Comparative study of thermal breakage of annealed and tempered glazing with different thicknesses under uniform radiation conditions

Glass façades are extensively used in high-rise buildings; however, glass panes may break and fall out when subjected to transient heating from a fire, likely altering the fire dynamics both within and outside the building. This work tested annealed glass (non-tempered) and fully tempered glazing with different thicknesses of 6, 10, 12, 15 and 19 mm under uniform incident radiation of 50 kW/m2. Parameters measured included glass surface temperature, breakage time, fracture behaviour, and incident heat flux to study the different phenomena between annealed and tempered glass. A finite element method (FEM) thermal and mechanical model of annealed and tempered glass considering the glass prestress field was developed to deepen the understanding of the coupling between the internal prestress and thermal stresses under uniform radiation conditions. The simulated glass surface temperature distributions are in reasonable agreement with the measured values, with relative errors of 1.8% (annealed glass) and 2.2% (tempered glass) in terms of breakage times, thus partially validating the glazing heat transfer model. A sequential thermal-mechanical method was then used to predict the glass breakage times, with average relative errors of 10.5% and 12.2% between the predicted and measured results for the annealed and tempered glasses, respectively, regardless of glazing thicknesses. It was established that maximum tensile stresses were located at the edge's centre on the radiation-exposed-side surface for annealed glass; for tempered glass, the maximum stresses were in a region approximately 0.3 of the thickness from the heated side surface, where the cracks are prone to initiating. The time needed to overcome the compressive prestress with thermal tensile stress resulting from transient heating is considered the primary reason for the enhanced ability of tempered glazing to resist fracture, which was approximately double that of annealed glass under the conditions studied in this work.