By Chuck Knickerbocker
The First United Bank recently opened its doors to a stunning new building in Moore, Oklahoma. Designed by Gensler, a global architecture firm, the building creates a welcoming space that supports the comfort and well-being of all occupants, all while being sustainable and timeless. These design goals help the building embody one of First United Bank’s key values of serving the community.
The 40,000-square-foot three-story building integrates innovative sustainable strategies, including rainwater harvesting, deep roof overhangs, efficient HVAC design and more. The design team extends the eco-conscious approach to its material palette, using a renewable mass timber structure and high-performance steel-framed curtain wall system to create a simple yet contemporary cuboidal form.
Central to its intent, a steel-framed structural silicone glazing system (SSG) wraps around the bank’s facade, spanning the three floors and over 12,000 square feet. The high-performance structural silicone glazing is held to the steel frame by toggles, creating uninhibited sightlines to facilitate community engagement while providing durability to maintain its aesthetic appeal over time.
The Material Behind the SSG System
Structural engineers are familiar with the critical role framing materials play in enabling expansive curtain wall facades. While glass occupies a larger surface area, it is the framing that supports the system. Among the commonly used materials, steel stands out for its exceptional stiffness, which is nearly three times greater than aluminum. (It has a Young’s modulus of approximately 29 million pounds per square inch (psi) versus aluminum at 10 million psi). This inherent strength allows steel to carry heavier loads and resist deflections more effectively than aluminum.
For example, consider a curtain wall with a 5-foot mullion spacing subjected to a wind load of 30 pounds per square foot (psf). A typical aluminum mullion measuring 2.5 inches by 7.5 inches, including the glass and exterior cap, can span up to 12.5 feet but may experience a deflection of about 4.396 inches.
By replacing aluminum with steel, using 2.375-inch by 7.56-inch mullions under the same wind load and spacing, the deflection reduces to nearly 1.5 inches—about one-third of that of aluminum. This increased stiffness and load-bearing capacity enables project teams to extend the length of steel mullions, allowing spans of almost 16.33 feet—a 30% increase compared to their aluminum counterparts. In the case of the First United Bank, steel’s strength is what enabled the curtain wall to use sleek profiles that span greater lengths without requiring intermediate supports. The other contributing factor was advanced steel fabrication techniques first pioneered in Europe.
Modern Manufacturing Makes It Possible
The expansive curtain wall at the First United Bank relies on a laser-welding method that utilizes long, flat carbon or stainless plates (generally 38 to 49 feet in length, and up to 1 ½-inches thick), which are first laser-cut out of flat plate products into bars or strips of the required size. These commonly include rectangles, channels, Ts, angles, square tubes, and I-beams. In particular, the bank employs 10mm (0.394 inch), 12mm (0.472 inch), and 20mm (0.787 inch) thick T-profiles.
Once cut, the edges of these precise sections are laser-welded to create composite shapes that eliminate the need for field assembly of any plates into desired shapes. This is because the profiles arrive on-site in their final forms, complete with the necessary machining for fitting horizontal and vertical sections. These pre-formed components are then assembled into curtain wall grids on-site by the glazing subcontractors. Consequently, the precise off-site manufacturing makes custom shapes that don’t fall into conventional categories also possible—including non-standard shapes for wall assemblies such as rectangles with inset walls or Ts with a round bar at the bottom of the web.
Additionally, when working with architecturally exposed structural steel (AESS), hot-rolled steel sections typically require grinding any exposed welds smooth and removing any visible markings. This is eliminated with modern fabrication because laser-welding produces profiles without exposed welds, mill roll marks, or identification marks. This advanced manufacturing method also allows for sharp corners (0.02 inches) in profiles, creating joints with no visible welding beads or fasteners. Another process allows for larger sections of thin-gauge carbon or stainless material, ranging from 24 gauge (0.0235 inches) to ¼-inches to be roll formed from steel coil stock into a variety of shapes. The rolling process allows for some complexity, such as gasket races and screw bosses (to name a few available features) to be formed into the completed shapes.
An Engineering Marvel in the Entryway
Extending a glass curtain wall from the ground to the roof on a three-story facade, without any intermediate support can be a significant challenge for any project’s engineering team. Transferring wind loads in a tall assembly without any floor plates or beams is a challenge that not every curtain wall material can address. Additionally, the glazing system’s primary framing member also needs to factor in aspects such as glass size, design loads, and connection or anchor points.
To achieve a curtain wall of the desired magnitude, the engineering team at the First United Bank project specified custom, heavy-duty, laser-welded “T”-shaped profiles as vertical mullions and rectangular framing profiles for the horizontal members, leveraging the advantages of modern steel fabrication techniques. The anchorage details were tailored specifically to the project environment, considering the project employed mass-timber structural systems. The result is steel-framed SSG curtain walls that span 12 feet, 23 feet, and 39 feet in height, with no intermediate vertical support points or splices in the main lobby at the First United Bank.
Two factors are at play that allow steel frames to not require splicing. First, the aforementioned strength and load-carrying capacity of steel enables single profiles to extend to such a height. Second, appropriately designed curtain wall systems with continuous steel-backed members can handle up to 38-foot spans without splicing. In fact, upcoming technology allows manufacturers to produce even longer lengths, but these profiles may be challenging to finish and ship to the site. That said, variables such as the center-to-center location of verticals, span length, and structural loads influence the size of the framing member required to support the project’s performance criteria (see sidebar). Still, when longer-length members are desired, it is possible to splice individual members together to form continuous framing members, enabling curtain walls of any height and length.
Anchoring the Curtain Wall
While evaluating mullion spacing and profile sizes is important for structural stability, detailing the connection point where the curtain wall anchors to the structure is equally important. Typically, in concrete buildings, a curtain wall’s frame is connected to the structure using embedded anchorage plates or devices, that allow welded or bolted connections of the anchor to the building frame. In steel buildings, anchors can be welded or bolted directly to the building’s structure or to the face or edge of the floor.
However, because the First United Bank’s floor decking system is primarily cross-laminated timber (CLT), conventional anchorage details were not applicable. Instead, the curtain wall manufacturers worked closely with the structural engineering team to customize the connections between the anchorage, framing, and the building’s decking, something that is not far afield from conventional connections to steel or concrete structures. However, one of the goals for this meticulous detailing was to ensure that the connections were inconspicuous and did not interfere with the finished floor slabs.
Among these details, the anchor plates connecting steel frames to the building’s structural system were intentionally designed to be slightly larger to distribute the load evenly across the timber members, since the primary connection methodology between anchor and structure are wood screws, keeping in mind the capacity of the wood to hold the screws. Each anchor plate is secured with four or more screws, providing the necessary tie to the building’s structure for the curtain wall system. And while column or structure foreshortening (shrinkage) will generally not be included in the curtain wall specifications, this information is critical for the curtain wall designer to know how to properly design and specify anchorages, splice connections, and sealant perimeter joints.
Uninterrupted Glazing With Modern Steel Profiles
The First United Bank’s project team utilized the advantages of steel’s material properties to enhance not only the system’s structural strength but also the user experience. “Establishing a connection to the outside and maximizing natural light are two traits proven to promote the well-being of building occupants,” Jorge De Loera, one of the architects on the project, reports. Building on this evidence-based approach, the team at Gensler integrated biophilic design, which connects occupants to the natural world, by opening the interiors of the First United Bank to uninterrupted views of the landscape outside.
To achieve this goal, slender steel profiles empowered Gensler to facilitate larger opening sizes without the need for additional support. This in turn increased the surface area for light penetration, filling the interiors with natural light and offering sweeping views of the Oklahoma prairies and skies. The narrowness and shallowness of the metal profiles further reduced interruptions to sightlines and created a more monolithic facade. Detailing it further, the project team enabled an uninterrupted, frame-free facade from the outside by specifying a structural silicone glazing (SSG) curtain wall system.
In SSG systems, structural silicone attaches compatible lites to modular carrier frames or cassettes. These carrier frames are then pressure-glazed via the use of toggles against continuous gasketing mounted on the steel framing. The gasketing is the primary means to resist air and water penetration to the building interior while keeping the water off the face of the steel, eliminating the need for exterior pressure plates and caps typical in curtain wall design. The SSG system enables a high-performing building envelope at the First United project, while still utilizing steel’s strength.
Another critical factor is the amount of sealant required to hold the glass to the frame; this is called the bond line width. Its dimension is determined by the size of the glass lite and the anticipated design wind load. In addition to the amount of gasket or glazing tape and the exterior weather joint between lites of glass, the bond line width generally sets the width of the framing member.
Steel Supports High-Performance Curtain Wall Design
Beyond the user experience, sustainability and energy efficiency were key considerations for the First United Bank. The design teams naturally opted for high-performance insulated glass units (IGU) to accompany the steel framing. The four different high-performance curtain wall configurations in the project have glass thicknesses ranging from 1-inch to 1-1/8-inch and weigh between 5.8 and 9.4 psf. While traditional framing may have required larger profiles to support additional weights, steel’s inherent strength allows it to improve energy efficiency using smaller sections.
Additionally, steel has a thermal conductivity (31 British thermal units per hour [BTU/H]) nearly 74 percent less than its aluminum cousin (118 BTU/H). This is equivalent to that of thermally broken aluminum frames. Moreover, some advanced steel frames do not need a traditional thermal break due to their profile designs. Likewise, the system at the First United Bank optimizes steel’s ability to support high-performance glazing and “minimize thermal breaks,” adds De Loera, augmenting the bank’s larger sustainability goals.
Benefits of Sleek Steel Frames at the First United Bank
The sleek steel-framing profiles at the First United Bank enable longer spans, allowing the SSG curtain wall to blur spatial boundaries and transform the way visitors and staff interact. “People in the lobby can see the people in the café, all the way to the mezzanine, making these common spaces feel full of energy,” De Loera explains. By establishing visual connectivity through the high-performance glazing system and spatial planning, steel frames facilitate an interconnected, communal environment, enabling engagement and interaction.
Steel’s material capabilities and strength support the engineering teams at the First United project to deliver a high-performance glazing system that shapes the bank’s values of promoting community engagement and sustainability. While doing so, it also helps them materialize the design team’s pursuit to “create monumental spaces,” positioning the bank’s building as a market differentiator for years to come. ■