Seismic Performance Of The Rocking Steel Concentrically-Braced Frames With Column Uplift

To enhance the city’s disaster prevention and mitigation capabilities,the design and construction of earthquake-resilient structures are one of the effective ways.In steel structures,the soft-story failure is prone to be triggered and plastic hinges may be observed at the column bases under earthquakes.This results in poor ductility,low energy dissipation capacity,and obvious residual displacement,which are detrimental to the repairability of the structures after earthquakes.In the process of promoting steel structure buildings,it is of important theoretical and practical significance to control the damage distribution and residual deformation of the structure to improve its earthquake resilience.This research proposed novel rocking steel concentrically-braced frames with column uplift that consist of two rocking steel concentrically-braced frames connected by corrugated steel plate links.The proposed lateral force-resisting system is a type of gravity-controlled rocking steel concentrically-braced frames.The rocking interface between the steel concentrically-braced frame and the foundation is connected by the rocking column base with coupon-type "fuses".When subjected to lateral loads,uplift is observed at columns in tension.The tensile forces generated from the "fuses" at the rocking column bases together with the gravity loads that existing in the steel concentrically-braced frames tend to recenter the upper structure.Thus,both the rocking and self-centering mechanisms are achieved.The rocking interface between the two steel concentrically-braced frames is connected by the corrugated steel plate links.The corrugated steel plate links have the characteristics of excellent ductility and energy dissipation capacity,and thus the energy dissipation mechanism is achieved.When subjected to earthquakes,the "fuses" at the rocking column bases together with the corrugated steel plate links absorb seismic energy.This means that the damage distribution in the structure can be controlled.In this research,the seismic performance,as well as the earthquake resilience of the rocking steel concentrically-braced frames with column uplift,are systematically studied by means of experimental researches,theoretical analyses,and numerical simulations.The contents of the research work and main conclusions are summarized as follows.(1)A rocking column base with coupon-type "fuses" was designed,which allows the separation between the bottom of the column and the foundation and satisfies the objective of being easily repaired when subjected to lateral loads.It is beneficial for the steel concentricallybraced frame to stimulate rocking and self-centering mechanisms.The contributions of the axial forces provided by the coupon-type "fuses" and anchor rods,as well as the axial load ratio to the moment strength of the rocking column base were quantified according to the resistance mechanism.To validate the theoretical model and investigate the effects of the width-thickness ratio and cross-sectional area of the "fuse",as well as the axial load ratio on the seismic performance of the column base,five two-thirds scale specimens were tested under quasi-static loading.A simplified computational model of the rocking column base was developed by using OpenSees.The coupling effect of the lateral and axial loads applied on the column base could be successfully achieved by adopting the proposed model.The model has been validated by the test results thus can be used in the numerical analysis of the whole structure.Besides,the seismic performance of the rocking column base subjected to compression and tension axial loads was compared by using numerical models.(2)A type of energy dissipation device with a trapezoidal corrugated section was developed,which is called the replaceable corrugated steel plate(CSP)link.It is intended to improve the ductility and energy dissipation capacity of the steel links and thus achieve the energy dissipation mechanism of the rocking steel concentrically-braced frames with column uplift.Eleven specimens with different geometries were tested under quasi-static loading.The influences of the aspect ratios of the CSP,as well as the corrugation sizes on the seismic performance of the CSP link,were discussed.Test results showed that the CSP link with the h/d ratio was greater than 1.0 and the b/H ratio was less than 0.1 has higher ductility and energy dissipation capacity,which can fulfill the required energy dissipation capacity of the rocking steel concentrically-braced frames with column uplift.Based on the experimental and numerical results,simplified equations were proposed to predict the lateral strength and initial stiffness of the CSP link.The accuracy of the equations was verified by the test results.Furthermore,a two-node link element was adopted to simulate the CSP link by using OpenSees.The CSP link model has been validated by the experimental results thus can be used in the numerical analysis of the whole structure.(3)The performance-based seismic design procedure that applies to low-to mid-rise steel structures was proposed to achieve the performance objective for each limit state.The numerical model of the rocking steel concentrically-braced frames with column uplift was developed by incorporating the computational models of the rocking column base and the CSP link.Nonlinear dynamic analyses were performed to examine the behavior of a four-story steel structure.The seismic performance of the rocking steel concentrically-braced frame is contrasted with a corresponding fixed-base structure,in which the column base is designed as a rigid connection.The results showed that the rocking steel concentrically-braced frame obtained from the proposed seismic design procedure was able to achieve the expected performance objectives and deformation mechanism.The comparison also indicated that the story displacement of the rocking steel concentrically-braced frame was dominated by the rigid body rotation.The rocking steel concentrically-braced frame exhibited uniform interstory drift distribution and negligible residual displacement and thus rocking and self-centering mechanisms can be achieved.The CSP link could coordinate the relative movement between the two steel braced frames and could achieve the energy dissipation mechanism.(4)The effects of the number of stories,the self-centering ratio(SC),and the coupling ratio(CR)on the seismic performance of the rocking steel concentrically-braced frames with column uplift were investigated.SC is selected to investigate the effects of the ability of the rigid body rotation of the steel concentrically-braced frame on the seismic performance of the structure.CR is selected to investigate the effects of the ability to coordinate the relative movement of the CSP link on the seismic performance of the structure.The results of dynamic time history analyses showed consistent design laws for the four-story and eight-story rocking steel concentrically-braced frames.To ensure the ability of the structure to resist strong earthquakes and ensure the structure to achieve rocking and self-centering mechanisms,the recommended range of SC is 0.6-0.9.To ensure the ability of the CSP link to coordinate the relative movement between the two steel concentrically-braced frames and ensure the structure to achieve energy dissipation mechanism,the recommended range of CR is 0.5-0.6.