2012 Volume 43 Issue 4 Pages 152-159
Numerous pipe-framed greenhouses (referred to as “pipe houses” hereafter) were damaged by tornados in Ibaraki and Tochigi Prefectures on May 6, 2012. We surveyed damaged pipe houses along the tracks of two tornados, and determined the relationship between the modes of failure of pipe houses and wind direction. Light-weight pipe houses do not have sufficient wind resistance to withstand a tornado that is F1 or more on the Fujita scale of tornado intensity. For wind direction perpendicular to the ridge, collapse was such that the windward side walls and eaves were pushed down. This collapse mode resembled that of a Typhoon blowing normal to the ridge. Pipe houses were destroyed by the wind in the horizontal direction in the circumference, rather than by the upward flow near the center of the tornado. The influence of the difference of covering film or structure on the collapse mode was not significant. In many cases, all types of covering films were not blown off in the wind. There was not enough time for the covering film to fail, because the wind velocity increased in an exceedingly short period. Therefore, the covering film transmitted the wind pressure on its surface to the frame of the pipe house causing it to fail. For wind parallel to the ridge the pipe house collapsed along the direction of its ridgeline. To avoid this inclination, it is necessary to increase the number of bracing elements and to modify the connections to prevent rotation of the arch pipe and the straight pipe elements crossing at a right angle.
