Seeing ClearlyJanuary 13th, 2023 | Category: Architects' Guide to Glass and Metal
Design, Collaboration & Engineering Combine in a Unique Structural
By Ellen Rogers
When you can go to work every day with extensive, beautiful views of water and mountains, why would you want to stay home? Maybe that’s what Expedia was thinking when designing its new Seattle headquarters. Completed in 2020, the campus spans 40 acres and features several sustainable, LEED-certified buildings that provide views of the Puget Sound, the Olympic Mountains, downtown Seattle and the working port.
With Expedia’s “open and connected” strategy and principles in mind, biophilic design played a key role. Biophilia provides a way to connect people with nature—whether through views of the outside or surrounding the interior with lush greenery and plants. Natural light is also essential, making a massive cable net wall ideal for this project. Designed by architects ZGF and installed by Benson (now called MiTek), the main entrance of the headquarters features two structural glass, insulating cable net walls—a structural glass vestibule and a structural glass canopy—that were designed, engineered and supplied by Sentech. One of the cable walls spans the full height of the building, while the other side incorporates the canopy and doorways.
“The cable net wall encloses the north and south facades of the entrance lobby with an essentially uninterrupted ‘window’ on the internal and external centers of activity for the campus,” says Patrick Gordon, AIA, principal at ZGF Architects, who led the Expedia project. “The space forms the central heart of the campus and reveals the interactions of employees, partners, and visitors, while also providing a view through and beyond to the waterfront campus, its expansive landscape, and the incredible natural environment beyond. The result reinforces a sense of anticipation for
what lies beyond, and establishes an undeniable point of entrance and arrival.”
He adds, “We were looking to enclose the space with as little vertical or horizontal interruption as possible and reveal as much of the internal space as possible, while having to address the seismic constraints imposed by linking the new and existing structures.”
For the entrance façades, Sentech supplied its VetraNet series system, which reaches more than 48 feet tall and 148 feet wide and totals more than 10,900 square feet of structural glass.
North Glass supplied 190 panels, with the maximum size spanning approximately 12 feet tall. The units measured 15/8-inch thick and were constructed with low-iron tempered insulating glass units (IGU) with a SentryGlas Plus laminated inboard and outboard lite. Pre-tensioned cables create an optical net effect and provide flexibility, durability, and resistance to high environmental loads.
While the Expedia project included the renovations of several existing buildings and some new construction, the cable net wall is the stand-out feature. Benson was responsible for all of the buildings. For project manager Brian Cox, this was his first cable net wall.
Cox says Benson worked in a design-assist relationship with ZGF and brought Sentech into some of those meetings.
“[Sentech] came up with the design and installation plan and sequence based on the tension of the cables,” says Cox. “Everything went smoothly, and there was no real moment where we had to scramble and figure things out.”
Aesthetics and Performance
Sustainability and energy efficiency were essential; so was the glass needed to meet stringent performance requirements. That brought a unique component to this system—the use of IGUs, which is rare in cable net walls.
According to Sentech, the IGUs undergo large warping deformations that are difficult to accommodate. This was a challenge during the design and engineering phase. The team had to balance the high amount of movement and flexibility inherent in pre-tensioned cable systems with the required deformation limits of the façade. This was critical to maintaining seals in the IGUs. The company designed unique, flexible edge conditions to minimize panel warping and ensure the IGU seals were not overstressed.
“Cable net walls are pre-tensioned and flexible, which is great for blast design since the flexibility allows for their use in high impulse load applications,” says Dave Dunham, Sentech’s director of engineering. “Insulating glass can’t handle large edge deflections or warping deformations, as they can lead to a break in the edge seal, and the units will fog up. Balancing this restrictive edge deflection is a unique challenge in cable net walls. Typical corner panels and panels adjacent to entrance vestibules are critical elements in the design of this type of structure because they have rigid support on two edges and they typically undergo large warping deformations. The design of edge elements is difficult to balance in these applications and it often requires varying tension loads and carefully balancing the stiffness of the perimeter members to minimize warping deformation.”
He continues, “The behavior of cable walls is highly dependent on the stiffness and behavior of the perimeter structure. As the wall deforms, the tension increases, which causes the building to deform. The building deformation, in turn, relaxes the cables and creates more deflection of the facade. Coordinating how much load you’re applying and how the structure responds, and harmonizing the stiffness and behavior of the façade and support structure required extensive coordination and communication between the design team and our engineers.”
In the Details
The hardware and fittings were also essential. Dunham explains that most cable walls are designed using panels with an aspect ratio close to one(fairly square), which makes it easy to accommodate warping deformation and results in highly transparent solutions, Dunham explains.
“For this project, we had some glass panels that were taller than they were wide, so we used mid-span patch fittings to optimize the glass thickness design,” he says. “This allowed us to remove joints in the cable wall that are in the opaque zone of the glass and increase the height of the IGU. The glass support system afforded by a cable net wall is flexible, and there’s no support with zero deflection. The novelty of this design is that we used IGUs within a non-rigid support system that results in high warping IGU deformations; our engineers had to design for gradual changes of the
support stiffness to ensure that the IGU deformation was within allowable limits.”
He adds, “The IGUs were argon-filled and allowed us to use a very high energy-performing façade system. The cable wall allowed for increased transparency, so it was highly thermal while still connecting to nature and adjacent views. The systems allow increased daylight without overtaxing the HVAC.”
The design of the fittings also affected the installation. Cox says the glass panels had to be installed in a specific sequence, even the imperfect pieces.
“We rejected only one and had one broken piece, but we had to install them anyway, or we couldn’t keep going because of the patch fittings,” he says, explaining that Sentech designed the fittings to accommodate the glass in a precise order. “The glass wasn’t caulked, though, until we had the replacements. Then we were able to swap them out.”
On the Plus Side
Some structural applications have waned in popularity, such as point-supported and spider-fittings. Dunham says designs have shifted from the once-popular spider fittings, which can have a mechanical aesthetic, to a more clean and continuous design that blends into the surroundings.
“Cable net walls have a place because of their advantages in flexibility and span. They can be designed as tall as you want; the design possibilities are almost unlimited,” he says. Also, he adds, the use of patch fittings on the Expedia headquarters allowed for the use of high-performance coatings, which wouldn’t have been possible if a design with drilled holes and spider fittings had been used.
The Expedia job was also a learning opportunity for Cox, having never worked on a cable net wall before.
“Our installers and glaziers had done these jobs before, but it was a first for me,” he says. “Pre-tensioning the cables based on the loads of the glass and the roof was unique, as well as the different sequencing [and knowing the right] cable tension to what percentage. This gave me insight into pre-tensioning and loading for the building.”
Projects like this can be intimidating, but they can also serve as a learning opportunity.
“At first, you can be skeptical and nervous,” says Cox, “but it also gives you confidence knowing that engineering works.”
Ellen Rogers is the editor of USGlass magazine.
Follow USGlass on Twitter and Facebook.
To view the laid-in version of this article in our digital edition, CLICK HERE.