Experimental Investigation of Modified Steel Braces for Ultra-low Cycle Fatigue Endurance

This paper studies the Ultra-Low Cycle Fatigue (ULCF) behavior of modified steel braces. Concentrically Braced Frames with modified braces (CBF-MB) is a Seismic Resistant System developed by the author and popularized through the INNOSEIS project in 2016 and 2017. As a continuation and upgrade of these studies, in 2018, an experimental program was conducted at the University of Architecture, Civil Engineering and Geodesy (UACEG) to investigate the endurance of modified diagonals subjected to tension and compression cycles causing large plastic strains. Three series of three full scale specimens each, subjected to different constant amplitudes of axial displacement were developed and tested. A total of 9 tests were conducted and experimental correlation between applied axial displacements and the number of cycles to failure was derived. The mechanism of the nucleation and development of cracks until fracture was observed and described. Typical stages of damage development were identified and reported. The results revealed that the braces can withstand a significant number of cycles with large plastic strains before fracture and that the CBF-MB system possesses good hysteresis performance. Based on the research carried out, a simplified analytical model is proposed for the relationship between axial displacement and the maximum value of the transverse displacement after buckling. Comparison between analytical results and experimentally measured values are presented. Some conclusions are drawn and discussed.