Impact resistance to ballistic ejecta of wooden buildings and a simple reinforcement method using aramid fabric

Abstract Many of the casualties in the phreatic eruption of Mt. Ontake, which is located in central Japan, on September 27, 2014, were climbers near the summit. The main cause of death was injury by lapilli and block ejecta (ballistic ejecta). The absence of shelters at the summit and along the climbing trails may be a major factor in the large number of casualties attributable to ballistic ejecta, and establishing and improving shelters on active volcanoes is an urgent need. The climbers who evacuated to wooden mountain huts survived the 2014 Mt. Ontake eruption, this research focuses on the impact characteristics of wooden materials. We therefore performed an experimental investigation into the safety of wooden materials under impact by simulated ballistic ejecta with a mass of 2.66 kg (equivalent to a diameter of 128 mm) as by assembling cylindrical abrasive travelling at up to 300 km/h (approximately 83 m/s). With abrasives acting as ballistic ejecta, the simulated shelter facility components serving as targets were wooden structures composed of Japanese cedar, which found on volcanoes at high elevations in Japan. The components of the fabricated roof were Japanese cedar with a thickness of 15 mm as roofboard installed on the surface waterproof sheet and 0.4 mm thick Galvalume® steel sheet with an additional rafter installed. The impact absorption energy of the fabricated roof increased with simulated ballistic ejecta impact velocity because the localization of bending deformation progressed at the impact part and the frequency of splitting failure increased with increasing impact velocity. This is attributable to variation in the mode of splitting failure of cedar boards and the dependence of the material strength on projectile velocity. We also experimentally investigated simple reinforcement methods in order to obtain basic data on conversion of mountain huts to shelters. The reinforced wooden structure could withstand simulated ballistic ejecta impacts of up to approximately 13 kJ by incorporating two layers of aramid fabric between a waterproof sheet and a cedar board. Review of the roof structures of existing wooden buildings based on these results promises further advances in safe impact-resistant design.