Volume 41, Issue 5 - May 2006
On Solid Ground|
Brief Discussion the Design & Guidelines for Glass Floors
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.
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.
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.
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.
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