Study On Seismic Behavior And Design Methods Of Beam-to-column Weak-axis Connections With Concrete Slab In Steel Frame

Most of the previous researches dealt with steel moment connections for the strong-axis bending connection,and always for the bare steel joints,while for the weak-axis composite connections,limited research has been done so far.But obviously,the mechanical performance of the strong and weak axis of the I-section column,and the composite joints and bare steel joints,has much difference.And the traditional weak-axis wedge-shaped stiffened plate connections have poor plastic rotational capacity and easy failure of the column web.Hence,this paper conducted the experimental study and theoretical analysis for the composite joints in weak-axis of I-section column with strengthened panel zone?hereinafter to be referred as the novel weak-axis joint?,which is the improvement and promotion of the existing research,and could develop new ideas and directions to the research in this field,and also provide a reliable weak-axis connetion selection for engineering design.This paper reports an experimental research and numerical simulation analysis on the novel weak-axis composite joints.The experimental research includes horizontal cyclic loading test on the top of the column of five bare steel middle joint specimens and five composite middle joint specimens,uniaxial tensile tests of steel products and deposited weld metal,and compressive strength test of cube concrete block.The beam end section forms were designed with standard?without treatment?,reduced beam section?RBS?,and reinforced beam section?plate covered,widened flange,and haunched?.The finite element software ABAQUS 6.14-1 was adopted to conduct numerical simulation analysis,including the simulation verification of the tested specimens and the parametric analysis of the novel weak-axis composite joints,and the novel weak-axis composite frame with asymmetry steel beam section of narrower beam top flange and wider beam bottom flange,and reduced beam bottom flanges were proposed.Secondly,the elastic-plastic dynamic time history analysis for weak-axis composite steel frames were conducted,and the traditional weak-axis wedge-shaped stiffened plate joint,the novel weak-axis joint with standard beam end,the novel weak-axis joint with asymmetry steel beam section and reduced beam bottom flanges,and the box column joint,designed based on the principle that the axial compression ratio was basically the same with the I-section column,were considered,and the dynamic response differences were analyzed.Thirdly,the component method was adopted to deduce the initial rotational stiffness of the weak-axis composite joints.Based on the test and FEM results,the distribution assumption of stresses for composite beams more in line with the actual mechanical behavior conditions was made,and the bearing capacity calculation method was deduced.Through comparing to the test results,the applicability of the derivation method was verified.Lastly,according to the failure mode of tearing in the skin plate,the yield line theory was used to deduce the thickness of the key component“skin plate”in the novel weak-axis joint,and the corresponding countermeasures and suggestions for seismic design were put forward.The test results indicated that the composite slab could shift up the beam neutral axis,thus increasing the stresses of beam bottom flanges.The influence of the composite effect should be considered when checking the principle of“strong column and weak beam”.The novel weak-axis composite joints under rational design could accommodate the inter-story drift ratio of 0.04 rad with the measured beam flexural resistance larger than 0.8 M_p.The reliability of the workmanship and welding quality were primarily required to avoid the brittle failure of joints,and also the Z-direction steel plate was recommended for the skin plate material to resist the lamellar tearing.The considered parameters for parametric studies included the detailed sizes of the weld access hole,slab reinforcement ratio,slab thickness,asymmetry steel beam section,and the reducing parameters of the RBS?Reduced Beam Section?.The analysis results showed that the beam flange welds were in three-direction tensile state.The strength check of the skin plate corresponding to the beam bottom flange welds region should be focused on when designing the skin plate.In terms of the novel weak-axis composite joints,the bearing capacity calculation method by the Chinese code was less of security.Using the steel beam section with narrower top flange and wider bottom flange could reduce the stress concentration degree and the brittle failure risk of the beam bottom flange welds.When the RBS treatment was adopted for the beam bottom flange in the asymmetry steel beam section,the value of distance between the starting position of weaken region and the skin plate a can be set to?0.65⁰.90?b_bf,the length of the weaken region b can be set to?0.65⁰.90?h_b,and the depth of the weaken region c can be set to 0.20b_bf to move the plastic hinge away from the connection and reduce the risk for the beam bottom flange weld failure,where b_bf and h_b are the beam bottom flange width and beam section height respectively.The elastic-plastic dynamic time history analysis results showed that the stiffness of the novel weak-axis composite frame was much larger than that of the traditional weak-axis composite frame and the steel frame with box column connection.Compared to single composite steel frame,different joint forms had a little influence on the floor drift,story drift,and bottom shear of the whole structure,and the maximum story drift angle of each frame model did not exceed the limit values in the Chinese code.Based on the component method,the initial rotational stiffness of the weak-axis composite joints under the positive and negative bending moment was deduced.The compression of the concrete slab,the tension of the reinforcements,and the tension,compression,and shear of the column flanges was considered.Then derivation formula was verified through comparing to the FEM results,and the deviation was no more than 20%.The distribution assumption of stresses more in line with the actual mechanical behavior conditions was made that only the beam bottom flange reached the tensile or compression strength design value f of the steel material under the positive and negative bending moment.Then the ultimate flexural capacity of the composite joints under the positive and negative bending moment was deduced.And the comparison with the FEM results was conducted,which indicated that the ultimate flexural capacity of the composite joints could be calculated conservatively by the proposed methods.The yield line theory was used to deduce the thickness of the skin plate to prevent it from tension fracture,and it can be calculated conservatively according to the formula of?6.56?.