Nonlinear Behavior of Diaphragms in Concentrically Braced Steel Frames with Ground Floor Out-of-Plane Offset Irregularities

Diaphragms, beyond supporting gravity loads and transferring them to vertical structural elements, play a crucial role in collecting lateral forces and distributing them among lateral resistant systems. In some cases, due to architectural considerations in the design of braced steel building frames, structural designers are compelled to shift the braces in the structure's plan, classifying them under structures with out-of-plane offset irregularity. In such structures, the horizontal diaphragm between the shifted braces must effectively transfer the shear forces induced by earthquakes. Therefore, investigating the behavior of these diaphragms is imperative for the analysis and design of these structures. The objective of this research is to investigate the seismic behavior, drift demands, over-strength factor, and the degree of vulnerability of discontinuous concentrically braced frames (CBFs) when subjected to earthquakes. To achieve this goal, two three-dimensional CBFs of 3 and 6 stories, both with and without out-of-plane offset irregularity, incorporating inelastic diaphragm behavior, were taken into account through pushover and nonlinear time-history dynamic analyses using ABAQUS. The examination of the results indicated that the concrete diaphragm existing in the discontinuity region, with a thickness of five centimeters, has been damaged at a 45-degree angle. It lacks the ability to transfer the shear force resulting from the lateral load of the braces. This is in contrast to thicknesses of 10 and 20 centimeters, where the diaphragm in the mentioned area remains undamaged. Given the information provided, it seems that, with the minimum thickness specified in Iranian National Building Code (part 9), was vulnerable to the transferred shear forces. Additionally, it was found that the inter-story drift demands of the frames exceed the estimations proposed by the building code.