Bent and curved glass terminologies are loosely used in facade design and construction, leading to internal debate, project miscommunication, and production errors. This article will discuss the terminologies, standard specifications, and generally accepted practices in bent glass design, production, and installation.

Bent glass is a flat glass shaped while hot into a body with curved surfaces (ASTM C162). Bent is a general technical term, while curved is a specific definition for a particular glass shape. Loosely defined, bent glass is commonly known as curved glass. It is a specialized type of glass shaped by heat and a metal mold to assume a curved - single curved form rather than the standard flat plane. Or a cold bent process to shape a compound bent glass panel.

Architecture uses bent glass for various applications, including glass domes, canopies, handrails, revolving doors, and display cases. It is because it adds a modern, futuristic, sleek, and elevated aesthetic to building facades, making it a popular choice for facade designers and architects.

How is bent glass made?

There are two primary methods of manufacturing bent glass: slump method (gravity force) and force method (mechanical force). I'll go ahead and elaborate.

1. The slump method, also known as using gravity force in bending/curving glass panels, is an approach where a regular glass is heated to the required temperature until it softens. The softened glass slumps onto a curved mold, where it cools and hardens.

2. The force method, also known as using mechanical force in bending/curving glass panels, generally known as cold bending, requires specialized equipment, including a heating station and roller or a press, with the application of mechanical force in guiding the bending process. The force method is commonly used for complex designs, compound bends, or serpentine bends.

Pros and Cons

Assisting the architect or specifier in choosing the most practical method depends on the aesthetic and performance requirements, and the schedule and budget allocated for the project is the prime responsibility of BEN Cladding; thus, in this section, the pros and cons of each method will be discussed.

The slump method is straightforward and requires minimal specialized equipment. It is ideal for less intricate designs like single and gentle curves. However, the slump method is not always suitable for structural applications, particularly where there is susceptibility to thermal shock and a requirement to improve visual clarity. The challenge lies between the structural strength and clarity of the bent glass, as it is advisable to temper the glass chemically. Chemical-strengthened glass shall be in accordance with the standard specification ASTM C 1422.

Considering this complex process, including the coating limitation and expensive molding equipment, the slump method would further drive up costs. Thus, it is not advisable for glazing projects with limited resources, tight deadlines, and constraints with budget.

On the other hand, the force or bent method allows for complex and precise designs while sustaining the increased load-carrying capacity due to structural redundancy, enhancing stability in the final glass structure. However, the force method may require more material, compatibility, and information, leading to increased costs, but it can be mitigated when properly designed, particularly in consideration of stress distribution.

There was an alternative solution in producing double-curved thermally toughened glass, but glass manufacturers are still experimenting with the feasibility of employing the hot bending method. Visual clarity is the primary challenge as it rapidly degrades when increasing or decreasing glass panel thickness and changing the curvature.

What are the classifications of bent glass?

Before going further, bent glass is classified according to ASTM C 1464 with the following terminologies as accepted in general practice.

1. Kind BA is a bent-annealed flat glass, either transparent or patterned, following ASTM C 1036.
2. Kind BCS is a chemically strengthened bent glass, shall be flat glass, either transparent or patterned, following ASTM C 1036 and ASTM C 1422.
3. Kind BFT is a fully tempered bent glass, shall be flat glass, either transparent or patterned, following ASTM C 1036 and ASTM C 1048.
4. Kind BHS is a heat-strengthened bent glass, shall be flat glass, either transparent or patterned, following ASTM C 1036 and ASTM C 1048.
5. Kind BL is a laminated bent glass, shall be flat glass, either transparent or patterned, following ASTM C 1036 and ASTM C 1172.

Fabrication Guidelines

Bent glass is produced from flat glass, which is permissible in fabrication processes, including cutting glass to the overall dimension, edgework, drilled holes, notching, grinding, sandblasting, and etching. All these fabrication processes shall be completed before the strengthening stages, including the chemically strengthened, heat-strengthened, or fully tempered. A bent glass panel intended for safety glazing application shall be permanently marked with the manufacturer's trademark and application designation to track the maintenance and reordering stages.

Optical Distortions On Fabrication

Distortions can easily be measured for flat panels, but the same method cannot be used for curved distortion. Special techniques are recommended to use following ASTM C 1464 and BS ISO 11485-2: 2011. The common methods to measure distortion for curved panels are the following:

1. Shape Accuracy (PC) can be determined by placing a template on the concave surface of the bend and measuring the deviation between the template and the glass.
2. Cross Bend (CB) can be determined by placing a straightedge along the horizontal edge (perpendicular to the curved) on the concave surface of the glass and then measuring the distance between the glass and the straightedge.
3. Panel Twist (PT) can be determined by positioning the panel with its upper edge on a flat surface. The twist value is the distance that the other corner is away from the surface.
4. Edge Straightness (ES) can be measured by placing a straight ruler on the glass and measuring the distance between the ruler and the glass.

Other common optimal distortions are the roller waves, edge lift, and local edge bow.

1. Roller waves are a surface distortion that occurs in a regular wave pattern (300mm to 400mm) and essentially reduces the surface flatness. The quenching process freezes the glass, locking this distortion. Generally, the thinner the glass, the more likely the roller wave distortion will be noticeable, while the larger the glass sizes may appear to have more distortions.

2. Local edge bow refers to a distortion that occurs over a relatively short distance on the edges of tempered glass. It can be determined by placing a straight ruler parallel to the glass edge, approximately 25mm away from the edge. The gap between the glass and ruler is measured, and the maximum value represents the local bow of the glass.

3. Edge lift refers to a slight bow or dip at the edges of the thermally tough glass. It is commonly observed in flat panels, but codes and standards do not specifically mention the geometrical deviation for curved glass panels.

Optical Distortion on Installation

Distortions observed due to production are inherent causes, yet additional distortion during glazing assembly and panel installation contributes to the bent glass distortions. The causes of distortions due to assembly and installation are not easy to identify as other factors contribute to the facade's overall distortion. The following are some factors that contribute to perceived distortions.

1. The out-of-plane frame is when two edges of an opening are not parallel, causing the glass to twist, leading to reflected image distortion.
2. The effect of climatic load in Insulated Glazing Unit (IGU). The isochoric pressure expresses the magnitude of the climatic load. The elevation difference, air pressure difference, and temperature difference between the conditions at production and installation. The glass panel may deflect due to barometric pressure change and variation.

3. The curvature of the glass impacts how light rays are reflected. Concave curvature caused inward projection, making the reflective image appear shorter or thinner. Positive or negative bows (outward or inward) can create optical distortion in the reflection.

4. Spot faults caused by bubbles and deposits can contribute to optical distortions. To minimize spot faults, it is imperative to properly melt the glass to minimize bubble formation, ensure uniform distribution of raw materials to prevent impurities, optimize the mold design to reduce trapped air and surface defects, and control the cooling rates to minimize bubbles and deposits.

Bent Glass Testing Methods

Testing bent glass involves different methods to evaluate its strength, performance, and visual clarity. Testing includes dimensional inspection, visual inspection, four-point bending test, guided-bend test, and others.

Dimensional Inspection (ASTM C 1464)

• The shape accuracy is determined by placing a template on the concave surface of the bend and measuring the deviation between the template and glass .
• The twist is determined by setting the glass in a 90-degree test fixture inclined five to seven degrees from vertical. The glass is positioned with the bottom corners in tight to the back surface. The twist value is the distance that the other corner is away from the surface.
• Crossbend is determined by placing a straightedge along the vertical edge (perpendicular to the arc) on the concave surface of the glass and then measuring the distance between the glass and the straightedge.

Visual Inspection (ASTM C 1464)

• Positioned the specimen in a vertical position. The viewer shall look through the specimen, using daylight without direct sunlight or background light suitable for observing blemishes. View at 2 meters.

Four-Point Bending Test (EN 1288-3)

• The method supports a glass plate at two points using support rollers.

• Two bending rollers apply force from the other side, creating nearly constant edge and surface stress.

• The glass stress is calculated based on the applied vertical force, considering factors like thickness and displacement.

Guided-Bend Test (ASTM E 290-22)

1. A mandrel or plunger forces the mid-length of the specimen between two supports. The bending force is applied to the glass while it is in contact with the mandrel.
2. Semi-guided ben, free-bend, and bend-and-flatten tests are variations of this method.

Conclusion

The article addressed the confusion surrounding bent glass terminology in facade design and construction. By clarifying the terms and outlining standardized practices, misunderstanding, and project errors can be minimized. Bent glass is a general term for flat glass shaped into curves, compound curves, serpentine bends, and elliptical bends.

There are two primary production methods: slump (gravity) and force (mechanical). Choosing the right method depends on design complexity, lead time, and project budget.

The article also covered classifications, fabrication guidelines, and testing methods for bent glass. Understanding these aspects ensures that the final product and facade project meet the aesthetic and performance requirements planned by facade experts and architects.

References

• ASTM C 162 Terminology of Glass and Glass Product
• ASTM C 1036 Specification for Flat Glass
• ASTM C 1048 Specification for Heat-Treated Flat Glass - Kind HS, Kind FT- Coated and Uncoated Glass.
• ASTM C 1172 Specification for Laminated Architectural Flat Glass
• ASTM C 1422 Specification for Chemically Strengthened Flat Glass.
• ASTM C 1464 Standard Specification for Bent Glass
• ASTM E 290-22 Standard Test Methods for Ben Testing of Material for Ductility
• Fadel, C., (2023), Effect of Climatic Load on IGU, DOI: https://www.linkedin.com/pulse/effect-climatic-loads-igu-cris-fadel/
• ISO 1288-3:2016 Glass in BUilding - Determination of the Bending Strength of Glass, Part 3: test with Specimen Supported at Two Points (Four Point Bending).
• Naomi, S. (2019), Quality Control and Specification for Distortions of Curved Glass, Eckersley O'Callaghan Ltd.
• Peters, T., Hof, P., Schuster, M., Schneider, J., Seel, M., (2024), Determination of the Bending Strength of Glass Via Four POint Bending, Glass Structure & Engineering.