Thermally Induced Stresses in Glazing Systems

Abstract

A thorough examination has been performed of the thermally induced stresses on a window in an enclosure-fire environment. Analytical and numerical methods establish the importance of bridling stress (due to axial elongation) and flexing stress (due to normal deformation). Most previous studies omitted the consideration of flexing stresses. Maximum stresses, being the sum of bridling and flexing stresses on the side, have been calculated for varying aspect ratios of a rectangular window and for varying ratios of shaded width to the window sides. For uniform width and heating (e.g., represented, by constant high temperature in the heated region and ambient temperature in the shaded region), the maximum stresses are bridling and equal to those of an infinite strip shaded on two sides when the shaded width is less than 40% of the short side. For nonuniform shading (e.g., due to radiation blockage) or nonuniform heating, flexing stresses contribute to the total stress by an increase up to 50%. These new results have been applied in predicting the magnitude of stresses and the location of the first crack in well-controlled experiments and measurements. These results are important because they extend and delineate the limitations of currently used relations for determining thermally induced stresses and times of first cracking in windows.