Seismic Behavior Of Steel Frame With Expanded Flange Connections At The Beam End

Nowadays，more and more steel frame constructions are used in the design of multi-layeror high-rise steel buildings. Some researches have suggested that the properties ofbeam-to-column joint in a steel frame can directly influence the whole behavior of the frameunder load (especially the dynamic load). However, the conventional welded joint of beamand column of steel frame system has bad ductility and high residual stress, which areimportant reasons for the brittle break of the whole system. As a result, the optimal seismicdesign of beam-to-column joint in a steel frame and the development of new forms ofbeam-to-column joint should be the key to improve the earthquake resistance of themulti-layer or high-rise structures. In this paper, the energy dissipation capability and theearthquake resistance mechanism of the steel frame whose beam flange is strengthened at thebeam end is systematically introduced by experimental research and theoretical analysis,which can serve as the theoretical and experimental foundation of the design for the newkinds of ductile steel structures. The major sections of this paper are as follows:(1) Through pseudo-static experiments on two kinds of steel frame specimens withexpanded beam flange connections including direct arc strengthened beam flange connectionand welded side-plate connection and specimens with traditional bolted and weldedconnections, the deformation regulation and failure mode have been achieved under cycleload. And the hysteretic behavior, skeleton curves, deformation capacity and energydissipation of specimens with different kinds of beam-to-column connections have beenanalyzed comparatively. The results show that the specimens with reinforced beam ends haveobvious plastic deformation, and plastic hinge is far away from the end of beam-to-columnconnection, therefore, the brittle failure of beam-to-column connection near the welding seamcan be avoided. The capacity of energy dissipation has been obviously improved, and ductilityfactors are all above3.0. As a result, it is good to achieve “strong connection weakcomponent” design idea. In addition, the results obtained by analysis show that the specimenswith arc expanded flange can transfer load more smoothly, and the plastic hinge could beexactly located, which means that the performance of such kind of specimen is better than thatof specimen with welded side-plate connection. So the connections with arc expanded flangeshould be firstly recommended in practice.(2) Based on the large finite element analysis program ANSYS, through analysis of theelastic-plastic cycle load, the influence of connection forms and various parameters on theseismic performance of the specimens have been studied. And from the view of stresstransmission and plastic development, the mechanical behavior and energy dissipation capability of reinforced beam-to-column connection under seismic load has been analyzed.Compared with the analysis of experimental results, it can be informed that both of the twokinds of specimens have the same failure form. The load-displacement curve, componentbearing capacity, yield load and ductility coefficient are similar, which can prove thereliability of finite element analysis method. The calculation results show that the stress levelnear the welding seam in the beam end can be obviously reduced by reinforcing the flangeplate in the beam end. The widened flange can delay the growth of beam end’s stress andreduce the beam end’s stress level in component yield stage, and the stress of the plastic hingesection designed can be fully developed, which can result in the out-shift of the plastic hinge,the large plastic deformation and improvement of energy dissipation. The results obtained byfinite element analysis also show that the comprehensive performance of arc expandedconnection is better than side-plate connection. Based on the sensitivity analysis of thestrengthened parameters (strengthened length, length of the transition section andstrengthened flange width, etc) of the beam ends of reinforced connection, and connectionparameters’ reasonable value ranges have been given out.(3) Based on the research of the anti-seismic properties of the beam-to-columnconnection, the dynamic response and the anti-seismic properties of widened beam flangereinforced connection in big scale steel-frame has been studied by the pseudo dynamic tests,and the stress reaction level and development of the deformation of the key part in the framehave been obtained under earthquake load. The results show that, in the elastic stage,experimental steel frame has a good anti-seismic capability because its maximum horizontalrotation is less than the limit value of the seismic design code under the frequent earthquake,which is equivalent to9degrees of seismic fortification intensity. The time curve ofacceleration versus obtained from the top and bottom of the experimental model has a similartendency, yet the occasion when their wave crest and trough came out is hardly identical; eachlayer has the same displacement valve and peak position. With the increase of the peak valueof earthquake acceleration, there would be a change of displacement peak position and areduction in peak value of top layer’s acceleration. The shear in bottom would get rose withthe increase of seismic intensity, reflecting that structural seismic responses depend onseismic intensity in the elastic state. Meanwhile, the results show that the maximum responsetime of the shear in bottom is not stable. The hysteretic curve of bottom shear to topdisplacement is spindle, and the seismic response is good.(4) Through quasi-static tests, the force transmission mechanism, energy consumptionmodel and its failure modes of steel frame test model with the reinforced beam flangeconnection have been studied under the repeated horizontal load. The results show that, in theelastic range, reinforced connection steel frame is consistent with the general frame in theinternal force and deformation. After entering the plastic stage, the steel frame will firstlyform plastic hinges at the end of the reinforced segment of beam, which can achieve thepurpose of the plastic hinge away from the joints domain and protect the complex structureregion of the beam end from destruction effectively. It has strong energy dissipation capacityand structure deformation capacity which reduce the base shear. And the overall structureseismic performance is good.(5) Using the finite element analysis program ANSYS, respectively through theelastic-plastic time history analysis and cyclic load analysis, the mechanical behavior of the steel frame with reinforced beam flange connection at the beam end under seismic loads hasbeen studied. The calculation results show that the maximum stress of the steel frame firstlyappeared at the reinforced section of the bottom flange end in the first layer of frame beam,which can effectively move plastic hinge outside. It is consistent with the experimental results.Layer shear—layer displacement hysteretic curve in the second layer showed plump spindle,no "pinch" phenomenon and hysteretic performance is good. The hysteretic curve in the firstlayer is very plump, it explains that the structural plasticity to be fully developed. It is farmore than the actual test results, the main reason is that the finite element analysis ignores theinfluence of the bolt slip, weld residual stress and sliding and rotating of column foot. Theindicators of the structural bearing capacity and the equivalent viscous damping coefficient ofthe finite element model are greater than the actual test values. In addition, the impact ofdifferent design of the plastic hinge location on the overall seismic performance of steel framewas analyzed and compared. The results show that the horizontal shear capacity of the framehas been improved with the increase of distance between the plastic hinge location and beamend, however, the structural deformation and ductility factor decreases.