The use of curved glass may bring endless creative opportunities, but it requires careful consideration and team coordination to achieve results that balance aesthetics and performance goals.
Curved glass can be an effective choice for structural glass applications. The material has high load resistance, and the curved shape minimizes deflection. Bent tempered glass also has excellent mechanical strength and durability against temperature changes. Although recent improvements in glass manufacturing technology have made products safer and cheaper, curved glass is still a customized material.
There are no standard-sized curved glasses, and the curved shape must meet the specifications of the project. Although any glass installation requires careful handling, curved glass can exacerbate the challenge. Typical glass suction cups cannot be used for curved glass. Conversely, special suction cups with deeper holes provide better grip around the glass radius.
When the demand for safety glass is minimal, annealed curved glass appears most often in windows and display cases. Tempered curved glass is stronger and more shock-resistant, and is often found in stairs and other indoor applications. Only laminated curved glass can provide glass retention after breakage and can be used for internal and external applications. The curved glass properties can be customized for different colors and finishes.
Coatings (such as Low-E) cannot be used after the glass is bent. Instead, different hard coating and soft coating methods must be performed before bending. Even insulating glass units (IGU) can be built with curved glass to improve acoustic and thermal performance. Generally, the maximum bending angle is one-half of a circle. Glass can be bent in the factory using hot bending process or cold bending during installation.
Hot bending uses heat to create curved shapes, accounting for 95% of all glass bending. This can be done by heat treatment, bending and quenching. In the special configuration part of the heat treatment production line, quenching mechanically establishes a constant radius form. Alternatively, the glass can collapse and bend. Various types of steel molds are used to collapse flat glass into the desired shape. The glass is placed on the preheated mold and then placed in the kiln. At about 600°C, the glass begins to melt and bend, fusing into the shape of a mold. The furnace cools down very slowly, completing the curved glass.
During the installation process, cold bending forces the flat glass into the bending position. The process uses external stress to transform the glass into an out-of-plane geometry and uses a glass frame system to permanently confine it in that state. After the glass is tempered, it can be cold-bent to make the glass have the characteristics of safety glass.
Laminate bending combines hot bending and cold bending with lamination. Using the autoclave process, the sheet and the laminated glass sandwich are bent by heat and pressure. Laminated glass meets safety glass requirements while providing flexible shapes. However, laminated curved glass is susceptible to the spring effect, where the glass straightens slightly after the autoclave process, adding subtle glass relaxation over time.
Bent Glass Design, a professional manufacturer in Hatboro, Pennsylvania, provides architectural, laminated, and specialty glass that is bent or bent into any shape, including sharp bends up to 90 degrees and complex bends in complex shapes. Bent glass requires careful measurement and size determination. The company’s recommendations are as follows:
Unlike the length and width dimensions required to order flat glass, the manufacture of curved glass requires precise dimensions of the glass shape. There are two ways to describe the shape. The first consists of perimeter, height or width, and radius dimensions. When the radius is unknown, chord and depth dimensions can be provided. A flexible tape measure is needed to measure the outer circumference. Plane measurement is used to measure height/width, inner chord and inner depth. If the exact radius size cannot be provided, a cardboard or polyester film template with all the above sizes is required.
Bent Glass Design provides additional guidance and works directly with the architectural and glass teams to provide technical design assistance for curved glass projects.
AGI asked its member glass contractors and affiliated companies to provide insights on the challenges of using curved glass. Everyone agrees that this product provides ample opportunities for creativity in design and effective construction. However, there was a consensus on the main concerns:
Top 5 big curved glass problems 1. Delivery time 2. Tolerance 3. Adjacent 4. Distortion 5. Coordination
Joe Clabbers, president of National Glass and Metal Co. Inc., listed long delivery times as a major concern. Depending on the composition of the product, the delivery time for locally sourced glass may be 6-8 weeks, and the delivery time for overseas products may be more than 20 weeks.
John Norwood, the national project manager, emphasized that strict tolerances are a composite problem of curved glass. You must consider the size of the curved glass and its associated frame. When using pressure walls or other systems with uniform gasket sizes, glaziers may need to use wet seals to increase the dimensional tolerances between glass and metal.
When installing curved glass into a metal frame, care must be taken not to exert excessive pressure on the glass. If the radii are not perfectly aligned, the stress will cause future fractures. Stephen Strauss, vice president of CMS Strauss Glass, recommends correct sizing to take into account the groove setting, direction, and measurement on both sides of the curve. "If you measure from the inside or the outside, you will get different dimensions," he explained. "You must specify two sizes when ordering."
According to Clabbers, adjacent materials have a direct impact on the installation and performance of curved glass. The substrate above and below the window is a particular concern. Problems arise when the substrate needs to be mixed with glass or structural steel. "The base material must be precisely related to the curves and line segments," he said. "Some high points sometimes don't match the radius of the glass." Clabbers recommends planning and coordination in advance to overcome substrate issues.
Strauss added that when installing laminated glass railings, laminated adhesives should also be considered. Typical polyvinyl butyral (PVB) adhesives are suitable for internal railings and cannot withstand external elements. Instead, SentryGlas® ionoplast interlayer must be used. This element needs to be considered early, because laminated glass must be "sandwiched" with all interlayers and adhesives before bending.
Wiss Janney Elstner (WJE) Senior Assistant and Building Envelope Consultant Dana Landis, PE, advises design professionals to pay attention to the glass spacers. Typical aluminum spacers used in flat insulating glass units (IGU) may not be able to adapt to the curve of curved glass. Instead, use warm-edge spacers. Compared with metal gaskets, their thermal conductivity is poor, so the energy efficiency of the sealed glass window unit is higher.
These separate the glass panels and maintain the integrity of the air gap, protecting the building from heat loss through the windows. However, Landis warned that the spacers may change the visual appearance of the building. "For example, if metal and warm-edged gaskets are used at the same time, it is important to coordinate the color of the gasket." The partitions and frames of two adjacent systems—such as side-by-side curved and flat systems—should be coordinated so that the system fits in the envelope. Integrity and appearance blend seamlessly.
Adjacent flickers may also cause concern. The metal in the straight section must be aligned with the curved section. For IGUs with glass, the curve of the aluminum extrusion may not match the curve of the glass. Additional tolerances can be added to the structural sealant to accommodate the difference in curves.
Roller wave distortion describes the subtle wave pattern of glass passing through rollers during heat treatment and bending. Anisotropy, sometimes referred to as iridescence, describes the appearance of patterns and colored areas due to the difference in stress generated in the glass by heat treatment (heating and cooling processes).
According to Landis, most people have not noticed these effects on the finished glass. However, the anisotropy becomes more pronounced under polarized light — or through the lenses of polarized sunglasses — or at different times of the day. In this case, the effect can be a rainbow glare or a series of lines of reflection. "This is an inherent effect that occurs during the tempering process," she explained. "We can control it, but we cannot avoid it."
For example, glass sheets can pass through the tempering furnace in the same direction so that any visual distortion of all glass sheets on a facade is consistent. The design team and the manufacturer should discuss the maximum size of the glass sheet passing through the oven in advance to coordinate the consistent direction of heat treatment and bending of the glass. A visual model should also be provided to assess distortion and be used as a benchmark for comparison in the production process.
From measuring the curve correctly to reducing the risk of tolerance, to ensuring that anisotropy and rolling distortion will not negatively affect the aesthetics of the project, design teams using curved glass benefit from early communication and coordination. Strauss recalled a project with a dramatic semicircular display. When the curved pieces arrive on site, they include a four-inch straight flat piece at the end of each curve. When the glass sheet is bent, the flat sheet accommodates the manufacturer's fixture. This detail-and the associated correction time and cost-could have been avoided by discussing it in advance.
For more information on curved glass: Expert Series: Guardian Glass's Bent Glass Bent Glass Design, Inc.'s Bent Glass overview.
Penn Medicine Pavilion | Philadelphia, Pennsylvania
The newly built 1.5 million square foot, 17-story inpatient facility at the University of Pennsylvania Hospital uses curved glass exterior wall cladding, combining a unitized curtain wall and pressure wall system, and integral back-painted glass.
The team includes AGI member National Glass and Metal Co., Inc., which executes some external glass ranges. National Chairman Joe Clabbers described long lead times for curved glass, increased pre-project planning and tight tolerances as major concerns. "CAD and REVIT help to coordinate to ensure that all radii match," he explained. Project manager John Norwood added that in addition to carefully recording the radius when using curved glass, designers must also pay attention to manufacturing restrictions that limit the angle of the radius. "Early communication with the manufacturer will help ensure that you specify the correct product and understand the restrictions."
For this project, European manufacturer Interpane provided Cricursa with coated glass for bending. Wiss Janney Elstner (WJE) Senior Assistant Dana Landis, PE, provided building envelope consultation and commissioning for the exhibition hall. Her WJE team worked with Enclos (the glass contractor responsible for the tower section of the building) and National. "We solved the aesthetic-related issues, determined the direction of the glass through the oven to limit visual distortion, and ensured that different glass systems match in terms of visual and performance requirements."
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