A strong understanding of how construction materials behave at high temperatures gives builders and engineers important information which enables them to accurately predict how structures will respond to a fire. The performance of steel at high temperatures has been the focus of extensive research, as it is often used to form the building frame. This research has created a comprehensive collection of conventional steel thermal and mechanical properties at various temperatures, however information pertaining to the high strength steel bolts used to form connections in the structure is lacking. These pieces transfer loads between components of the structural system, therefore if these pieces become compromised during a fire, the entire structure could become unstable. Kodur et al. (2012) investigated the thermal and mechanical properties of two high strength steel bolts over a large temperature range to determine how their properties change when exposed to high heat, and quantified the difference between their performance and that of conventional steel.
Figure 1. Steel bolts are an important part of the building structure.1
Two high strength steels, A325 and A490, were used in this research. A conventional steel, A36, was tested as well so that it could be used as a comparison. Thermal conductivity, specific heat, thermal expansion, stress-strain response, yield, and ultimate strength of the samples were investigated. Measurements of thermal conductivity and specific heat were performed using the Hot Disk Transient Plane Source 2500. This instrument is a powerful tool for thermal conductivity measurement; it measures values between 0.005 and 1000 W/mK, and can perform in temperatures ranging from -160 to 1000°C. The Hot Disk system uses a nickel wire sensor encased in either Kapton or mica as both the heating element and the temperature sensor, which gives an absolute measurement with no contact agent required.
Thermal conductivity and specific heat measurements were performed between 20°C and 735°C. Testing occurred during both the heating and cooling periods. This temperature range was achieved by placing the samples and Hot Disk sensor inside of a furnace for testing (Figure 2). The Hot Disk nickel and Kapton sensor was used for the room temperature measurement, and all subsequent measurements were taken with the nickel and mica high temperature sensor.
Figure 2. Photos depicting a high temperature testing scenario in the Thermtest lab. The furnace is able to test multiple samples at a time, and heat proof cables connect the sensors in the furnace to the equipment outside.
Thermal Conductivity and Specific Heat of High Strength Steel Bolts
Thermal conductivity measurements revealed that there was a decreasing trend in the obtained values as the temperature rose. Values obtained during the cooling period closely matched those from the heating curve, and the samples regained their original values when returned to room temperature. The amount of carbon present in samples played a role as well.
Specific heat followed the opposite trend of thermal conductivity, and increased along with temperature. The temperature had a significant impact on the rate of increase of the specific heat. As with thermal conductivity, the values obtained during the cooling cycle closely matched those produced by the heating phase.
Results of strength and thermal expansion tests found that the thermal strain increases with temperature, and the bolts lose a significant amount of their strength at high temperatures. Kodur et al. (2012) found that the high strength steel retains strength better than conventional steel at lower temperatures (under 400°C), however at higher temperatures it has higher strength losses. The team concluded that high strength steel is much more sensitive to temperature than conventional steel.