Seismic evaluation of Low Rise Hybrid steel frames with different patterns of semi-rigid connection

Document Type : Original Research

Authors
M. Ghassemieh
N. Vahedi
School of Civil EngineeringUniversity of Tehran
Abstract
The 1994 Northridge earthquake motivated the researchers to overview the conventional design philosophies. At that point, one of the safest lateral load resisting systems was the fully restrained welded steel moment frame and it had been the dominant design choice in the seismic regions. The confidence in the fully restrained frames has been decreased by brittle failures of welded connections. Thus, the researchers introduced the new seismic structural systems; braced frames and hybrid frame.

Hybrid steel frame is a new lateral resistant steel moment frame that is designed based on introducing the new energy dissipating mechanism. In order to enhance frame’s seismic performance, selected rigid connections are replaced with the ductile energy dissipating semi-rigid connections. This concept at the first glance is similar to the eccentrically braced frame. In the eccentrically braced frames, structural fuses are isolated links distributed throughout the frame, while in the hybrid frames fuses are semi-rigid connections placed at the selected locations with particular patterns. The seismic performance of hybrid frame is in such a way that story drift results in the rotation of the semi-rigid connections. Thus, for a properly designed connection that behaves in a ductile manner, the rotation is absorbed by angle or plate yielding without bolt or weld fracture. It would lead to excessive end plate or angle distortion at ultimate rotation that can be retrofitted after earthquake.

In this research, the ductile semi-rigid connection is used in hybrid frames. Finite element modelling of the hybrid frame is conducted in OpenSees computer program. The semi rigid connections are implemented in OpenSees by nonlinear plastic rotation ends. The nonlinear hinges are modelled by using Ibarra Krawinkler deterioration model. The panel zone is also modelled by Krawinkler model proposed in the FEMA 355C. Then, several different patterns and locations of semi-rigid replacements within 3 story benchmark SAC frame are selected. All the frames are subjected to nonlinear static analysis as well as cyclic displacement analysis. For the assessment of the frames subjected to seismic excitations, nonlinear dynamic history analyses are conducted subjected to 40 Los Angeles records. The finite element numerical model of the SAC frame is also verified by comparing the results with the technical literature. Normalized base shear, energy dissipation capacity, and maximum story drift angle under 40 Los Angeles records are obtained for each frame. Finally, based on the mentioned parameters and design criteria the frame with desirable performance is selected.

In general, hybrid frames have less base shear, less energy absorption, and more drift compared to frames with rigid joints (SAC frames). The lower energy absorption of these frames is due to the fact that in beams that are connected to the column in a rigid manner, the plastic joint of the beam is not activated and the semi-rigid connection is responsible for energy absorption. The geometric characteristics and cyclic behavior of semi-rigid joints are such that they absorb less energy than plastic beam joints. The data obtained from different analyzes on hybrid frames are slightly different and this shows that the effect of semi-rigid joints in short-term frames is not significant.

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Modares Civil Engineering journal
Volume 21, Issue 1
March and April 2021
Pages 149-163