Channel High Design: Part two

by Erik Missio | November 16, 2015 2:48 pm

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Using channel glass in curved and serpentine applications
by Jeff Razwick

As shown in Part One[2] of this series, channel glass’ distinctive, self-supporting, U-shape makes it possible for design professionals to use glazing in new ways. Its versatile depth and profile can follow a building’s shape, step around fixed objects, change elevation, and soar uninterrupted up to 7 m (23 ft). Its linear form also opens up a whole new realm of design possibilities by enabling what few other glazing materials can—curved and serpentine applications.

The slender frames and narrow glazed segments in channel glass systems allow for radii as tight as 1980 mm (78 in.). These compact arcs allow architects to push the form of buildings in ways impossible with conventional glazing, from tall curved partitions to ethereal glass walls that ripple like ribbon.

While these installations may appear complex to design professionals who are new to channel glass, a basic understanding of how the material performs in curved and serpentine applications can simplify specification. To aid in this process, this article offers considerations for using the material to add ‘curve appeal’ to buildings.

Curved channel glass basics
In curved channel glass systems, the aluminum framing is stretch-formed into a radius. The cast-glass channels then segment around the framing to form a curved or serpentine configuration. The three primary ways the framing can be stretch-formed to achieve a desired outcome are:

• curving the head and sill frames parallel to the ground—this creates a standard curve;
• curving the head frame perpendicular to the ground—this creates an arch at the head of the channel glass application; and
• curving raked head or sill frames parallel to the ground—this creates a curve that gradually changes dimensions.

The simplest way to stretch-form framing is to curve the head and sill frames parallel to the ground. In such applications, the glass is cut square at each end and then segmented around the framing. Where aesthetics drive channel glass applications that rise or fall in height as they curve, or that feature an arch at the head, the manufacturer or supplier will cut the glass segments at an angle to accommodate the change in dimension and ensure the planks fit within the frame profile. This process is more challenging and may require additional planning to ensure channel glass segments lay out appropriately within the system. In such instances, one should consider consulting with the manufacturer or supplier early during the design and specification process.

In all the above configurations, the channel glass segments are installed vertically (horizontally glazed applications cannot be installed in a curve/serpentine, as that would require the glass channels to be bent, which is not done). The planks can either be dual or single-glazed, depending on project needs. Single-glazed channel glass systems allow for greater light transfer, and are well-suited for interior partitions or façades where energy performance is not a concern.

Dual-glazed channel glass systems provide greater sound, thermal, and energy performance, and can accommodate insulating aerogels to improve thermal performance (as discussed in Part One[2]). Both single- and dual-glazed curved channel glass systems can incorporate high-performance coatings to improve functioning.

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Beyond the curve

Due to channel glass’ versatile shape, it can do more than form a simple curve. The glazing can serve as a tall, sweeping façade that encloses interior rooms, transitions between straight and curved sections, and integrates windows. While the design freedom the material provides can push the form and shape of modern buildings, it is important to consider how the geometry of these complex curves can impact the design and specification process.

Tall curves
As is the case with all vertical channel glass systems, the cast-glass channels in curved systems can span up to 7 m (23 ft) without interim framing if allowed by a building’s design-imposed wind loads. When working to extend the elevation, either for aesthetics or to combat wind load constraints, it is important to note wind load anchors are not compatible with the system. The irregular geometries created between the nested channel glass segments as they curve prevents the anchor from running between the planks and attaching to structural elements behind the glass.

To extend the curved channel glass elevation, it is necessary to use stacked joints. Stacked joints use a structural member to split the elevation into two separate openings, which are then individually framed in an aluminum perimeter frame. They allow design teams to elicit the effect of tall, continuous spans of glazing while ensuring a structurally stable system. A case in point is the Oceanaire Seafood Room in Houston, designed by GHA Architecture. The design team specified a three-tiered channel glass system that stretches approximately 10.7 m (35 ft) high and curves around the corner of the restaurant.

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Unusual transition conditions
When achieving tight radii or working with unusual transition conditions, such as straight-to-curved sections, it can prove challenging to align channel glass segments within the geometrical constraints. To achieve the appropriate configuration, the design team can use a combination of different standardly available channel widths, ranging from 232 to 331 mm (9.13 to 13.03 in.), with consistent flange depths.

In the case of the University of San Francisco John Lo Schiavo Center for Science and Innovation, designed by NBBJ, unusual transition conditions were necessary for the cast-glass channels to animate and follow the curve of the building’s adjacent walkway. To account for these transitions, the design used a combination of raked and curved channel glass heads and sills. This effectively made the size and positioning of each channel glass piece unique, allowing the façade to transition between curved and straight sections. In some instances, the linear glass channels step around architectural elements that intersect the walls at unusual angles.

Integral vision lites
Where desirable to incorporate integral vision lites or other glazed systems in curved channel glass systems, it is often necessary for the window manufacturer and supplier’s in-house engineering team to work together to overcome design-specific challenges. With appropriate design and specification, this custom condition can contrast with the channel glass’ translucent form to add movement to the façade, extend line of sight, or enhance daylight transfer.

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For example, in the University of Virginia’s Claude Moore Medical Education Building, the design team and supplier worked together to create a multi-story, curved façade with integral vision lites that wraps around lecture rooms. The two parties coordinated to eliminate the normal need for structural isolation between the channel glass system and adjacent window systems, as well as the jamb framing at those locations. From a design and performance standpoint, this reduced sightlines from framing to allow more natural light to stream into the space for students and faculty.

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Serpentine channel glass systems
While the same basic design principles apply to serpentine channel glass applications, one technical challenge is ensuring a homogenous appearance when transitioning between a convex or concave radius. The joint connection between convex and concave radii creates a fixed transition point. Careful planning and precise installation are necessary to ensure the channel glass segments do not collide at this fixed point, but rather flow smoothly into adjacent sections.

To create a seamless transition at the joint connection, it is possible to use flange-cut pieces (L-shaped pieces of channel glass) and channel glass segments in a combination of different widths, as is done in stepped transitions. This was the case in Canada’s Bradford West Gwillimbury Leisure Centre, designed by Salter Pilon Architecture. To ensure the channel glass segments did not interfere with the joint connection and interrupt the channel glass wall’s uniform look, flanged L-shaped channel glass planks were installed on one side of the joint transition and full U-shaped channel glass planks on the opposing side. This configuration allowed the channel glass framing head and sill components to be stretch-formed into a smooth, continuous structure with a precise radius.

As serpentine channel glass systems may require manufacturer or supplier involvement, so it is important to consider early collaboration during the design and specification stage. Conversations during the initial phase can help conserve time and costs during installation, and leave critical time for problem-solving and design adjustments.

Staying ahead of the curve
While curved and serpentine channel glass systems can add visual interest and engagement to curtain walls and façades, their translucent channels can also redefine how light plays into a space. Varying the translucency of the cast-glass channels or adding bold polycarbonate laminates to the channels can create eye-catching façades that double as privacy screens.

Color-coated metallics and custom decals can help design professionals achieve decorative walls and partitions. It is also possible to set lights between the primary wall and channel glass system to create gently curving partitions or façades that glow at night, while taking advantage of sunlight by day.

Before moving forward with the final specification, one design option is to consider how light, texture, and color can add to the visual appeal of curved and serpentine channel glass systems to create a façade, entry, curtain wall, or partition that sets the building apart.

In the final article[7] in this three-part series, this author explores high-performance systems that address meeting hurricane, sound, and thermal insulation requirements.

[8]Jeff Razwick is the president of Technical Glass Products (TGP), a supplier of fire-rated glass and framing systems, and other specialty architectural glazing. He writes frequently about the design and specification of glazing for institutional and commercial buildings. Razwick is a past-chair of the Glass Association of North America’s (GANA) Fire-Rated Glazing Council (FRGC). He can be contacted via e-mail at jeffr@fireglass.com[9].

Source URL: https://www.constructionspecifier.com/channel-high-design-part-two/

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