. Forming the curves of the "guitar" is a 14-acre, vegetation-covered rolling roof that is the largest of its kind in the Southeast that had to remain watertight to protect the occupied space below.
The use of steel joist over the lobby wall areas enables the desired curve lines to be followed effectively. In addition, open web steel joist are an economical solution for a special profile roof situation.
The entire project shows off the design possibilities of steel construction, while putting a spotlight on the engineering of “special profile” steel joists for dramatic aesthetic impact and cost savings.