We were requested to perform a forensic investigation on a steel tube chair that had failed when a 180 pound individual sat down. The evaluation was accomplished using generally accepted engineering design calculations and modern finite-element elastic-plastic large-deflection three-dimensional computer analyses.
The failure was due to gross plastic deformation of the material in the bend, which had been deformed during fabrication. The failure occurred when the bending moment applied to the tube exceeded its plastic moment capacity. A three-dimensional finite element analysis was performed to evaluate the structural integrity. The results were compared to the relevant American National Standard, requiring that dynamic loads be included in the structural adequacy of such chairs. The results showed that the chair did not meet these requirement in the “as-built” condition, and was structurally defective per this American Standard.