Failure Analysis of 100-Year Old Timber Roof Truss

On January 21, 2002 the bottom chord of a timber roof truss at Nettelhorst School in Chicago suddenly fractured. This fracture caused the partial collapse of the roof structure and fourth floor suspended ceiling at the school. The roof truss that failed was one of four timber trusses that support the roof over the fourth floor gymnasium at the school. The school was built in 1893. Eyewitness accounts, the available physical evidence, and structural analysis indicated that the failure occurred at midspan of one of the interior trusses at the bottom chord panel point. The failure of an individual truss did not result in the catastrophic collapse of the entire roof structure because of the inherent redundancy in the hip roof system. There was no extraordinary superimposed load such as snow on the roof at the time of the truss failure. The truss appears to have failed under its own self-weight, and the weight of the roofing, sheathing, purlins, rafters and plaster finishes it supported. The estimated stress in the bottom chord member at the time of failure was greater than the current code-prescribed allowable, but less than what we would anticipate to be the ultimate (failure) stress. The failed truss was subject to a relatively high dead (permanent) load. Based upon the absence of any extraordinary superimposed load and an estimate of the level of stress in the failed chord member, the failure was determined to result from a combination of high permanent load and long-term time-dependent loss of strength of the chord member. The reduction in the strength of a wood member when subjected to sustained loads for long time periods is related to a process generally known as damage accumulation or creep rupture. As a result, fracture occurs at a stress level that is lower than the ultimate strength of a member. The transfer of the load in the bottom chord at splice locations was also determined to be inadequate by current design standards. The relatively high bolt forces at the bottom chord splices contributed to the fracture of the chord member.