Wind and Snow Load
Pilkington provide the service of determining the glass thickness and type to withstand wind and snow loads when the information is not available in the standards. This is a commonly requested service that is quick and easy to provide. To enable us to calculate a suggested glass thickness we need some basic information:
- The design wind load and snow load
- The dimensions of the glass width and height
- The angle of glazing
- The number of supported edges of the glass
The design wind load is derived from the basic wind speed and factors that affect the way wind imparts load to the building. The load can be wind pressure or more often suction. The height and shape of the building will change the loads as well as the location. Coastal locations and out of town sites suffer greater loadings than city centres inland. The type of terrain and the placement of the building on a slope or ridge may also change the result. Pilkington are unable to determine the design wind load for any building because we do not have the required familiarity with all of these factors. An architect or engineer should be able to work out the design wind load that we will need in our calculations. Typically for the UK the average design wind load is 1200 N/m2 and the snow load 600N/m2 however using the average may cause the glass to be over specified or even worse underspecified for the task. The design wind load can be calculated using BS6399-2. For low rise buildings an abbreviated method is in BS6262-3:2005. This also contains charts that allow common glass configurations to be chosen.
Note that the aspect ratio i.e. length to width is considered in the calculation because oblong glass shapes can be stronger than squares. The deflection as well as the stress in the glass is considered to ensure that the glass does not appear too flex when exposed to winds. For this reason the minimum design wind load used is 600N/m2. Even in internal locations like shopping malls we use the minimum loads to allow for localised air movement that may otherwise make the glass look vulnerable.
For snow loads refer to BS5516-2 Patent glazing and sloping glazing for buildings and BS6399-3.
The angle of glazing places additional stress on the glass. After a certain point snow is likely to settle on the glass and apply a constant load. The ability of glass to withstand stress changes with the duration of the load. Snow load and the self weight of the glass provide extra challenges. In some cases thickening the glass adds to the stress and the prediction of the likely durability gets worse. Sloping glazing is also likely to be overhead glazing and there are additional safety considerations. A thicker outer toughened glass pane and thinner laminated inner pane often form a good combination in an insulating glass unit for sloping glazing.
Suitable glass for Overhead Glazing
Pilkington T Plus (heat soaked toughened glass)
Pilkington Optilam and Pilkington Optilam I
Combinations of the above.