Development of an Objective Brace Fatigue Model

To dissipate energy, steel braces in concentrically braced frames are expected to sustain large deformations due to yielding in tension and global buckling in compression. During seismic events, braces can experience very large strain demands that can lead to brace rupture. Thus, characterization of behavior and evaluation of braced frame performance depends on adequately simulating force re-distributions following both buckling and fracture of the brace elements. In the past, the fatigue life of braces has been estimated by using the Coffin-Manson relationship and Miner’s rule to evaluate the accumulation of damage based on strain or deformation histories. Typically, this model for low-cycle fatigue is considered only at the material level. However, when using force-based beam-column elements, strains at the material level are nonobjective and depend on the numerical model inputs, e.g., location and number of integration points used to model the brace. Therefore, using the same fatigue model with different numerical modeling parameters for the brace does not always produce consistent results. In contrast, deformations at the element level are little affected by the numerical model inputs. An objective fatigue model based on deformations at the element level is developed to calculate damage and fracture in brace elements. Results are compared to similar fatigue models implemented at the material and section levels. The new section and element brace fatigue models have been implemented in the source code of OpenSees.