AGG - 2008 - May/June

Volume 22, Issue 3 - May/June 2008

Glass Tech
The ABCs of Glass Floors and Stairs

Glass floors and stair treads are growing in popularity because of their aesthetic appeal as well as for the daylighting benefits they bring to a space. While there are manufacturers that provide engineered and tested flooring systems, most glass floor and stair tread projects require the input of a structural engineer to provide load calculations and an experienced installer to provide glazing details.

Because pedestrian safety is a critical objective, the glazing in-fill must provide an acceptable level of slip resistance. If there is a possibility for inappropriate lines of sight, the glass will require greater opacity or translucency to address a concern for modesty.

Combining Loads
The design of glass floors and stair treads typically calls for load calculations that are, typically, the responsibility of a structural engineer. The load requirements are taken from the applicable building code or, if none, from the American Society of Civil Engineers (ASCE) Standard 7 Minimum Design Loads for Buildings and Other Structure. The uniform loads include live, snow, dynamic and dead loads. Other loads include impact and point loads, but careful consideration should be given to applying point and impact loads to glass. Special consideration should be given to high loading conditions, such as dance floors.

An important consideration is the ability of the walkway to support the design loads, even after breakage. Glass is a brittle material by nature and surface damage can occur by impact from hard objects, which can greatly reduce the load carrying capacity. Redundancy and suitable safety factors must be used in the design of glass flooring. The edge support conditions must be identified as four-, three-, or two-sided in order to understand the load-carrying capabilities of the glass.

Recently, an ASTM International task group, E 06.56.04, was formed to begin work on a standard practice for the design and performance of supported glass walkways. Once finalized, this document will assist the designer in better understanding the elements related to performance, design and safe behavior of glass walkways that include interior and exterior walking surfaces constructed and intended for pedestrian use, including floors, ramps, sidewalks and stair treads.

Several types of glass products are used in floors and stair treads, including laminated glass and glass block systems.

Providing Slip Resistance
Slip resistance of a walking surface is an important safety consideration. The Occupational Safety and Health Administration (OSHA) requires a minimum slip resistance, expressed as a static coefficient of friction of 0.50. However, special activities, such as dancing, may require a different level of slip resistance. Glass floors used near entrances that may get wet require special consideration.

There are a variety of ASTM test methods that measure slip resistance using specific test equipment under dry or wet conditions. These are:

F 609 Standard Test Method for Static Slip Resistance of Footwear, Sole, Heel, or Related Materials by Horizontal Pull Slipmeter (HPS);
F 1677 Standard Test Method for Using a Portable Inclinable Articulated Strut Tester (PIAST);
F 1679 Standard Test Method for Using a Variable Incidence Tribometer (VIT); and
D 2047 Standard Test Method for Static Coefficient of Friction of Polish-Coated Flooring Surfaces as Measured by the James Machine.

Other industry standards, such as ASTM F 1637 Standard Practice for Safe Walking Surfaces, ASTM F 1646 Standard Terminology Relating to Safety and Traction for Footwear and Underwriters Laboratory (UL) UL 410 Slip Resistance of Floor Surface Materials, address the safety issues of walkway surfaces from a more general point of view.

Processes designed to roughen the top surface of the glass to provide slip resistance include sandblasting, acid-etching, ceramic frit and embossing. It is important to note that sandblasting may reduce the strength of the glass by as much as 50 percent; therefore, glass flooring should never be sandblasted in the field without a complete engineering analysis.

A Few More Things
Modesty becomes an issue when glass floors are found on upper levels and inappropriate lines of sight are created from spaces below. When modesty is a concern, it may be necessary to incorporate a ceramic enamel finish or a decorated or translucent interlayer in the glass to create opacity.

Any sealants or caulking used must be checked for compatibility with the glass flooring. The framing and supports for glass floors comprised of laminated glass must provide drain holes or weep systems to avoid the possibility of water or cleaning fluids building up in the system.

Because glass floors are not normally fire-resistant, careful consideration should be given to maintaining adequate fire barriers between floors.

Testing
Glass floors can be tested for strength or impact resistance. Test methods that are used include:

ASTM E 72 Standard Test Methods of Conducting Strength Tests of Panels for Building Construction;
ASTM E 695 Standard Method for Measuring Relative Resistance of Wall, Floor, and Roof Construction to Impact Loading;
ASTM E 2322 Standard Test Method for Conducting Traverse and Concentrated Tests on Panels Used in Floor and Roof Construction.

This material was adapted from an informational bulletin developed by the Glass Association of North America (GANA), which is based in Topeka, Kan. For more information, visit www.glasswebsite.com.