| In order to achieve the fortification goal of “No collapse under strong earthquake” for reinforced concrete frame structures,design guideline of “strong column and weak beam” and a series of related strengthen measure were proposed in current seismic specifications around the world.However,experimental result and earthquake damage investigation show that damage characteristics dominated by “column hinge” were usually observed in frame structures conform to specifications.Frame structure,as statically indeterminate structure,produce significant spatial constraint effect on frame beams.In current design and analysis: 1)cast-insitu floors slabs are usually regarded as a unified effective flange width while actual boundary constraint conditions are not taken into consideration,which may underestimate the effect of floor slab.2)frame beams are usually designed as flexural component,however,reinforced concrete beams produce nonnegligible axial elongation during cyclic loading,the passive beam axial force generated under the structural spatial constraints will significantly change the mechanical performance of beams.The above-mentioned factors cause the seismic performance of frame beam in actual structure to be greatly changed compared with the results of the component test,which hinders realization of the design concept of "strong columns and weak beams".In view of the shortcomings of existing research,spatial constraint effect in frame structure has been studied systematically from the aspects of working mechanism,design method,component experiment,numerical model and structural system application.The main research contents and conclusions are as follows:(1)Research on mechanism of the integral RC spatial frame structure with cast-in-situ floor slab was performed by using fine finite element method.Multi-scaled fine finite element models of cast-in-situ RC spatial frame structures was built in general finite element software ABAQUS.Yielding order of reinforcements,component deformations,force transfer mechanism of floor slab and beam axial force distribution of frame structure under lateral loads were discussed,the results indicate that: 1)cast-in-situ floor slabs change the yielding order of reinforcement in beams and columns,delay the formation of plastic hinge at beam ends and retard the development of plastic hinges significantly.2)Regardless of the yielding mechanism of “strong column and weak beam” or “strong beam and weak column”,the reinforcements at the column bottom of the first story yield in tension at the initial stage of frame yielding,and the yield moment is located around 1/130～1/100 of the first story drift.3)Reducing the negative reinforcement at beam end accelerates the yielding of negative reinforcement,however,yielding mechanism of frame structure does not change significantly.Increasing the size of column section is an effective method to change the yielding mode to "strong column and weak beam".4)Stress distribution of floor slabs shows significant spatial effect.The tensile stress areas of slab on the negative moment side of joints exhibit obvious shear lag,and the compressive stress areas on the positive moment side of joints are spindle-shaped and concentrated near the frame column,and the specific stress distribution pattern is related to the joint location.5)Significant differences in slab reinforcement strain distribution were observed between middle joint and edge joint.Slab reinforcement strain develops more sufficiently at middle joint,which reflects the influence of boundary constraints on the effect of floor slabs.6)Unevenly distributed axial forces along the beam span generate in the rectangular section of frame beam due to the presence of the cast-in-situ floor slab.Axial force behaves as axial tension at the beam end subjected to positive bending moment,while behaves as axial compression at the beam end subjected to negative bending moment.Beam axial force increases with the lateral displacement of the frame,but the growth trend gradually slows down with the increase of the lateral displacement.(2)A concept of “flexural margin” at beam end was proposed to characterize the variation of effective slab width with beam end rotation.Quantifying the contribution of slab to beam end flexural resistance by the effective flange width.It was found that the development of effective flange width is not strongly correlated with the inter-story drift,but is strongly correlated with the beam end rotation.“Flexural margin” mainly depends on the section and reinforcement of the main beam,transverse beam and floor slab at the joint,that is,floor slab itself and the constraint conditions around the floor.The utilization rate of “flexural margin”depends on the column connected to the joint.The higher the bearing capacity and ductility of frame column,the larger the rotation of beam end,and the higher the utilization rate,which means that more slab reinforcements participate in the flexural resistance of beam end.(3)The calculation formula of effective flange width and the calculation formula of beam stiffness amplification factor considering structural spatial constraint effect were established.Based on the parametric analysis of the multi-scale fine finite element frame model,it was found that the parameters that have a significant impact on the "flexural margin" are the height of main beam section,the width of transverse beam section,the width of floor slab,the reinforcement ratio of slab and the strength of slab reinforcement.Based on the major influence factors,a large-scale parametric finite element analysis was carried out,the calculation formula of effective flange width and the calculation formula of beam stiffness amplification factor were established respectively for different joint types.The comparison with the fine finite element results proves that the calculation formulas have good accuracy.(4)A cyclic loading test of axial restrained RC beams was carried out.The test included 3rectangular section RC beams without axial restraint,3 T-section RC beams without axial restraint,and 9 T-section RC beams with axial restraint.Seismic performance of RC beams,such as failure mode,hysteretic curve,ductility,energy dissipation capacity,stiffness degradation,axial deformation,and passive axial force,was analyzed.The results show that the axial elongation of RC beam increases with the lateral drift of the specimen,and the axial elongation phenomenon is more obvious in the specimens with large shear-span ratio.The maximum elongation reaches 11 mm,accounting for 2% of the section height.Axial restraint significantly reduces axial elongation of the specimens,and generates passive beam axial force which increases with the loading displacement.Floor slab and axial restraint significantly change the seismic performance of RC beams,the bearing capacity can be increased by36%～87%.RC beam affected by the structural spatial constraint effect is actually a compression-flexural member rather than a flexural member in general design assumption.(5)An improved plastic hinge model considering the effect of structural spatial constraint was proposed.This model is based on the IMK restoring force model in the finite element platform Open SEES,and combines the calculation formula of the effective flange width proposed in this paper with the deformation performance index of RC component to consider the bearing capacity improvement induced by structural spatial constraint effect.By comparing with the RC beam experiment in this paper,the results show that the improved plastic hinge model can better reflect the influence of floor slab on bearing capacity,simulate the degradation of strength and stiffness,reflect the relationship between skeleton curve and hysteretic curve,and show good agreement with test results,which verifies the validity and rationality of the improved plastic hinge model.(6)The effect of structural spatial constraints on the yielding mechanism and collapse resistance of RC frame structures was studied.The improved plastic hinge model proposed in this paper was applied to the elastic-plastic analysis of RC frame structure system,and the results show that: 1)The structural spatial constraint effect significantly increases stiffness of the structure,which affects the actual seismic action on the structure,and also affects the frame deformation pattern and the position of weak stories,resulting in overall shear deformation effect of the frame and the downward trend of weak stories.Generation order,development level and spatial distribution of plastic hinges all change significantly as well.2)The effective flange width of 6 times the slab thickness,which is recommended by the specification,fails to fully consider the actual floor spatial effect.3)Collapse margin ratio CMR is not only related to the yielding mechanism of structure,but also to the fortified ground motion intensity.Structural fortification intensity has a significant effect on CMR,while changing the number of story and span has little effect on CMR. |
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