Moment-resisting-frames under cyclic loading: Large scale tests and validation of plasticity and damage numerical models

Large scale cyclic tests on two-story Moment Resisting Frames (MRF) were conducted in the Laboratory of Steel Structures of NTUA. The test frames had welded joints and were designed according to weak beam-strong column criteria, either by weakening the beams by providing reduced sections (RBS) or by strengthening them via haunches. Cyclic load of increasing amplitude was applied reaching interstory drifts up to approximately 5%. Damage was observed in form of crack initiation and propagation in the beam flanges, especially at holes which were located far from the most stressed regions. When such stress concentration drivers were missing, the structure remained intact even at large numbers of full cycles at 5% drift. Accompanying numerical calculations were performed in order to simulate the tests and validate innovative numerical models that concern non-linear material behavior and damage initiation in the ultra-low-cycle fatigue range. A good agreement between experimental and numerical results was observed. The investigations show that steels with high toughness, much higher than the minimum requirement of delivery specifications, as currently produced in Europe can sustain large inelastic cyclic deformations with little vulnerability to damage when significant stress concentration drivers are absent.