Performance of structural glass at elevated temperatures

Abstract

Nowadays glass is more commonly applied as structural element, either as a replacement for, or in collaboration with conventional building materials such as steel, aluminium, timber and concrete. Key arguments to opt for glass in structural design are sustainability, aesthetics and transparency. As for all structural elements, glass elements are required to be structurally safe, even in extreme conditions such as during a fire. Building regulations demand a structural element to be able to withstand a fire for at least 30 minutes. There is however a lack of experimental results on the material properties and performance of structural glass elements at elevated temperatures. One of the key material properties that determines the performance of glass at elevated temperatures is its stiffness (Young’s modulus). In this research the Young’s modulus of both Sola Lime Silica (SLS) and Borosilicate (BS) glass is experimentally determined for temperatures up to 700°C. The transmittance time of high-frequency sound waves (ultrasound) is measured for both BS as SLS glass rods during heating in a tube furnace. From this data the Young’s modulus has been derived mathematically. Additionally, the effect of a low-E coating and an intumescent coating on the performance of laminated glass panels is experimentally determined and assessed. During heating of only one side of the laminated glass panels, the time until the interlayer starts to form gas bubbles is recorded, whereas the temperature of the glass has been measured on both sides of the glass panels. From the experiments it is observed that glass performs well up to temperatures of 500 °C, provided there are no distributed thermal stresses present in the glass. Given this condition, the remaining stiffness at a temperature of 500 °C is 90% for SLS glass and 102% for BS glass. As for structural steel this is only 60%, it is concluded that both SLS and BS glass outperform steel in terms of relative stiffness at elevated temperatures. Between 500 and 600 °C a sudden deformation is observed. Therefore the structural safety of a glass element cannot be guaranteed for temperatures beyond 500 °C. The heating of a glass element can be delayed by the application of an intumescent coating. For low-E coating, an improvement in fire resistance is however not proven in this research. Instead, a negative effect has been observed and therefore the application of low-E coating is not recommended. It was shown that the PVB interlayer is the weakest point of the glass element. At temperatures of 200 °C the PVB interlayer is already completely molten, while at 500 °C the glass is still intact. The fire resistance can be improved by applying glass without an interlayer.