Many casualties have been caused by the collision of lapilli and blocks ejecta (ballistic ejecta) following the phreatic eruptions of Mt. Ontake in September 2014 and Mt. Moto-Shirane, which is the southern part of Kusatsu-Shirane volcano, in January 2018 in Japan. Therefore, studies on shelters that provide protection from ballistic ejecta are necessary. However, reinforced concrete and steel shelters are heavy and are not suitable for mountains with high altitudes where transportation is limited. Thus, it is expected that wooden buildings such as mountain huts, which are an existing facility, can play a role in providing shelter. In this study, we propose a staggered structure and a cross structure for roof, built using wooden materials, as a means to reinforce mountain huts. The specimens were made with Japanese cedar roofboards; one sheet with a thickness of 15mm (boundary energy with and without penetration: approximately 1200J) and another with a thickness of 18mm (boundary energy: approximately 1300J) were prepared. All structural reinforcement specimens comprised of the roofboard installed on a surface waterproof sheet and a Galvalume® steel sheet with 0.4mm thickness. A test of the impact of simulated ballistic ejecta was conducted with a cylindrical abrasive (2.66kg) using the pneumatic impact test apparatus in these structural specimens. We observed that the boundary energy increases due to an increase in the thickness of the roofboard, when the same type of structural reinforcement was being analyzed. On the other hand, the cross structure showed higher boundary energy than the staggered structure when roofboards of the same thickness were utilized. It was experimentally clarified that a cross structure using a roofboard with a thickness of 18mm could withstand a simulated ballistic ejecta impact of up to approximately 3000J. In conclusion, we recommend the usage of a cross structural reinforcement on the roof of a wooden building, constructed by stacking two cedar boards, as a simple method of impact resistance against ballistic ejecta.
