Effect of Temperature on Thermal and Mechanical Properties of Steel Bolts

When steel-framed structures are exposed to fire, connections play a crucial role in transferring forces from highly stressed members to less stressed members. The performance of bolted connections, under ambient and fire conditions, depends on the strength characteristics of bolts. At present, the fire resistance of bolts is evaluated by assuming the high-temperature properties to be same as those of conventional mild steel. This is due to lack of data on high-temperature properties specific to high-strength steel bolts. To overcome this drawback, high-temperature thermal and mechanical properties of Grade A325 and A490 high-strength steel bolts are evaluated. Thermal conductivity and specific heat of A325 and A490 were measured in the temperature range 20-735°C, whereas thermal expansion was measured in the range 20-1000°C in both the heating and cooling phases of fire. Steady-state single shear and tension tests were carried out on A325 and A490 bolts at 20-800°C. Results from these tests were used to generate thermal conductivity, specific heat, thermal expansion, shear, and tensile strength relationships as a function of temperature for A325 and A490 bolts. Test data indicate that strength properties of A325 and A490 steel degrade faster than those of conventional steel and also that A490 steel exhibits slightly higher strength and stiffness properties than A325 steel at 20-800°C.