One of the many highlights from this year’s conference was a presentation yesterday by Chris Barry of Pilkington North America titled “How to Calculate Glass and Sealant Stress in Insulating Glass Units (IGUs).” Barry, who recently retired from Pilkington after 45 years in the industry, offered attendees several methods and observations for evaluating the stress loads and potential breakage of IGUs.
According to Barry’s presentation, methods for calculating stress are to:
“1. Assume starting sealed air space pressure = barometric pressure;
2. Change air space proportionally to the assumed change in pressure (with no resistance from glass bending and no pressure difference across each lite);
3. Calculate the ‘flexibility’ of the size, shape and thickness of glass for that change in gap dimension;
4. Calculate net pressure difference across each light from given equations for wind, temperature and barometer differences,;
5. Extract new (more accurate) air space pressure with glass flexibility from Step 3;
6. Repeat steps 2, 3, 4 and 5 with the new air space pressure, from step 5, until final air space pressure does not change; and
7. Calculate glass stress and deflection.”
Barry says he uses a “pressures and stresses” calculator, available in a free spreadsheet format on FDR Designs’ website, to help calculate these figures.
He also made several observations noting, “IG seal certification sizes (14” x 20”) can easily break glass (~5 percent reported) while in test,” and adding, “We should be testing what we install, not 14- by 20-inch [units].”
Stay tuned to www.usgnn.com™ for further updates on the IGMA Board of Directors meeting and information on the newly elected president.