Numerical investigation of braces and replaceable links for steel frames

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2021-2-10
Kazemzadeh Azad, Sina
Concentrically braced frames (CBFs) and eccentrically braced frames (EBFs) are among popular lateral load resisting systems for steel structures. The present study investigates different aspects of these systems. The part devoted to EBFs begins with a comprehensive review of research where 22 future research needs are identified and presented. This is followed by an experimental study on the low-cycle fatigue behavior of links, which are the most important members of an EBF. Results of the experimental study are then combined with a comprehensive database of previous experiments available in the literature and synthesized to calibrate a cumulative damage law for estimating the exhausted as well as the remaining life of a link during an earthquake. A simple 8-step algorithm was outlined which utilizes the developed damage law for estimating the accumulation of damage as well as the instant of link fracture in nonlinear time history analysis under earthquake-induced loading histories. The algorithm can also contribute significantly to the decision-making process of the post-earthquake replacement of EBF links. In addition, in the EBF part of the thesis, a replaceable link detail with a mid-splice connection (referred to herein as the detachable replaceable link concept), recently developed at METU, is numerically investigated. After reviewing the previous experiments conducted on the concept, a comprehensive parametric study is carried out and a design equation is developed for estimating the axial force that the midsplice connection can experience due to large link deformations and end restraint effects. In order to further investigate the application of the proposed detail for larger (deeper) shear links than those previously tested, another set of sophisticated simulations is conducted. The analysis results further suggest the potential of the detachable concept for practical applications. For the CBF part of the thesis, first, the seismic design rules recommended by American and European provisions are thoroughly reviewed and compared. A series of CBF archetypes are then designed based on American and European provisions and subjected to a large set of earthquakes in order to investigate and compare their seismic performances. Results of more than 800 nonlinear time-history analyses reveal that the differences which exist between these provisions can lead to significant differences in the observed seismic performances. The most notable difference is the occurrence of soft-story behavior at the top stories of the CBF archetypes designed according to European provisions. Possible reasons for this phenomenon are investigated and recommendations are provided. In the last part of the thesis, the effect of rapid shortening of braces during earthquakes on the seismic behavior of CBFs is investigated. Axially loaded members might experience compressive forces above their static buckling capacity as a result of dynamic buckling under such rapid shortenings. A theoretical background on the topic is provided followed by a comprehensive parametric study considering several CBF configurations under a large set of earthquakes. Results of a total of 1600 nonlinear time-history analyses are utilized to demonstrate the frequency of occurrence and importance of this phenomenon particularly for the capacity design of columns, beams, and gusset plates of CBFs. The implications of these extra forces were discussed, and a simple formula was developed which can be used for the estimation of the ultimate brace force (considering the dynamic buckling effect) during the capacity design of CBF systems
Citation Formats
S. Kazemzadeh Azad, “Numerical investigation of braces and replaceable links for steel frames,” Ph.D. - Doctoral Program, Middle East Technical University, 2021.