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Experimental and numerical studies on replaceable links for eccentrically braced frames

Özkılıç, Yasin Onuralp
Eccentrically braced frames (EBFs) are extensively used as a steel lateral load resisting systems in high seismic regions since EBFs simulate ductility and high energy absorption capacity of moment resisting frames (MRFs) and high stiffness of concentrically braced frames (CBFs). High stiffness and high ductility of EBFs are obtained from diagonal braces and yielding of link element, respectively. This thesis reports findings of a three phase experimental and numerical research program on replaceable links for EBFs. The first research program was conducted to investigate four-bolt extended end-plate connections for replaceable shear links. Extended end-plate moment connections are used in a number of applications including the beam-to-column connections in seismic moment resisting frames (MRFs) and replaceable link-to-frame connections in eccentrically braced frames (EBFs). While the extended end-plate connections have been extensively studied for MRF applications, little is known about their performance in EBFs. The loading conditions and the acceptance criterion are different for the same connection when used in MRFs or EBFs. An experimental and numerical study has been undertaken to investigate the performance of four-bolt extended unstiffened and stiffened end-plate connections used for replaceable shear links. Pursuant to this goal, 10 nearly full-scale EBF tests were conducted where the thickness, width and stiffening of the end-plate were considered as the variables. The results showed that the design recommendations given in AISC guidelines and Eurocode provisions provide conservative estimates of the end-plate thickness. Finite element simulations were conducted to investigate the bending strains for different plate thicknesses and to determine the sources of conservatism in the capacities determined using the design guidelines. Modifications to the AISC design guidelines were proposed to more accurately determine the required end-plate thickness. The second research program was performed to develop novel detachable links for eccentrically braced frames. Post-earthquake replacement of links enables the use of eccentrically braced frames (EBFs) after a seismic event. Recent years have witnessed the development of numerous replaceable links. The extended end-plated replaceable links are the most efficient among the developed details. The use of these links enables to minimize the size and the weight of the part to be replaced. In addition, the performance of end-plated links is similar to the conventional links. Research reported to date showed that these links have disadvantages in terms of removal and replacement. Large axial forces can develop within the link member which may require using hydraulic jacks for the removal operation. More importantly is the difficulties associated with the link replacement under residual frame drifts. A novel detachable replaceable link is proposed in this study which employs a splice connection at the mid-length of the link. The splice connection consists of saw cut I-sections welded to both parts of the replaceable link. The detail provides an erection tolerance which facilitates easy removal and enables replacement under residual frame drifts. Proof-of-concept testing of the proposed links was performed on 3 specimens where the type of force transfer in the splice connection was considered as the prime variable. All specimens failed at link rotation angles that are significantly higher than the link rotation angle required by AISC341 and demonstrated the potential of the proposed link concept. Complementary finite element parametric studies were conducted to validate the design procedure developed for the proposed replaceable link concept. The third research program was carried out to enhance replaceable links by introducing frictional dampers. The main target was to extend the low-cycle fatigue life of the replaceable links by dissipating energy through the link yielding and frictional resistance provided faying surfaces of the side plates. Pursuant to this goal, brass shims were placed between saw cut I-sections and side plates which were drilled with slotted holes, and bearing type connection was utilized to promote bolt slippage. The proposed frictional mid-spliced connection was validated through a comprehensive experimental study. A total of eight specimens with proposed connection were tested considering slot size, the number of bolts and the number of slotted holes as the primary parameters. Moreover, three specimens with extended end plated connection were tested to compare low-cycle fatigue life with the link having the proposed mid-spliced connection. The link with proposed frictional mid-spliced connections exhibited significantly higher rotation capacity that the required link rotation. Furthermore, the link with the proposed connection excessively increased low-cycle fatigue life and energy dissipation capacity of links.