Evaluation of the progressive collapse of steel structures with beam to column-tree connections

Progressive collapse begins by removing the local bearing capacity of a small part of the structures and causes structural failures which are not directly affected by the initial local event. In this research, the behavior of 3 and 6-story moment resisiting structures with tree column bending connections and span length to story height ratios (L/H) of 1.0, 1.5 and 2.0 were investigated under the effects of progressive collapse. In comparison between nonlinear static and dynamic analysis in terms of performance, the structure has almost the same response in two analysis. The deformations are slightly higher in nonlinear static analysis due to the fact that the dynamic load factor is more than actual in static nonlinear analysis. The maximum deformation occurs at the corner scenarios, which is higher if it is located in the middle story scenarios. Structures with more stories show better performance against progressive collapse, in other words, more structural elements in removed column connection area and upper stories means the structure has more alternate paths to carry and transfer the extra load. Therefore the number of critical members will decrease. Increasing the L/H ratio raises tensions and deformations in removed column connection zone. Three-story structures with L/H ratio of 1.0 are throughtly resistant to progressive collapse.and their resistance decreases by increasing L/H ratio. Six-story structures with L/H ratio of 2.0 are relatively resistant, because they have more structural members to carry extra load although their L/H ratio is relatively high.