Roofing Types

Long span: The majority of today’s arenas of up to 25,000 seats are long-span truss roofs. In addition to supporting video boards and concert rigging, the roof will often participate in the acoustic objectives for the facility (i.e., foster good interior sound, address sound escaping the venue, etc.),” says Dickson.

“Long-span truss roofs can support large loads close to the oculus and facilitate construction by providing a support line without the requirement for temporary works below,” states Peter Chipchase, M.Eng., C.Eng., MICE, MIStructE, PE, director, WSP, London.

Because the NBA and NHL maintain stringent temperature, humidity, and sports lighting requirements for their venues, the roofs are typically closed and fixed.

Tension/compression ring system: This is a lightweight, long-span solution that can internalize the inherent forces within itself, much like a bicycle wheel, Correa explains. “This often creates advantages for a sympathetic retrofit of an existing venue by reducing the impact on the current structure, or creating a demountable system that can be removed, replaced, or modified independently—with minimal impact on the internal program—to promote venue flexibility and help future-proof a design,” he says.

Retractable roofs: Though not typically found in arenas, retractable roofs offer the authentic experience of playing football, baseball, or soccer outdoors, while maintaining a comfortable, temperature-controlled environment on especially warm, cold, or rainy days.

“A retractable roof can provide the best of both worlds, allowing for the venue to be either an open-air or enclosed stadium depending on the weather and the preferences of the fans,” Miller says.

Buro Happold Principal Derrick Roorda, SE, points out that designs must accommodate both open and closed loading conditions and have a mechanism for opening and closing that can be accommodated by the structural system.

Cantilevered canopies: Though more common in stadiums, cantilever roofs are a signature architectural element, enhancing the fan experience with unobstructed views and creating a greater sense of intimacy in the seating bowl, according to Miller.

Offering the benefit of modularization and standardization, this enables mass production. Consequently, the project benefits from fabrication efficiencies, repetitive construction, and extensive optimization to reduce steel tonnage while maintaining performance, explains Chipchase. As self-supporting units, they can be installed on multiple work-fronts, thereby shortening the construction schedule.

Fabric roofs: Also more common in stadiums and ballparks, these lightweight tensile structures utilize membranes to enclose the primary structure. “Clear and translucent roofing and facade materials, such as ETFE and PTFE, create an outdoor atmosphere in a permanently enclosed, conditioned space,” Miller says. The lightweight roof materials reduce structural support requirements and cost.

Fabric and cable roofs are sometimes used in pure sports venues where blackout conditions, acoustic performance, and high rigging loads are less important, but Aryes would not normally recommend them for an entertainment-led, commercially driven venue. These roofing types “tend to come into their own on larger stadium projects where minimizing self-weight is even more important, climatic control is less important, and a lighter, airier structure is more architecturally attractive,” he says.

Stadium Roof Considerations

In determining which roofing system is best for a particular project, a number of factors must be taken into consideration.

“The needs of arenas and stadiums and environmental effects (such as rain, wind, snow, and ambient temperature) as well as the future readiness of the venue weigh heavily on the selection of the appropriate long-span roof: fixed closed roof, partially closed roof, or retractable roof,” explains Cerone.

Additional factors for determining the ideal solution for a long-span roof, according to Miller, include geometric alignment with architectural design intent, structural efficiency that minimizes materials and self-weight, repetition in detailing and assembly, ease and speed of erection, the capacity, expertise and competitiveness of local contractors, fabricators, and erectors, and site considerations, such as crane access and removal.

While weighing these factors will help guide the design decision, there is no single formula that always yields the best solution.

Offering some general guidance on typical arena configurations, Aryes relates that most modern commercially driven arenas have roofs with a primary span in the range of 100–130 meters (325–425 feet) with an aspect ratio of around 1.2–1.5. “They require a fairly dense node configuration to accommodate rigging, and they tend to be blacked out,” he explains. “As a result, they almost always end up being some form of one-way truss, two-way truss, or space frame configuration since these options tend to favor relatively low roof profiles and are relatively easy to build using easily handleable components.”

When Aryes’ team embarks upon a long-span roof project, one of the first questions it asks is what the roof will look like once it is half built since the cost of erection and temporary works can easily outweigh the material costs. Unlike some roof types that are only stable once complete, the benefit of trussed roofs is that they can be assembled in robust, stable, handleable components and tend to be self-stabilizing with minimal temporary works. Consequently, the roof construction does not paralyze the entire site in the way that a dome or cable-net roof might.

Another major consideration is the roof’s ability to support future flexibility and retrofits. As market needs are constantly changing, arena owners are looking to maximize the flexibility of their venues. For example, the infrastructure required for network broadcasting is quickly evolving and needs frequent updating. “Revenue models evolve as well, requiring the rebalancing of fixed seating, suites, concessions, and other spectator experiences,” adds Tracy.

That said, steel structures generally add the most flexibility when it comes to renovations. For example, demolition can usually be achieved using cutting torches, diamond saws, and other small tools, Tracy explains. And connections to the existing structure can be made through welding and bolting, which are both methods that can be reliably inspected and afford a high level of confidence.

At the same time, it is not as common to retrofit the long-span roof structure itself because accessing the trusses to perform structural work is exponentially more expensive once the facility is opened; the work is highly field intensive, and room for logistics is limited at that point, says Callow. Consequently, it is important for the project team to carefully evaluate the desired allowances for show rigging and potential scoreboard upsizing to strike a balance between minimizing future retrofit work and day-one construction cost.

Barclays Center

Taking a look at the way structural steel was designed and fabricated for a high-profile multipurpose arena project, Brooklyn’s Barclays Center, designed by AECOM and SHoP Architects, is home to the National Basketball Association’s (NBA’s) Brooklyn Nets and one of the home arenas for the National Hockey League’s (NHL’s) New York Islanders, in addition to a host for concerts, conventions, and other sporting and entertainment events.

Photo courtesy of Forest City Rater Companies

A 350-foot long-span structural steel roof crowns Barclays Center in Brooklyn.

The building design was heavily influenced by the Bankers Life Fieldhouse in Indianapolis, which is rated as one of the premier basketball viewing facilities in the country, and provides excellent sightlines, as the roof structure is much higher than the seating, explains Callow, whose firm did the structural engineering for the project.

But one noted difference between the two arenas is that Bankers Life was a concrete superstructure with a conventional arched truss, whereas Barclays Center was more suited for a steel superstructure and a tied-arch system. This determination was based on initial schedule and cost studies that showed the tied-arch system was an efficient way to achieve the 350-foot roof span.

Working on a congested urban site, the choice of steel also eliminated the potential scheduling challenges of having too many trades on location at one time. On the other hand, with limited real estate for on-site building, the steel structure did not inherently have sufficient lateral stiffness to resist the arch thrust forces alone. The solution was introducing a tension tie to balance the arch forces. Consequently, the thrust forces imposed on the superstructure are greatly minimized.

“This strategy also simplified the overall roof system, as two ‘super’ tied-arch trusses were provided to span the long direction of the arena, while shallow trusses span between the super trusses and the perimeter,” explains Callow. “With the pure arch trusses, it was preferable to distribute that thrust across the whole building, whereas the self-contained load path of the tied arch trusses allowed them to be centralized.”

Several analytical studies were performed to both understand and design for some component of the overall thrust that results due to the stiffness of the building, he adds. Construction sequencing with member leave-outs were performed to minimize the amount of the thrust that ‘leaked’ to the primary structure to maximize the superstructure’s efficiency.

Two main tied-arch trusses run in the long east-west direction, spanning approximately 350 feet. Each comprises a 12-foot-6-inch-deep arched upper truss, with chords made up of W14s that vary across the section and a tension tie, consisting of a 14x311 wide-flange member, which occurs approximately 50 feet below the top of the arched truss and 10 feet above the lower ends of the truss for optimized sight-line preservation. The truss configuration was largely dictated by the fabricator’s shipping limitations. The tie is hung by a series of eight, 8-inch vertical HSF pipes. Sixteen smaller trusses connect with the large longitudinal trusses and provide the free span of the arena with chord sizes varying from 14x90 to 14x159. Dead loads and lateral loads are resolved into the steel superstructure, with thrust forces from the trusses largely balanced by the ties.

Photos courtesy of Thornton Tomasetti

Pictured here is the erection of primary roof trusses at Barclay Center. Four temporary shoring towers were provided to allow for construction of the tied arch truss elements.

Callow explains that one of the project’s main structural challenges was connecting the tied arch to the steel superstructure that rings Barclays Center and forms its street-side concourse. In a conventional design, where the tension tie would connect at the ends of the truss, the truss would rest on a roller or bearing support that could allow the truss some movement. However, for Barclays Center, the connection had to be able to transmit some lateral forces to the superstructure. While the introduction of the tension tie significantly reduces the amount of thrust imposed on the superstructure, it does not eliminate it completely due to strain compatibility between the tension tie elongation and the superstructure lateral displacement. To minimize this effect, Thornton Tomasetti specified a construction sequence involving leave-outs of elements to disengage the arch thrust resistance of the superstructure for de-shoring of the roof structure. This forced the tied-arch action to resist the initial dead load. After the roof was de-shored, these leave-out connections were completed, leaving the superstructure to resist thrust forces under future environmental and live load conditions.

Another unique aspect of the design is its cantilevered canopy. To find the balance between the desired geometry and project budget, Thornton Tomasetti and SHoP Architects performed numerous parametric studies. “To create the 85-foot cantilever, backspan trusses were run from the entry structure storefront all the way back to the primary bowl support columns,” explains Callow. “The three-story entry structure to the west of the primary bowl was lightly loaded, and thus it was desired to tie the cantilever backspan back to the heavier bowl structure, like the back anchorage of a diving board.”

Coordinating the cladding attachments to the cantilevered structure was one of the more challenging aspects. To address this, Callow’s team communicated anticipated movements of the cantilever under dead load, snow, thermal, and wind loads to the facade subcontractor to ensure their joints were able to accommodate the movements. The structure was pre-cambered to accommodate some of the anticipated vertical deflection. In addition, three-dimensional models of the anticipated structural range of positions during cladding panel installation was provided to the cladding manufacturer to ensure they had sufficient range of tolerance in attachment points.

“One of the less obvious benefits of the canopy structure was that the creation of the back span trusses that were needed to support the cantilever allowed for elimination of columns in the entry structure VIP restaurant region, as the back span truss was able to span over the region,” says Callow.