Glass deflection: annealed and tempered glass

The equation

E1300 gives an equation for the center of glass deflection of a rectangular lite with four sided support. It is in Appendix X1 of E1300 12a, 16 and 24 and in Appendix X2 of E1300 09a. It is w = t·exp(r0 + r1·x + r2·x²), where x = ln(ln p̂) and p̂ = p·(a·b)² / (E·t⁴). Here 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 depend only on the aspect ratio AR = a/b.

No glass type in the equation

The equation has no term for the glass type. Annealed, heat strengthened and fully tempered glass of the same size and thickness get the same deflection under the same load. The equation uses the actual design load, not the load resistance.

What does change is the load. Fully tempered glass has a load resistance up to four times that of annealed glass for a short load, so it can be used at loads where annealed glass would break, and at those loads the deflection is larger. A design that passes on strength can still need a check on deflection.

Example

E1300 Example 9 is a lite 1500 mm by 1200 mm and 5.60 mm thick under 1.80 kPa. The deflection is about 12 mm, which is more than twice the glass thickness. It is the same for annealed, heat strengthened and fully tempered glass. Deflection does not grow in proportion to the load. A thin, large lite stiffens as it bends, so doubling the load gives less than double the deflection.

How accurate is it

The equation is a curve fit to test data. It is approximate and can differ from the true value by about 10 percent. Where deflection governs, verify it by an independent method. GlassLoad.com compares w with the allowable deflection w_allow that you enter, on a line of its own that never changes PASS or FAIL. Use the limit of your project or your glass supplier.

Deflection warnings

• Load below one third of the load resistance: the equation is least reliable at low loads, so treat the value as rough.
• Aspect ratio above 5: the deflection is calculated as if the ratio were 5, and the real deflection can be larger.
• Deflection larger than the glass thickness: the value is rounded to the nearest millimeter (nearest 0.04 in). E1300 12a, 16 and 24 require this. E1300 09a does not state the rule, and GlassLoad.com applies it under every edition.
• Very low load: the equation does not apply, so no deflection is reported.

In GlassLoad.com

Open the calculator to see the deflection, with its warnings, next to the load resistance. Read how to calculate load resistance, the guide on short and long duration loads, 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.