Difficulty of the prediction of the mechanical properties of wood in fire due to the possible influence of thermal decomposition of wood, moisture and its transfer on the mechanical properties of wood is one of the major obstacles for the development of rational fire-safe large scale wooden buildings. Previous studies (Kaku, Suzuki et al., 2014 and 2015) on Cryptomeria japonica and Zelkova serrata revealed significant dependence of mechanical properties of wood on temperature, moisture content and suggested its notable dependence on tree species. In this study, bending strength and Young’s modulus of various tree species at elevated temperatures are measured using 20 mm × 20 mm × 320 mm specimens cut from a single log for two purposes, i.e. for the establishment of database of mechanical properties at elevated temperatures and various moisture conditions of major tree species common for structural members in Japan and for examining predictability of the mechanical properties of any tree species at elevated temperatures from simple material properties. For the first purpose, mechanical properties of Chamaecyparis obtusa, Larix leptolepis and Pseudotsuga menziesii, the most common tree species for building structures in Japan other than the two studied by Kaku, Suzuki et al, are measured to prepare comprehensive table for their moisture content dependence at various temperatures below the boiling point and temperature dependence below 250°C. Furthermore, the mechanical properties are measured on Picea sitchensis, Tsuga mertensiana, Castanea crenata and Quercus myrsinifolia are chosen as specimens for similar bending tests at limited temperature and moisture conditions selected to examine the governability of bone-dry density of wood on the mechanical properties at elevated temperature and moisture content: the tree species are chosen to cover range of bone-dry density as wide as possible for both softwood and hardwood.
From these tests, the following conclusions can be drawn.
① Mechanical properties of wood become generally more dependent on moisture content at higher temperature irrespective of tree species.
② Mechanical properties of wood for oversaturated moisture condition (roughly >30%) is nearly constant according to tree species and temperature range.
③ Bending strength and Young's modulus show positive but different correlation against density for softwood and hardwood at room temperature.
④ Density dependence of the mechanical properties of wood becomes weaker at elevated temperature irrespective of moisture content.
⑤ Bending strength and Young's modulus remaining ratio (Young's modulus at tested condition divided by that measured before test at room temperature) are within relatively narrow range irrespective of density once they are significantly reduced due to high temperature near the piloted ignition or oversaturated moisture condition near the boiling point.
