Experimental And Theoretical Study On Seismic Behavior Of CFSST Column-steel Beam Irregular Joints And Frame Structure

Concrete filled steel tubular structure are widely applied in multi-story and high-rise building and long-span spatial structure.However,irregular joints may be required in frame structures to meet the specific spatial requirements of architectural structure.The mechanics character of irregular joints are obviously different from those of the conventional joints due to the different depths and axes of left and right beam.Theoretical research and design method of irregular joints are not quite perfect yet.This paper is devoted to studying the seismic behavior of CFST irregular joints and frame structures by mean of experiments,numerical simulation and theoretical analysis.The research contents and results are as follows:(1)Experimental research on seismic behavior of irregular joints of concrete filled square steel tubular(CFSST)column and H-shaped steel beams,three unequal-depth beam-to-column joints and three staggered beam-to-column joints was tested under monotonic and cyclic loading protocols.The result show that irregular joints exhibited good seismic performance and flexural failure occurred firstly at the beam end,the different depths and axes of left and right beam has a great influence on the deformation and stress distribution in the core area,and the positive load of hysteresis curves is not symmetrical with the negative load.As the beam depth ratio decreased,the unequal-depth beam-to-column joints have a seismic performance decline.The seismic behavior of staggered beam-to-column joints improve with the increase of axial pressure ratio,but shear deformation occurred in its core area.(2)Experimental research on seismic behavior of CFSST column-steel beam frames with irregular joints.A two-bay,three-story CFST frame with unequal-depth beam-to-column joints,a two-bay,three-story CFST frame with staggered beam-to-column joints,and a conventional CFST frame with conventional joints were tested under monotonic and cyclic loading protocols,respectively.The test result show that the test frames presented the expected failure mechanism of a beam hinge,the hysteresis curves are full and shuttle-shaped and the strength degradation and stiffness degradation both slowed after the peak load,which reflect that the test frames have excellent deformation capacity and good energy dissipation capacity.The conventional CFST frame exhibite better seismic performance energy dissipation than other frames.According to the trilinear skeleton curve model,restoring force mode for CFST frame structure was established.The finite element analysis were carried out using ABAQUS,Open Sees and SAP2000 software,respectively,and the simulation results were in good agreement with the test results.Based on the simulation,the working mechanism and damage process of frames with irregular joints through onlinear full-range analysis,as well as the location and appearance sequence of plastic hinge distribution of beams and columns are investigated,and the parameter analysis on seismic performance of CFSST frames ith irregular joints were carried out.(3)Shear capacity analysis of irregular joints of CFSST column and H-shaped steel beams.Base on the test results and finite element analysis of the irregular joints and the frames,the proportion of shear force shared by various force transfer mechanisms in different stages is obtained.On the basis of the adapted equivalent diagonal strut mechanism,the calculation formulae of the shear capacity for unequal-depth beam-to-column joints and staggered beam-to-column joints were established,as well as the mathematical calculating relations among the beam height ratio,beam span ratio and maximum shear bearing capacity of unequal-depth beam-to-column joint.(4)Analysis of damage evolution process of CFSST column-steel beam frame structure with irregular joints.The damage quantification analysis of the test frames were carried out based on different seismic damage models.The damage evolution process of the test frame were analyzed according to test phenomenon,stress distribution law and damage index,at the same time,corresponding relation between damage index storey drift ratio of the frame was established.Based on 33 sets of test data,quantitative evaluation standard of seismic damage for CFST column-steel beam frame were proposed.(5)Analysis on seismic fragility of CFSST column-steel beam frame structure with unequal-depth beam-to-column joints.The incremental dynamic analysis(IDA)was carried out for carried out for the 9-story CFST 2-D frame model and 3-D frame model,respectively,with PGA as the ground motion intensity parameter and the maximum storey drift ratio as the EDP,IDA curves and seismic fragility curves have been plotted.The analysis indicated that the exceedance probability of the 3-D frame under bi-directional earthquake action is sequentially greater than that of the 3-D frame under one-directional earthquake action and greater than that of the 2D planar frame at each performance level of structure.