Tensile Pavilion in Boston
A four-story stage house, clad in metal siding and housing an aluminum rigging-truss, was to cover 5,400 square feet (500 square meters), with a performance area of 3,600 square feet (330 square meters). The truss would cantilever over the audience 26 feet (8 meters), supporting a load up to 40,000 pounds (18,000 kilograms).
Continuous over the stage house and audience seating area, the fabric roof would integrate the spaces, both structurally and visually.
While developing the seating plan, however, we quickly realized that the three masts would obstruct views of the stage for a substantial number of seats. The financial consideration of such loss of potential ticketing revenue was a key criterion for design. After a design meeting, the general manager joked about eliminating all the masts. This sparked our internal re-evaluation of the design.
Our solution came during a late-night brainstorming session together with the project engineering firm, Buro Happold Consulting Engineers (BHCE).
Craig Schwitter, BHCE's, lead engineer on the project, came up with the idea to try an arch, and we tested the idea immediately with a coat hanger, pizza box, and panty hose. By midnight, we'd confirmed the feasibility of the structural concept.
Cooperation led the design process that night, as business principles, engineering, and aesthetics were synthesized to yield the final music pavilion design concept.
Based on the preliminary architectural drawings and physical model, BHCE developed a computer model to size the primary structural components such as the supporting steel, cables, fabric, and foundation.
This computer model was then transferred to the acoustical consultant, Cavanaugh Tocci Associates, Inc., to aid their critique and analysis of the general shape and materials of the proposed structure. Their analysis showed the shape was quite acoustically friendly, and only the back sloping portion of the roof needed to be treated with an acoustical batten.
The design easily passed all subsequent approval processes. Because the architectural design concept had appropriate engineering input during schematic design, we were presenting, from day one, a feasible, buildable, scheme. And at the same time as we designed the building, we were designing the construction process.
During the design phase, we met repeatedly with rigger Ocean State Rigging Inc. to figure out how the structure would be erected, effectively fusing their construction expertise with our expression of the building form.
In September of 1998, Beacon Skanska was retained as the general contractor. They analyzed the architectural/ engineering design scheme and developed a construction budget and an aggressive schedule to meet an early July 1999 opening date.
A.Form Architecture and BHCE were retained to proceed with design development and to produce an architectural/ engineering bid set tailored for a tensile design/ build contract. In January of 1999, Span Systems Inc. was awarded the tensile portion of the contract. Work immediately started with full coordination between owner, architect, engineer, construction contractor, and fabricator.
This tensile structure is a balance of the forces of tension and compression through five major components:
First is the tensile fabric, a structural material, which can span long distances in double-curved surfaces. Tensile fabric comes in different structural grades and colors. Selection is based on a structural engineering analysis, environmental considerations, and architectural program intent.
The fabric selected for the FleetBoston Pavilion was Ferrari 1502t, a vinyl-coated polyester, with an additional coating to ease cleaning and enhance durability. Ferrari has created a recycling process for their PVC products.
The fabric usually comes on 72-inch- (1.8-meter-) wide rolls. We patterned the fabric as a clothing tailor would, to create the right shapes and use the roll economically. The pieces were welded into the large, properly shaped sheets with radio frequency welding machines which melted the polyester.
The second important component of the tensile structure is the compressive steel frame which serves to push and pull the fabric into the desired shape. The steel skeleton is usually formed of a system of masts and struts which the fabric and cable connect to.
In this case, the frame was a single large truss arch, the signature structural feature of the amphitheater. It spans 260 feet (80 meters) at the base and is 100 feet (30 meters) high. The main members are 12 inches (30 centimeters) in diameter with 8-inch (20-centimeter) web members.
The third structural component is the cable web, which stabilizes the steel arch and forms the boundaries of the fabric skin. The cables are in tension and assist in transferring forces throughout the structure.
For the FleetBoston Pavilion, we used doubled-up cables one inch (2.5 centimeter) in diameter. We used two cables instead of one to keep the cables to a manageable size and with stock diameter and standard end fittings and hardware.
Fourth are the complex-shaped steel plates sandwiching the fabric and cables so they can be connected to the steel frame. The membrane plates are a pure expression of engineering and geometry but can be stylized by the architect.
The cables and membrane plates are part of a connection toolkit of pins, shackles, turnbuckles. These combine to create connections that provide adjustability and accommodate movement. For detailing, we borrow both hardware and concepts from bridge construction, sailing, or the theatrical world.
Fifth are the compressive concrete foundations to support the reaction loads required to resolve the structural system. These footings can be formed as sculptural elements.
The Pavilion Opens
In July 1999, the FleetBoston Pavilion opened to the public. For us, it represents a collective commitment in design and function, a willingness of the entire design and fabrication team to work together and share ideas.
The inherent aesthetics of a tensile structure — free of extraneous elements — are the direct expression and visual resolution of structural forces through architecturally designed components. The tensile approach to structure is not applicable for all architectural programs, but when designed and used appropriately, tensile structures can be stunning.
Andrew Formichella is principal of A.Form Architecture in New York.