Volume 41, Issue 5 - May 2006

On Solid Ground

A Brief Discussion the Design & Guidelines for Glass Floors
by A. William Lingnell

The use of glass as a material in floor systems has been in practice for many years. Such an architectural statement requires a special knowledge of the fundamental characteristics of glass, including material strength of the various types of fabricated glass, glass composites that supply the required safety for the end use, deflection conditions during the expected performance of the floor, support methods for the glass, proper glazing methods and materials, manufacturing tolerances and a host of other details. 

Engineering Analysis 
One of the most important considerations in the proper selection of a glass floor is the engineering analysis of the glass. This ensures that the proposed design or thickness determined will meet the load requirements selected for the project. Glass is an elastic brittle material that behaves in an elastic manner up to the point of fracture. Glass does not exhibit a yield point. This is a major difference in glass compared to many other solid materials with which engineers deal. This means stresses are not redistributed as with materials that do exhibit ductile features and can still reach a plastic zone. Glass has no plastic zone. The fracture of glass is abrupt and, in many cases, unpredictable due to flaws, surface or edge damage and other stress-raising items that affect the overall strength or load-carrying capacity of glass. 

An engineering analysis should be performed by an engineer who is familiar with plate analysis and the response of glass to the type of load that is to be accommodated in the design. Glass is used as a structural material in sidewalks, corridors, bridges, dance floors, stairways, landings, balconies and other areas that are subject to pedestrian traffic and material storage. The glass engineer needs to have the correct design load information from the architect, specification writer and the structural engineer or design professional responsible for the glass floor system design. In many instances, the appropriate building code will supply the necessary loads. However, it should be noted that these are usually minimum loads and whether or not special conditions relating to the loads for the specific project may exceed the code requirements should be explored. Items such as long-term loads, concentrated loads, impact loads, combinations of uniform live loads and concentrated load and other conditions not addressed by the codes need to be considered.

The engineer analyzing or designing the glass floor system may use classical engineering mechanics for flat plates or, in some cases, choose to use a finite element analysis (FEA) method to determine the stresses and deflections of the system. Support methods for the glass, dimensions of the glass, shape and the specific details on how the glass is to be placed in the floor system are important to understand as boundary conditions needed to proceed with the engineering portion. Many glass floor systems are not just squares or rectangles and are of unusual shapes such as circles, ovals, triangles and trapezoids. Some also have holes, notches and cut-outs with which the engineer must deal in order to ensure areas with stress concentrations are considered. Once the glass engineer has the information needed, an appropriate study of the stresses encountered, along with the deflection characteristics of the glass used for the design, can be made.

Glass Construction
The glass used for this type of application should be a laminated safety glass consisting of two or more plies of glass that meet the strength criteria required by the engineering review. The type of glass used can consist of annealed, heat-strengthened, tempered, chemically-strengthened or other glass products that have well-defined mechanical properties. Types of interlayers used as the glass laminate can be polyvinyl butyral (PVB), ionoplast, liquid resin and/or epoxies. The properties of the interlayer materials (such as shear modulus, transition temperatures and effective thickness criteria) should be available to assist in determining the method of analysis. The allowable stress in the glass with an appropriate safety factor is applied to the glass under consideration for each design. The maximum allowable stress values are determined based on the type of glass being used. 

The glass thickness for the plies is usually the same for each ply; however different thicknesses can be used in a composite and various treatments can also be used in the overall laminate construction. This needs to be included in the engineering analysis to ensure none of the plies are overstressed during the application of the design loads. It is interesting to note that the stress in a flat plate varies according to the square of the thickness, and the deflection varies according to the cube of the thickness. When the composite laminate is composed of lites having more than one thickness the engineering evaluation becomes somewhat more complex since the load sharing of the various thicknesses must be taken into account. 

Glazing Details
The support of the glass is important to a successful design and installation of a glass floor system. Pay special attention to the materials used to support the glass. The bearing surface of the glass should always have a cushion between the bottom surface and the material supporting the laminated glass lite. The support members may be aluminum, steel, wood, concrete or other hard material that provides firm support and limits edge or perimeter deflections. The glass can be protected from hard or metal surfaces through the use of materials such as neoprene or silicone rubber that has a Shore A Durometer hardness of 85 ± 5. This ensures a bearing cushion between the glass and the support. It is also important to protect the glass from adjacent materials. Details relating to this area have shown that silicone construction sealants, for example, have been used successfully to protect edges and provide a seal between the glass and adjacent materials. Check the compatibility of the finish on each material to make certain proper bonding occurs and that the laminated glass does not encounter any adverse conditions.

A slip-resistant floor surface is an important consideration on some occasions. These treatments available incorporate ceramic frits, sandblasting or acid etching to overcome some of the concerns relating to the coefficient of friction for various footwear and other materials contacting the glass surface. 

Glass floor systems require that special attention be paid to the materials selected, the engineering review, the glazing details, the supporting members and the loading conditions that will be placed on the glass. The flexibility of the laminated glass plates and the amount of deflection that occurs under design loads must also be considered. Thin glass lites may have large deflections and still meet the load criteria expected, but because of large movements or “bouncy” conditions, they may appear unsafe. Therefore, glass strength is the primary concern for the safety of the application and deflection characteristics should also be considered in order to have a satisfactory design. 

A. William Lingnell, P.E., is an engineering consultant specializing in glass and glazing applications with Lingnell Consulting Services in Rockwall, Texas. 

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