Boston – When it opened at the turn of the 20th century, the 15th century Venetian-style palace housing the Isabella Stewart Gardner Museum welcomed a mere 2,000 visitors per year. Now, more than 200,000 visitors annually enter the museum doors. Ms. Gardner may no longer reside in the palace apartment, but plenty of preservation, restoration and administrative specialists were shoehorned into the aging building. A tiny café and a bookstore also were wedged into the space. And the museum’s glorious tapestry room was being used as a makeshift performance space.
According to Gardner’s will, the palace galleries could not be changed, but the contemporary demands on the museum clearly needed relief if it were to survive into the new century and beyond. To provide such relief, the museum enlisted Renzo Piano Building Workshop as architect and Boston’s Buro Happold as the structural engineer and mechanical, electrical and plumbing (MEP) engineer for a $114 million expansion, which was completed in November of last year with its grand opening in January.
The expansion, which is situated in the museum’s rear gardens, houses museum-related facilities, administrative offices and back-of-house functions, a greenhouse, two apartments, a special exhibition space, and a 300-seat performance hall.
The greenhouse, entry lobby, and palace link are highly transparent structures with large areas of glass and minimalist detailing. Because of the architect’s extensive use of façade glazing and the desire for an elegant expression, bulky light and view blocking structural elements were highly discouraged.
To accommodate this desired design aesthetic, Buro Happold created a structural solution that integrated the gravity and lateral systems into the mullions, precluding the need for redundant columns and beams to carry the weight of the superstructure and exterior wall. As a safeguard, the systems were conceived and vetted at an early stage and carefully coordinated with the facade contractor during design and construction to ensure the constructability of the structure.
The structural engineer specified architecturally exposed structural steel (AESS) throughout these structures to control tolerances and appearance. The same specification also was used for the main stair, exterior emergency egress walkways, skylight trusses, and music hall balconies – places where the exposed minimalist structural systems are a key design element. Slender plates and rolled sections were prevalent in the design with rod hangers in select locations to reduce structural depth.
Matching the appearance of the many different AESS elements was further complicated by the variety of design and procurement methods. The integrated structural façades and exterior walkways utilized a design-assist process with the curtain wall fabricator. The more typical design approach delegated for miscellaneous metal and stairs was supplemented by fully designed sections and connection forces by Buro Happold.
The scope of each of the sub-contractors was carefully coordinated and supervised by the general contractor, Shawmut Design and Construction, Boston, to ensure
an aesthetic unity.
The two primary spaces in the addition are the special exhibition gallery and the music hall, which is comprised of a series of three “floating boxes.” The boxes are supported by W36 upturned structural steel cantilevered over 8-in. columns, which are, in turn, inset from the exterior wall. The columns of the floating boxes are supported by slender concrete basement columns and subjected to overturning forces. Lacking the adequate space for anchor bolts, the basement column reinforcement was tightly coordinated to pass up through the ground floor and welded to the
column baseplates. The resulting weld condition was not pre-qualified and required submission to the American Institute of Steel Construction.
The upturned steel frame and a distribution route for the MEP systems were hidden within the raised acoustic flooring of the floating boxes. Discrete points on the steel cantilevers support the three-story concrete box of the music hall. A detailed strut-and-tie model was created to test its stability and redundancy.
In the music hall, three levels of balconies wrap the circumference of the 300-seat venue. The balconies feature a custom-designed acoustic precast panel hung from an AESS steel frame. The space was originally conceived with post-supported balconies, but the design evolved into a hanging rod solution resulting in a slender structure that completely eliminated the need for columns in the performance space.
Initially, the team was concerned that the large duct requirement for the music hall smoke evacuation would have overwhelmed the space. The structural engineer resolved this issue by combining structural and mechanical systems into concrete cast box girders on the roof. In addition to providing long-span roof support and a smoke exhaust plenum, the beams double as support structure for the three levels of balconies.
In the end, according to the museum, the new building is a distinctive architectural addition that relieves pressure on the historic palace’s spaces and features materials that respectfully complement the existing site and neighborhood.
This article was contributed by Buro Happold of Boston. For more on architecturally exposed structural steel (AESS) go to www.aisc.org/aess.