Volume 40,   Issue 8                                  August  2005

Learning Curves

From Inception to Completion, Planning and Teamwork 
are Essential for Successful Bent Glass Installations

by Steven D. Lerner

The beauty of curved glass can be found in many different exterior and interior architectural glass applications, such as handrails, skylights, interior partitions, curtainwalls, elevators and lighting fixtures. Curved glass offers an unusual, aesthetically pleasing alternative to traditional flat glass design. While today’s bending equipment is advanced technologically and handling methods are well defined, it takes effective planning and teamwork to move a bent glass project from inception to completion.

Initial Design Phase
Whether the product is being specified for safety, security, acoustical or thermal performance, or artistic presentation, it is essential that research and planning are part of the initial design phase. Because manufacturing capabilities and product availability may vary from manufacturer to manufacturer, it is important to discuss the feasibility of each new project from the onset. Reviewing the project with a sales or technical service representative allows the installer to work through not only pricing and scheduling issues, but also to evaluate the project from the practical standpoint to determine if the product can be made as specified. 

In instances where manufacturing limitations prevent the material from being produced as specified, it is helpful to identify and address these issues early. Often times, the manufacturer can suggest an alternative product as a solution. Additionally, this early discussion provides the installer with the opportunity to better understand what will be required with regard to final order information and/or templates that will help pave the way for a successful curved glass installation.

Measuring and Templating 
Needless to say, accurate measurement is critical to the manufacturing process. If the glass is not measured properly it will not fit into the frame during installation. We recommend that our customers take measurements three times in order to verify the accuracy of dimensions. One way to ensure that the manufactured contour matches the one depicted on the final drawings is to supply a physical template. The physical template can be the frame itself, a tracing of the contour (for cylindrical bends) or a replica of the opening or contour taken directly from the jobsite. The template, which serves as an actual representation of the required material, provides the manufacturer with precise information allowing them to produce the glass to clearly defined requirements.

Additionally, the template serves as a useful quality control tool to which the final product can be checked for accuracy. 

Handling and Installation
Once the glass has been produced per the specifications and templates, the challenge of installing the glass is presented. It is important to note the special needs of bent glass during installation. As with all glass products, installers must exercise care when handling curved glass. The center of gravity changes when glass is bent, which generally necessitates the use of additional people to carry and set the material. The tighter the curve, the greater the tendency to tip off balance. 

Slings or a power vacuum (of sufficient diameter and depth to produce a tight seal against the glass surface) should be used to handle large curved lites. Along with typical installation preparation, special consideration should also been given to prepare for handling bent glass.

When setting the glass, the installer must consider the fact that curved glass is rigid. The glass should experience a “free float” condition with adequate clearance on all sides, allowing it to fit comfortably in the frame without any stress points. This state is best achieved with a set-glazed silicone glazing system. Unlike pressure-type gaskets, this system does not create undue stress on the glass that can lead to breakage. 
Curved glass should never be forced into a frame. 

It is important to be aware of glazing tolerances as these may vary depending on the application. For example, a traditional ½-inch bite of flat glass installed in a handrail area changes to a 2½-inch bite when using bent glass. It is important to discuss this issue with the supplier prior to manufacture, as often times tolerance will be a key factor during installation; and knowing the requirements in advance allows the manufacturer to fine-tune the product to individual specifications as well as reduce the chances of complication during installation. 

Standards
A final point to mention is that, unlike in recent past where quality and tolerance issues varied from manufacturer to manufacturer, there is now an approved standard from the American Society for Testing Materials (ASTM), which helps to standardize the acceptability of curved glass products. ASTM C 1464-00 Standard Specification for Bent Glass, specifically addresses the uniqueness of bent glass and clarifies certain tolerances and other surface blemishes. Being aware of the standard and understanding its purpose will help installers and curved glass manufacturers set expectations and quantify exactly what results they expect in the finished product.

Most bent glass manufacturers will work with an installation team to review proposed glazing designs and answer questions or make recommendations for products that will meet the required specification. In addition, curved glass manufacturers can offer assistance on working with tolerances, proper glass handling, and approaches to installation, which will offer the maximum results for the particular curved glass product and application. 

When presented with a project involving bent glass, it is imperative that installers and manufacturers work together to successfully produce not only the product, but the installation as well.

Terminology
When dealing with bent glass, you are really dealing with parts of circles. Terminology plays a very large role in the quoting and fabrication process. When discussing bent glass, the terms used to describe the circle and it’s parts become very important. Below is a list of frequently used terms and their relationship to bent glass. Diagrams are shown to the left:

• Circumference: The distance around the outside of the circle.
• Diameter: A line drawn completely through the center of a circle.
• Radius: A line extending from the center point of the circle to the circumference. The radius is always the same from any point on the circle to the center.
• Angle: All circles are divided into 360 degrees. For example, 1/8 of a circle equals 45 degrees, and 1/4 of a circle equals 90 degrees. The angle of the bend helps us to determine the radius.
• Girth: Distance around the bend. Measurement should be taken with flexible tape around the outside surface of the glass.
• Chord: Distance straight across the curve, measuring from point to point. Measurement must be taken with a flat ruler.
• Depth: Measure from the center of the chord to the top of the curve.
• Height: Distance between the highest and lowest point of the glass. This is sometimes referred to as length. 

Examples of Common Architectural Curves
• Type A: Shallow cylindrical bend; arc does not exceed 58 degrees.
• Type B: Cylindrical bend; arc exceeds 58 degrees but not 90 degrees.
• Type C: Curve-plus-tangent with curve limited to 60 degrees. Tangent may not constitute more than 1/3 of the total girth.
• Type D: Curve-plus-tangent with curve not exceeding 381/2 degrees of arc. Tangent may not exceed the arc.
• Type E: Curve-plus-tangent type has a tangent not more than twice the curve, and curved portion does not exceed 90 degrees of arc.
• Type F: Curve exceeds 90 degrees but not 128 degrees.
• Type G: Non-circular curve. Shape is that of a shallow letter S.
• Type H: Central curve flanked by tangents that may be equal or unequal. The curve may not exceed 90 degrees, and both tangents together in length may not exceed 1/4 the total girth.
• Type J: Tangent-plus-arc-plus-tangent; curve is 90 degrees or less, the tangents together, equal or unequal, do not exceed 3/4 of the total girth.
• Type K: Curve exceeds 128 degrees but not 180 degrees.
• Type L: Two curves of like radius and each not exceeding 90 degrees with a common tangent between them.
• Type M: Deep-curve-plus-tangent; curve will exceed 90 degrees of the arc.

The Author:
Steven D. Lerner is the president of Bent Glass Design Inc. based in Hatboro, Pa.


USG
© Copyright 2005 Key Communications Inc. All rights reserved. No reproduction of any type without expressed written permission.