How to calculate glass load resistance to ASTM E1300

The steps

1. Write down the long dimension a and the short dimension b, the nominal thickness, the glass type (annealed, heat strengthened or fully tempered), the load duration and the design load. Pick the E1300 edition your project calls for.
2. Take the minimum thickness of the nominal thickness from Table 4. For 6 mm glass it is 5.56 mm.
3. Find the non factored load (NFL). This is the load that a lite of annealed glass can carry for a short duration, for that size and thickness. E1300 prints it as charts, one chart for each thickness. GlassLoad.com calculates it for your exact size and checks the method against the printed charts.
4. Find the glass type factor (GTF) from Table 1. For a short duration load it is 1.0 for annealed glass, 2.0 for heat strengthened glass and 4.0 for fully tempered glass. For a long duration load it is 0.43, 1.3 and 3.0.
5. Multiply to get the load resistance: LR = NFL x GTF.
6. Compare. The check passes when the design load is at most LR.
7. Check the deflection and the probability of breakage, described below.

Worked example 1: SI units

E1300 Example 1 is a lite 1200 mm by 1500 mm and 6 mm thick. The chart gives an NFL of about 2.5 kPa, and GlassLoad.com calculates 2.5 kPa. For annealed glass and a short duration load the GTF is 1.0, so LR is about 2.5 kPa. A design load p of 2.0 kPa gives p/LR = 0.80 and passes. The same lite in fully tempered glass has a GTF of 4.0, so LR is about 10 kPa.

Worked example 2: US units

A lite 120 in by 60 in and 1/4 in thick has an NFL of about 22 psf. Annealed glass with a short duration load has LR of 22 psf. Fully tempered glass has LR of 88 psf. With a long duration load the annealed factor is 0.43, so LR falls to about 9.4 psf. The guide on short and long duration loads explains why.

Deflection

E1300 gives the center of glass deflection in the lower chart of each four sided figure of Annex A1 and as the equation in Appendix X1 (Appendix X2 in E1300 09a). GlassLoad.com reads the chart of your edition for the nominal thickness and shows the equation beside it; where the chart gives no value, such as above aspect ratio 5, the equation gives the result. Both use the actual design load, not LR. The equation is w = t·exp(r0 + r1·x + r2·x²), where x = ln(ln p̂), p̂ = p·(a·b)² / (E·t⁴), p is the design load, a is the long dimension, b the short dimension, t the glass thickness and E is 71.7 GPa. The coefficients r0, r1 and r2 are polynomials in the aspect ratio. For Example 9, a lite 1500 mm by 1200 mm and 5.60 mm thick under 1.80 kPa, the deflection is about 12 mm. The equation is a curve fit to test data, so the result is approximate and can differ from the true value by about 10 percent. The deflection guide has more.

Probability of breakage

For annealed glass GlassLoad.com also shows the probability of breakage at the design load, as Pb = 1 − exp(−B). B is a number built from the stress in the glass over its surface, as described in Annex A2 of E1300 24. At the NFL the probability is 0.008, or 8 lites per 1000, which is the basis of the charts. It is shown for information. Pass or fail follows the load resistance.

Run it

Open the calculator to run these steps for your own lite. Every intermediate value is in the report and in the PDF. See the questions and answers or the pages for E1300 24, E1300 16, E1300 12a and E1300 09a. Results are to be verified by a qualified professional.