Abstract
Public buildings in Japan are being increasingly constructed from wood. Timber-based buildings have been considered from various perspectives, including management of forest resources and environment concern. Large-scale buildings in Japan are needed to be fire－resistive or quasi-fire-proof constructions to withstand fire damage. If a fire breaks out a large-scale timber building, the residual strength of structural members exposed to fire must be known for the fire safety. For this purpose, study on physical properties of wooden structural members are important and influence of charring, temperature shift under heating and water content at normal temperature of wooden structural members have been studied. However influence of water content on the structural properties of wooden members under high temperature is not clearly.
In this paper, Cryptomeria Japonica, a softwood commonly used in timber engineering, and Zelkova serrata, a hardwood normally used in traditional large buildings, were heated up to 95°C. The Young's modulus and bending strength of two species were measured at the elevated water content and temperatures. The results provide an engineering basis for the structural fire safety design of large-scale timber buildings. The following properties of the woods were analyzed and determined.
1. The effect of water content on Young's modulus and bending strength at elevated and normal temperatures.
2. The effect of heating on Young's modulus and bending strength at elevated water content.
3. Relationship between Young's modulus and bending strength in heated moisture state wood.
In the bending experiment, the mechanical properties of heated Cryptomeria Japonica and Zelkova serrate on a scale of 1 to 4 below water evaporation temperature (room temperature, 50°C, 80°C and 95°C). The specimens were hewed from the two logs without knot and sized 20mm square and 320mm span (Fig. 1). The water content of specimens was from 1 to 150% and divide into four groups. Group1 is low water content, under 5%, Group2 is medium water content, between 5% and 15%, Group3 is other medium water content, between 15% and 30%, Group4 is high water content, over 30%. We used, as needed, a small dry kiln heated to 60°C to drying. The bending test conformed to “The examination method for the bending strength” stipulated by JIS Z 2101 and was conducted in a compression testing machine with a heat-controlled chamber (Picture1). A flowchart of the experimental procedure is presented in Fig. 2.
The Young's modulus of two species at elevated water content and temperatures, was largely affected by water content over 5% and heating temperatures 80°C or higher. The bending strength at normal temperature or higher was highly dependent on water content, however two species show different declining trends with the rise of water content and temperatures. The correlation between Young's modulus and bending strength of Cryptomeria Japonica and Zelkova serrate keep to the same with the rise of water content and temperatures. This study reports an experimental assessment of structural fire safety on large-scale timber building.
