FATALITIES CAUSED BY SNOW IN HOKKAIDO AND ATTEMPTED COUNTERMEASURES: Judgment criterion of snow removal on roofs based on risk analysis for fatalities and structural damage

In Hokkaido, accidents involving people due to snow have been occurring in every winter. The fatalities caused by snow in Hokkaido on 2006 heavy snowfall or after is largest number in Japan. 60 to 70% of all accidents were caused by snow removal of roofs such as falling from roof and falling from ladder in contents of the damage factors. According to Building Standard Act of Japan, it is possible to reduce snow load on roofs conducted by snow removal only in the regions that to rise on roofs is traditional. However, the reduction of snow load isn’t recognized by prefectural ordinance. Nevertheless, those accidents caused by the snow removal have been occurring frequently. To reduce the accidents caused by the snow removal, the authors analyzed damage risk using statistics data of 2006 or after, and then analyzed performance against snow load subjected the wood-frame houses build in Hokkaido. The authors proposed also methods of the snow removal based on those results obtained in the risk analyses. The fatality ratio affected by snow conditions and aging ratio in each subprefectural bureau was focused in the risk analysis using statistics data for accidents involving people. As the results, the fatality ratio increased with normal value of annual maximum snow depth increased. Also, the aging ratio had a larger impact on the fatality ratio. In relationship between the fatality ratio and snow depth ratio, which is annual maximum snow depth divided normal maximum snow depth, the fatality ratio of 2014 to 2019 winter was larger than the ratio of 2006 to 2013 even if the snow depth ratio was similar both. This means that the damage risk is raising by recent social situation such as increasing aging ratio. It is also important to focus that the fatality ratio was large even if the snow depth ratio was smaller than 1.0. And then the authors analyzed relationship between the damage factors and accident period. The peak of accidents caused by falling from roofs appeared in February when snow load increased. The fact that snow removal on roofs was caused by the snow load was clarified through the analysis. In analysis of performance against snow load for wood-frame houses build in Hokkaido, the authors calculated damage probability related roof snow depth of girder and rafter in each constructed period. The snow depth when damaging in girder with the probability 5% was 1.0 m, and was unrelated the constructed period. This means that snow load on roofs was set conside