河北某镇中学1997年暑假期间建教室及办公用房60间,用木屋架36榀,屋架下弦采用的是建房时现砍伐的毛白杨。1999年春季有8榀屋架下弦严重腐朽,并断裂,裂缝最大宽度80mm(见图),致使房屋前后纵墙外裂。其他屋架下弦也出现了不同程度的腐朽,房屋被判为危房。房屋建成不足二年便成了危房,其主 During the summer of 1997, a town school in Hebei Province built 60 classrooms and offices, and used 36 wooden trusses. The lower truss of the truss used the white-wool poplar that was cut down when the house was built. In the spring of 1999, 8 truss pedestals were severely decayed and fractured. The maximum width of the cracks was 80mm (see figure), causing cracks in the front and rear walls of the house. Other roof sheds also showed varying degrees of decay, and houses were condemned as dangerous. When the house is built less than two years, it becomes a dangerous house.