Experimental study about composite frames under an internal column-removal scenario

• Experimental tests on composite frames under column loss were conducted.
• Internal composite frames can form catenary action well.
• The failure of internal composite frames started from beam-column joints.
• Flexural and catenary actions can be formed more significantly with more rebars.
• The recommendations about horizontal tie forces given by UFC 4–023-03 are evaluated.

During the past decade, numerous research works have been conducted about building structures against progressive collapse. Among the conducted beam-column joint tests, normally it was assumed that the inflection point was located at the middle of the beam span. However, from the composite joint tests conducted by the authors, it was observed that this assumption may not be correct. In this paper, a series of experimental tests has been conducted to estimate the structural behaviour of composite frames under a middle column loss. The bending moment distribution among the middle and the side joints was investigated at the frame level. Two types of connections, namely web cleat and flush end plate connections were both studied. The experimental results show that the internal composite frames can form catenary action well. Also, it is found that the load-carrying capacities of the internal composite frames would be reached when beam-column joint failed. In addition, flexural action and catenary action can be formed more significantly if more reinforcing bars were used in the composite floor system. The bending moment distributions among middle and side joints are identified. In addition, according to UFC 4–023-03 published by DoD, all the horizontal tie forces should be carried by slab, since none of steel connections can reach the rotation angle of 0.2 rad. According to this suggestion, more reinforcing bars were used in composite slab in the conducted experimental tests and the effectiveness of the enhanced tie forces was investigated in this study. Also, the rotational capacities of composite beam-column joints were obtained.