Vol:.(1234567890) International Journal of Steel Structures (2021) 21(4):1228–1241 https://doi.org/10.1007/s13296-021-00501-1 1 3 Comparing the Seismic Behavior of Various Knee Braced Steel Frames Based on Incremental Dynamic Analysis and Development of Fragility Curves R. Yahyapour 1  · S. M. Seyedpoor 1 Received: 14 March 2019 / Accepted: 1 June 2021 / Published online: 8 June 2021 © Korean Society of Steel Construction 2021 Abstract The main aim of this study is to assess the seismic behavior of various knee braced steel frames using nonlinear static and dynamic analyses. For this, structures with four different types of knee braces at heights of 5, 10, 15 and 20 stories are designed through ETABS and then are three-dimensionally modeled via PERFORM 3D. Firstly, various seismic parameters including response modification factor, ductility factor, deflection amplification factor and overstrength factor of the structures are compared via implementing the pushover analysis. Secondly, target models are subjected to five far-fault and five near- fault records. Fragility curves as well as curves obtained from the incremental dynamic analysis (IDA) are then compared, so as to determine the best type of bracing in terms of seismic performance. Benefiting results of non-linear static analyses, top knee, x-knee, chevron knee and double knee braced frames have the best seismic performance, respectively. Likewise, using IDA results in the far-fault area, x-knee, top knee, chevron knee and double-knee braced frames demonstrate the best performance, respectively. In addition, in near-field area at the height of 5 stories, the best performance is correspondingly attributed to x-knee, top-knee, chevron knee and double knee braces, respectively. However, at heights of 10, 15 and 20 stories, chevron knee, double knee, top knee and x-knee braced frames show a better performance, respectively. Keywords Knee brace · Seismic performance · Push over analysis · Fragility curve · IDA curve 1 Introduction Structures designed at regions with high risk of seismic- ity are required to satisfy two basic criteria. They need to have appropriate stiffness to control displacement in order to prevent structural or non-structural damage during the earthquake. Moreover, they are called upon having sufficient resistance and ductility againts severe earthquakes so as to avert structural damage (Balendra et al. 1994). Such design philosophy has been recommended by the recent seismic regulations of buildings. Moment frames and concentrically braced frames used as lateral load-bearing systems in steel structures, lack the simultaneous sufficiency of stiffness and ductility necessitated for structures (Popov 1983). Given that moment and braced frames can provide a good ductility and high stiffness, respectively, an enhanced system with acceptable performance againts earthquake can be achieved through combining both systems into a unique one, e.g. eccentrically braced frames proposed by Roeder & Popov (1977, 1978). Selecting an optimum eccentricity for the braced system, not only provides the stiffness needed for the system, but also it produces ductility for the link beam based on shear and bending behaviors. Regardless of whether the link beam can provide a desirable ductility to the structure, it causes a serious damage to stories of the structure. In addi- tion to the role of such link beams, as a portion of primary structural components, it needs to be replaced in the case of failure. To overcome this problem, Ochoa (1986) suggested a system termed knee brace. This system includes braced frame together with diagonal bracings connected to knee member. However, there were major drawbacks associated with the initial design of knee braces. Initial knee-brace sys- tems buckled under serious lateral loads which led to lower energy dissipation and slimming hysteresis curves. Further- more, non-linear deformation of buckled braces produced lateral instability, which culminated in a sudden change of Online ISSN 2093-6311 Print ISSN 1598-2351 * S. M. Seyedpoor s.m.seyedpoor@gmail.com 1 Department of Civil Engineering, Shomal University, Amol, Mazandaran, Iran