Residual strength assessment of a heat straightened ASTM A 7 Steel I-section member exposed to a fire event

Abstract

Heat straightening is an efficient and cost-effective method of repairing steel members that have experienced impact or fire-induced deformations. Postfire investigations have shown that fire exposure can cause considerable loss of residual mechanical properties. Beyond the fire event itself, additional heat applied to the member during the straightening process can, potentially, even further modify the mechanical properties of steels. This study aims to evaluate residual mechanical behavior at zones of maximum heat-induced distortion representative of both the fire event and the heat-straightening repair of a fire-damaged steel I-section. The study further aims to independently observe heating patterns, working temperatures, and external jacking forces during heat straightening. The fire-damaged member was restrained in three-point bending with fork boundary conditions simulating service conditions for a discretely braced bridge stringer. Results revealed that the cumulative effect of both the fire and the repair caused measurable differences in mechanical properties compared with an undamaged portion of the member, but mean properties still exceeded ASTM International’s ASTM A 7 product and the AASHTO nonfracture-critical toughness requirements. Supplemental metallographic analyses revealed no measurable alteration in metallurgical phase composition or grain size of the steel, indicating that heat straightening would have been a viable repair solution to keep the member in service.