| Light portal frame structure was widely adopted in low-rise buildings for goodeconomic benefit and aseismic performance, but there ware less anti-seismicresearches on the structure due to light dead-weight and small seismic force. However,the structure partial of portal frame was some extent damaged under severeearthquakes; the structural integer collapse could be caused by the serious localdeformation, which arose of people’s stress on its anti-seismic research.The paper conducted hysteretic tests for vertical, flat and bevel endplate boltjoints of1/2scale model and three specimens were prepared for each joint form. Theload-displacement curve, skeleton curve, and distribution of stiffness degradation andinternal forces of bolt were obtained by the tests. It was showed that the failure modesof the three joint forms were similar, which the local buckling occurred on the flangeabout1.5times of the height of large end of beam to the beam end, the memberbearing capacity declined quickly after peak value, and brittle failure modes wereformed. The load-displacement curves of all specimens were not plump, which showedsmall plastic development and poor energy-dissipating capacity of the light portalframe structure, so the structure should be designed with elastic theory. The jointstiffness was influenced by the thickness of endplate and the strength of bolt, which thejoint stiffness would be higher with thicker endplate and higher strength bolt,meanwhile, the stiffness would degenerate more quickly. The form of vertical endplateis recommended in design by the comprehension of the aseismic performance of three joint forms. In conclusion, the requirements for aseismatic design concept of “strongerjoint and weaker member” would be obtained according to the “portal frameregulation”.Through the quasi-static anti-seismic test of the entire portal frame structure onsingle-layer and single-span heel hinged endplate of1/3scale model, three “bucklinghinges” were formed: two hinges were located at the two ends of the beam and one atthe weak cross-section of beam span; and the situation of the hysteretic curve, skeletoncurve, stiffness, stiffness degradation and horizontal displacement of the structure wasobtained; and equivalent base shear method was adopted to calculate and check theearthquake bearing capacity. It was concluded that the aseismic performance of lightportal frame structure could not be improved by its plastic deformation for the lowductility and energy-dissipating capacity, but the structure could satisfy the requirementof seismic fortification criterion for light dead weight and small seismic action. Throughthe analysis of strain data, it was found that the strains of close and far ends of beamwere different in plastic stage, and the factors which affect overall structuralperformance was analyzed, such as local buckling, skin effect, taper, flange width-thickness ratio and web width-tapering ratio.The test model was verified by the finite element software and the aseismicperformance of the prototype structure was analyzed, by which the parameter analysiswas done, including the influence of flange width-thickness ratio, web width-taperingratio and taper on the aseismic performance of the structure. It was concluded that thebearing capacity and hysteretic behavior were influenced by the flange width-thicknessratio, web height-thickness ratio and tapering ratio of beam; and the initial stiffness,degradation stiffness and displacement were influenced by the flange width-thicknessratio, web height-thickness ratio and tapering ratio of column. Based on the parameteranalysis, the computation formula of yield load, ultimate load and peak load, as well asthat of stiffness degradation, negative stiffness and initial stiffness was obtained. Asupporting form specific to portal frame was presented, which would rise10%ofstructural bearing capacity and1.8times of energy-dissipating capacity compared tofinite element verified model.Based on strengthening bilinear model and the test, M-θ simplified model of vertical endplate joint was established, comparison with the overall quasi-static testsand finite element model, which could well calculate the yield load and maximum loadof the structure and had a high practical engineering significance. Finally, trilinearrestoring force model was established according to flange width-thickness ratio, webwidth-tapering ratio and taper, which was fitted with results of test, and could provide asimplified analysis method for seismic performance analysis of light portal framestructure.According to the above study, an all-round knowledge on the aseismic performanceof the joint of and entire portal frame structure was showed, which provided certainbasis for revising the relevant specifications and lays solid foundation for future study. |
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