Steel Moment Frame Damage Predictions Using Low-Cycle Fatigue

Abstract eng:
For over 40 years low-cycle fatigue has been known to be a cause of structural failure in steel frames. The utility of these findings was limited due to the inability of existing fatigue damage models to properly account for the complexities of connection behavior. Recent research has reinforced the significance of low cycle fatigue through identification of the state of stress in standard moment connections. In addition, testing has shown that fatigue life is measurable, repeatable, and varies significantly based on connection type. This paper presents a method for calculating the fatigue damage in steel moment frames. The results of nonlinear analysis are combined with experimentally obtained fatigue-damage curves to predict failure or the remaining useful life after an earthquake. Several example problems are presented where the response parameters of plastic hinge rotation and story drift are compared to FEMA-356 acceptance criteria and used to calculate damage using low cycle fatigue. Fatigue life predictions are presented for pre-Northridge, reduced beam section, and slotted web connections. The results indicate that fatigue life provides a more nuanced prediction of damage than hard deformation/deflection limits, and that the predicted damage correlates well with observed behavior.

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