Drywall assemblies that composed of light gauge steel studs and panels like gypsum boards or fiber reinforced cement boards are common in buildings because of high construction efficiency. The fire resistance of the walls is ordinarily designed and ensured by adding the thickness of boards. The drywall assemblies have been mainly used for partition elements of a building. Fire resistance of compartment elements mainly depends on the thermal tolerance of protection materials at high temperatures and increased gaps or cracks in the wall. Among fire protection materials, gypsum board products have relatively high fire resistance, and commonly used for compartment elements such as walls and floors. The purpose of this study is to grasp the influence of fire severities, thermal decomposition and degradation of gypsum boards, and thermal deformation of light gauge steel studs on the fire resistance of compartment elements. In this study, a series of fire resistance experiments of drywall assemblies were conducted from bench scale specimens to mid-scale specimens. Main parameters of the experiments were fire severities and specimen size. The fire severities ranged from α = 350 to α = 650, including the standard fire exposure.
The fire resistance of GB-F(V) is dependent on the endothermic reaction during thermal decomposition of crystal water in gypsum boards of which CaS04 2H2O is the main ingredient. A large amount of water vapor is generated during the thermal decomposition of crystal water, and it moves to the unexposed side of boards in lower temperature through micro-cracks or pores in boards. The water vapor to condenses through the board with lower temperatures, and then evaporate again. This phenomenon increases the latent heat of evaporation. The experimental results showed that increase in the latent heat of evaporation significantly greater when a thickness of board are greater than 10mm. When the thickness reaches 15mm, the effective specific heat increases by ΔH = 236 [kJ/kg] of latent heat of evaporation in calculation.
In addition, when the drywall assemblies of lightweight gauge steel studs are exposed to fire, the out-of-plane and axial deformation of walls caused by the thermal expansion of the steel. It was found that the stud’s length and the temperature difference between heating and non-heating sides of steel studs affected was the main factor of out of plane deformation. The thermal deformation coupled with the thermal shrinkage of gypsum boards also caused greater boards cracks or joint gaps. Then the inflow of hot gases from furnace through the joint gaps and cracks caused a rapid increase in the temperature of air cavity in walls. The thermal deterioration of gypsum boards overlaps with the thermal deformation of steel frame, resulting in a substantial reduction in the fire resistance of walls.
Finally, the effective thermal conductivity of gypsum boards considering the joint gaps and cracks was calculated by heat transfer analysis through a series of the experimental results in different fire severities. The identified effective thermal properties and specific heat gave a good agreement with fire resistance tests.
