Mechanical Behavior Of Beam To Column Joints With Cast Steel Stiffeners And Its Influence On Progressive Collapse Resistance Of Frame Structures

Joint with cast steel stiffeners(CSS)for H-shaped beam to square tube column connection has clear path for force transfer and easy construction.At present,the research on beam-column joints with CSS mainly focuses on bearing capacity and seismic performance.There are few researches on the stiffness of CSS joints,as well as the influence of joint mechanical properties on the progressive collapse resistance of frames.Moreover,the research on high performance macromodel is also insufficient.This situation limits the popularization and application of CSS joints.Theoretical analysis,finite element analysis,and experimental analysis are conducted in this study.The mechanical behavior of the CSS joints is investigated.The influence of the mechanical behavior of CSS joints on the progressive collapse resistance of frames with square tube columns is studied deeply.Based on this study,the performance of circular tube column frames with CSS joints for progressive collapse prevention is also investigated.The main research contents and conclusions are shown as follows:(1)The theory and a practical calculation method on the initial rotational stiffness of CSS joints are proposed,which provide a basis for judging the performance of the CSS joints.By analyzing the experimental data obtained by the quasi-static test of fullscale specimens and a large number of parametric numerical simulation results,the applicability of the calculation method on the initial rotational stiffness of the CSS joints was verified,and the degradation law of the stiffness was also obtained.The results show that the post-yield stiffness of the CSS joints for square tube columns is3.5%-6.0% of the initial rotational stiffness,and the mean value is 4.7%;the post-yield stiffness of the CSS joints for circular tube columns is 8.2%-10.6% of the initial rotational stiffness,and the mean value is 9.4%.The ratio of the post-yeild rotational stiffness to the initial rotational stiffness changes little in the parametric numerical simulation,and the post-yeild rotational stiffness is much smaller than the initial rotational stiffness.(2)The mechanical model of the bending moment-axial force interaction at the beam ends of the square-tube-column frames with CSS joints during progressive collapse was revealed;the influence mechanism of the CSS joints on the progressive collapse resistance of frames with square tube columns was clarified.Based on these analyses,a practical macromodel for progressive collapse analysis of CSS joint frames with square tube columns was established;the dynamic increase factor for CSS joint frames with square tube columns was proposed.The results showed that the beam end is gradually transformed from the bending mechanism to the catenary mechanism with the displacement increasing of the corresponding removal column;compared with the welded joints without stiffeners,the force transform path of CSS joints is clear.Because of relatively high stiffness,strength and large thickness,CSS joints can effectively bear the load,and the strain of CSS joints is small and relatively uniform distributed.The macromodel proposed in this paper,which considers the initial stiffness and the stiffness degradation law of the CSS joints,as well as the interaction of the moment and the force at beam ends,is verified to be suitable for the progressive collapse analysis for CSS joint frames with square tube columns.The dynamic increase factor(DIF)of1.6 was proposed by using the macromodel.(3)Method to remove a vertical support instantaneously was invented,and an experimental study to investigate the progressive collapse behavior of square-tubecolumn frames with slabs and CSS joints was conducted.This experiment was carried out on a two-story,two-bay by four-bay steel frame.For the convenience of comparison,the frame was divided into four vertical sections,and each section had a specific joint arrangement of(i)joints with CSS and(ii)welded joints without stiffeners.Seven loads of increasing magnitude were applied to the frame.Under each load level,the column removal device in each section is removed and replaced,subsequently.The results showed that the support removal method,which transforms the sliding friction into rolling friction by using bearing steel balls,can meet the demand of progressive collapse analysis to remove the vertical support instantaneously;compared to welded joint without stiffeners,the maximum displacement of the removal column is reduced by17.83% because of the CSS joints,and the maximum strain change of beam ends is reduced by an average value of 31.30% because of the CSS joints.This result indicates that the performance of the beam-square tube column-slab frame with CSS joints is superior than the frame using welded joints without stiffeners for progressive collapse prevention.Four finite element models were developed to simulate the test frame.The results showed that the model with slabs was in close agreement with the test results;the plane model is rather safe for the finite element analysis.The space effect and the slabs of the frame play an important role in the progressive collapse prevention of frames.For further analysis,an increasing load was applied on the model with slabs until the frame suffered large deformation.The result showed that the performance of the frame with CSS joints is still superior than the frame using welded joints without stiffeners under large deformation.(4)The research on progressive collapse performance of frames with circular tube columns and CSS joints was conducted.The results show that the progressive collapse performance of frames with circular tube columns and CSS joints is similar with the frames with square tube columns and CSS joints.The suitable DIF of 1.7 was obtained by the macromodel proposed in this paper.A dynamic nonlinear analysis of the beamcircular tube column-slab frame with CSS joints was conduced.The results showed that when the frame was elastic,elastic-plastic,or suffered large deformation after the column removal,the the performance of the frame with CSS joints was superior than the frame using welded joints without stiffeners.The initial rotational stiffness calculation method,macromodel and dynamic increase factor proposed in this paper are of great guiding significance to the engineering practice,and the study on the influence of the CSS joints on the progressive collapse resistance of frames is conducive to the application of this CSS joints.