 |
 |
Members | Affiliated Professionals | Consumers | Press |
|
|
        |
|
|
|
||||||||||||||||||||||||||||||||||||||||||||||
|
|
|
|||||||||||||||||||||||||||||||||||||||||||||
 |
|||||||||||||||||||||||||||||||||||||||||||||||
|
Technical Resources Designing Large Glass Walls Important factors which must be taken into consideration By Christopher J. (Chris) Barry The transparency of glass often leads designers to attempt the creation of an all-glass wall with no visible supports or stiffeners. Reality, however, imposes certain limitations; before the wall can be designed, the following values are required: · How great are the loads on the glass? · How much glass deflection under load can be tolerated? · What glass size and thickness limitations exist? · What glass framing or support systems are acceptable? The windload must be provided to the glass designer. Wind speed alone will not provide enough information. The shape and exposure of the building, plus the relevant building variables in the applicable building code are needed for the calculation of windload for a given wind speed. The glass designer/fabricator/supplier is typically not able to convert wind speed to uniform pressure load. For a plain rectangular opening, as illustrated in the accompanying photograph above, the glass is "simply" supported. This means that the edges can rotate slightly, or slide in the plane of the opening, but they cannot move in a direction normal (at right angles) to the plane of the opening. If this lite, 28 feet (8.53 m) wide and 8.33 feet (2.54 m) high is 3/4 inch (19 mm) thick annealed glass, it will withstand a uniform windload of around 25 psf (1.2 kPa). At 15 psf (0.72 kPa) the center of glass deflection is about 3/8 inch (10 mm). Glass supported on all four edges is stiffened by membrane action. As the glass bends under a uniform load it also has to stretch because the four edges, confined to the plane of the frame, provide resistance to this membrane or stretching action. While there is no limit set for center of glass deflection in the standards or building codes, it is generally accepted that deflections greater than 3/4 inch (19 mm) can give rise to aesthetic questions caused by distorting reflections and other effects. The ASTM E 1300 glass design standard limits glass edge deflections to edge length divided by 175 simply because the stress analysis method used in the standard is not valid at greater values. Simpler Design Method It would be very difficult and costly to find, fabricate, transport, and install the single glass lite shown in the photograph I've been discussing. The maximum size glass readily available today is 3/4 inch (19 mm) thick and about 17 feet (5.18 m) by 10.8 feet (3.30 m). A much simpler design method is to edge glue smaller vertical strips of glass with black silicone with only the two opposite edges (top and bottom) held in the frame. But glass supported on two opposite sides is generally much more flexible than that which is supported on all four sides. This is true when the glass width and height are nearly equal. But when the aspect ratio is large (long dimension is much greater than short dimension), as in the photo, then the stiffness under load will be about the same for both support conditions. For this installation, glass 3/8 inch (10 mm) thick, of any width of 5 feet (1.52 m) or less by 8.33 feet high would deflect 3 inches (76 mm) under the 15 psf (0.72 kPa) load used above, and it must be protected from pulling out of the frame as the top edge will pull down by 1/4 inch (6 mm) under load. (This size would also need to be heat-treated to have adequate load resistance.) Glass stiffness is proportional to the cube of its thickness. This means that if the thickness is doubled, the glass be-comes eight times stiffer or will only deflect 1/8 as much under the same load. Therefore, using 3/4 inch (19 mm) thick glass in the above example will reduce the deflection from 3 inches (76 mm) to about 3/8 inch (10 mm). Seldom-Considered Option A seldom-considered option is to change the glazing detail from "simple support" to a "clamped edge" design. The bottom edge of a glass balustrade set in expanding grout in the floor is an example of effective clamping. The top edge of our window requires different treatment: The glass edge must be free to expand and contract in its vertical plane to accommodate thermal expansion, building component creep and movement, etc. When the top edge is restrained from any rotation (but still allowed to slide vertically) then the glass will become five times stiffer; therefore, the deflection of 3/8 inch (10 mm) thick glass in the above example would be only approximately 5/8 inch (15 mm). The accompanying photo (top left) shows an adventurous method successfully used when the glass in question was only available in small sizes. The tempered glass lites were effectively held in place using slim silicone rubber spacers to keep the width of the structural silicone joints between the 1/4 inch (6 mm) lites to about 1/16 inch (1.5 mm). This installation would be much stiffer if the frame detail around the perimeter of the assembly could provide a clamped condition rather than simple support. A method seldom used, and usually then without the insulating glass fabricator's approval, is to install sealed i.g. units supported along the top and bottom edges only. The sealant must be dual seal PIB and silicone to withstand the solar UV exposure. Such i.g. units have shown that the seal can withstand the high sealed air space pressures generated on hot days without the clamping effect of a traditional glazing frame. While the windload will be equally shared between the two lites of the i.g. unit, a careful calculation must be made of the shear stress developing in the sealant. This stress could potentially cause the i.g. seal to fail long before the limiting stress for the glass was reached. The units in the photo (top right), made with two lites of 1/4 inch (6 mm) glass, felt very stiff when a hand load was horizontally applied, though a close examination did show the fog of a failed i.g. seal inside one of the three units. Glass Fins Glass fins are always available to stiffen a glass wall; but, if they are not acceptable the wall can be assembled from alternating corrugations of flat glass, silicone joined at the verticals, where each lite is alternating about 30° out of the plane of the wall. In this situation, each lite acts as a fin for the windload on its neighboring lite. This is only an extension of the mullionless all-glass corner where the lites typically meet at 90°. Again, the stresses in the silicone must be carefully calculated, as well as the in-plane buckling loads carried by the glass. Finally, lites of bent glass can often be used to supply a smooth, continuous glass wall without any visible frames or obstructions at eye level. In this case, adequate stiffness of the glass is achieved by the cylindrical bent shape. Christopher J. (Chris) Barry is director of technical services for Pilkington North America, Inc., the Toledo, OH., based glass manufacturer. Source: The Glass Guide for Architects and Specifiers, Glass Magazine, June 2001. |
|
|
| Copyright © 2007 NGA - National Glass Association. All Rights Reserved. |
Site Design by WebFirst |