Analysis Of Seismic Performance Of Infilled RC Frame Before And After Retrofitted By CFRP

During the WenChuan earthquake in2008, the RC frames with infill wall sufferedserious damages, and the failure mechanism of “strong columns and weak beams” forframe structures which was actively implemented by every country almost did not getany reflection. As a result of the constraint enhancement of infill wall to the flexuralrigidity of the frame beams, the plastic hinges appeared in the upper and bottom end ofthe column for the majority of infilled frames, thus leading to the collapse of the wholestructure, and the damage mechanism of “strong beams and weak columns” was showedby the frames. Simultaneously, failure of weak floor due to the discontinuous of infillwall along the vertical direction of the building, torsion damage due to the asymmetricplane layout of the infill wall, shear failure of the short column due to the presence ofthe wall between the windows, casualties and escape route blocking due to the collapseof the infill wall had been seen everywhere. The serious damages of infilled RC framesin WenChuan earthquake had aroused wide attention and thinking in the civilengineering filed of our country, and people start to review the collaborative workmechanism between the infill wall and frame, as well as the influences of infill wall onthe frame structures’ overall seismic performance.After the WenChuan earthquake event, the design requirements for infilled framestructures have been improved in our newly issued seismic codes, thus causing most ofthe existing infilled frames designed according to old codes to fail to meet therequirements of the new codes. Facing the possibility of earthquake disasters at any time,it is better for us to rehabilitate existing infilled RC frame structures before theearthquake rather than implement large-scale demolition and reconstruction in order toalleviate the seismic risks faced by existing structures. The seismic retrofit for existinginfilled RC frames before the earthquake happens has been made an extremely urgentmatter by the painful experience in WenChuan earthquake.In this paper, the influence law of infill wall on the frames and the appropriatemethods of seismic retrofit for infilled frame structures using CFRP were investigated,and the main contents of the study were summarized as follows:1. The equivalent diagonal strut model used to simulate the infill wall wasdetermined first, and then the quasi static and shaking table test was conductednumerical simulation analysis using Perform-3D software, respectively. The correctnessof pushover and time history analysis based on Perform-3D, including the equivalentdiagonal strut model was proved by comparing the calculated results to the test records;2. Based on Perform-3D software, the static and dynamic analyses for two kinds ofinfilled frames were conducted, and they referred to frames with infill wall arranged inevery story and only in two to top story, respectively. By comparing with the analysis results of bare frame, the influences of infill wall on the overall seismic performance ofthese two structures have been determined;3. Different retrofit schemes using CFRP were designed for these two structures,and the overall seismic performance of retrofitted frame versus frame without retrofitwere compared under different retrofit schemes, finally, the appropriate retrofit methodsby CFRP of infilled RC frames were determined.Based on the study above, several conclusions have been obtained:(1) For the firstinfilled RC frame, the lower vibration modes of the structure were significantlyinfluenced by the infill wall, and the reduction coefficient of natural periods of vibrationwere distributed between0.4and0.6;(2) When considering the effect of infill wall, thelateral bearing capacity and stiffness of the first frame was1.7and5times that of thebare frame, respectively, while for the second infilled frame, the lateral bearing capacitychanged a little;(3) Due to the stiffness contribution of infill wall, the deformationcapacity of infilled frame were reduced dramatically, especially for the second frame,the first story was more inclined to be the weak floor, and only columns in the first storywere the main energy-consuming components under the seismic action, the overallseismic performance of the structure was bad;(4) The peak acceleration of themaximum ground motion which the structure can resist was reduced while the effects ofinfill wall were included, and the seismic ability of the structure was weakened,simultaneously, the frame was more easily to form the failure mechanism of “strongbeams and weak columns”;(5) For the first frame, the beams and columns in the firststory need to be retrofitted by CFRP, while for the second frame, the overall seismicperformance of the structure can not be improved significantly by just retrofitting theframe beams and columns, so other more effective methods should be used to retrofitthis frame, such as adding the infill wall or shear wall at the first floor of the building.