Glass DesignJune 14, 2012
ASTM E1300, Standard Practice for Determining Load Resistance of Glass in Buildings, is the primary resource for specifying glass capable of standing up to design loads for a given project. It describes procedures for determining the load resistance of annealed, fully tempered, heat strengthened and laminated monolithic glass, as well as insulating glass composed of any of these types, when exposed to a uniform lateral load of short (3-second gust) or long (30 days) duration. It has applied to vertical and sloped glazing for which the design loads (due to wind, snow and self-weight) total no more than 210 psf (10 kPa). It is assumed that the lateral deflection of the framing members of length “L” supporting the glass edges does not exceed L/175.
Note that in the procedure, units are given in metric (SI) units, and inch-pound (IP) conversions are provided “for information only.”
The basic procedure set forth in ASTM E1300 unfolds in four steps:
- Determine the “non-factored load” (NFL) from the tables provided in the appendices of the standard. (The NFL is defined as the three-second duration uniform load associated with a probability of breakage of 8 lites per 1000 for monolithic annealed glass.)
- Determine the glass type factor (GTF) from the table provided. (GTF is defined as a multiplying factor based on load duration for adjusting the load resistance of different glass types.)
- Multiply NFL by GTF to get the load resistance (LR).
- Based on this LR, determine the approximate maximum lateral center-of-glass deflection from the provided charts. If the deflection falls outside the limits of the charts, alternate calculation procedures are offered. Different groups of charts are presented based on glass type and on whether the glass is supported by framing on four sides (most typical), three sides, two opposite sides or one side (cantilever).
- First, the overall load limit has been increased from 10.0 kPa (210 psf) to 15.0 kPa (315 psf). A curve for this load level was added to each of the NFL charts for monolithic glass supported along four edges in the standard.
- Section 1.3 that defines the types of glazing to which the standard applies was changed; rectangular shapes of monolithic, laminated and insulating glass supported on one, two, three or four sides was included. However, the new version separates out insulating glass, for which the standard now applies only to insulating glass with four-sided support. Language was also added to include triple-glazed IG units.
- In Section 18.104.22.168, two additional glass plies were added to the exceptions to laminated glass thicknesses. In addition to the definition of “two 6mm (1/4”) glass plies plus a 0.38mm (0.015”) or 0.76 mm (0.030”) interlayer … as 12 mm (1/2”),” laminated glass units consisting of two 2.5mm (3/32”) glass layers plus 1.52mm (0.60”) interlayer is defined as 5mm (3/16”), and two 4mm (5/32”) glass layers plus any thickness of interlayer is defined as 8mm (5/16”).
- Table 4, which lists minimum thicknesses allowed for “nominal” glass thicknesses, added an additional thickness of 25.0mm (1”) nominal, 24.61mm (0.969”) minimum.
- The committee added a NFL chart and a corresponding deflection chart for nominal 25.0 mm (1”) glass supported along four sides.
- The tables for Load Share Factors for IG units added a reference explaining that “values are approximate; use Vallabhan & Chou 1986 for better estimation.”
- There are various clarifications of dimensions (e.g., “nominal” added as a characterization) and appendices were revised to add clarifying notes and application guidance.