Designing Glass to Resist Wind and Snow Loads
Accommodating for wind and snow is one of the important upfront considerations in the design and specification process. There is a multi-step process that must be undertaken to help ensure the glass in your project is able to resist these loads.
First, the impact of wind and snow on a building can be significant, which is why the design requirements must be followed carefully. ASTM E1300 is the main source of technical information for architects and other design professionals to determine the right glass thickness and type to meet a project’s anticipated wind and snow loads.
ASTM E1300 Procedures
ASTM E1300 includes procedures that address:
- Monolithic, laminated and insulating glass constructions made with annealed or heat-treated glass.
- The number of glass edges supported. Typically all four sides are supported in a framed window or a four-sided Structural Silicone Glazing. However, some applications can be two-sided support, such as Butt Joint Glazing or sometimes even three-sided or one-sided supports.
- Short and long duration uniform loads. A wind gust against a building façade is the most common example of a short duration load, while snow on top of a skylight is an example of a long duration load.
- Calculating center-of-glass deflection. Excessive glass deflection can result in edge pullout or may cause concern to the building’s occupants when they see glass movement greater than ¾ of an inch.
The procedures operate under some basic assumptions related to glass fabrication and installation, including:
- The glass has been properly glazed, without any edge damage.
- The glass hasn’t been subjected to any abuse.
- The surface condition of the glass doesn’t have any significant damage.
- The framing design limits the lateral deflection of the edges to less than 1/175th of their length.
- The center of the glass deflection will not result in loss of edge support
Key Concepts that Address Wind and Snow Load Calculations
There are five key concepts that impact the wind and snow load calculations on a project:
- The glass type factor, which is the multiplying factor for adjusting the load resistance of different glass types.
- The specified design load, which is the magnitude in kPa or PSF, of the type and duration of the load that is specified by the building code. A wind velocity in MPH can be easily converted to PSF by a simple formula.
- The relative strength of glass for a given thickness, width, height and number of supported edges.
- The load share factor, which is the multiplying factor derived from the load sharing between two lites of equal or different types or thicknesses.
- The resulting load resistance, which is the uniform load that a glass construction can sustain without breaking.
Basic Design Procedures
The assumptions and calculations result in these basic design procedures that should be followed:
- Start with the glass dimensions and the specified design loading. The design load needs to be in psf or kPa.
- Think about the glass thicknesses and types you’d like to use and formulate a trial design.
- Use ASTM E1300 to determine the load resistance and center of glass deflection.
If the load resistance is greater than the specified design loading and the center of glass deflection is reasonable, then the trial design is fine and can proceed. If the load resistance is less than the specified design loading, or if the center of glass deflection is excessive, then the trial design needs to be modified and re-tested.
While it’s important to understand how these complex calculations work, any Vitro Certified™ Fabricator can do these calculations for you. Visit PPGCFP.com to find a certified fabricator near you.