Research On Seismic Failure Mechanisms And Key Aseismic Measures For Masonry-Infilled RC Frame Structures

It is commonly believed that RC frame structures have strong seismic capacity, andthe seismic design problems of them have been resolved. However, seismic damageinvestigations of the destructive Ms8.0earthquake in Wenchuan show that, thereoccurred a series of disastrous failure modes beyond the designed expectations, such as“strong beam weak column failure”,“weak story failure”, and “short column failure”,etc. Based on the typical damage of the RC frame structure building of Xuankou school,which was collapsed in the Wenchuan earthquake, pseudo-static tests of typicaltwo-span RC frame with in-filled masonry walls and earthquake simulation shakingtable tests of RC frame space models are conducted in this paper. In addition, withtheoretical analyses and numerical simulations, the effecting mechanisms of the infillwalls on the failure mechanism of the RC frame structure are studied, and the influencelaws of the cast-in-place floor slab on the realization of “strong column weak beam”failure mechanism are analyzed, and then the technical measures and design suggestionsto effectively improve the anti-collapse mechanism are put forward. The main contentsof the paper are as follows:1. Taking the collapsed classroom building of Xuankou middle school inWenchuan as prototype, four masonry in-filled RC frame with floor specimens,1/2scaled, with two kinds of masonry materials and three kinds of arrangement, were made,and the quasi-static tests were conducted. The influence of the infill panels on theseismic performance are studied, such as strength, stiffness, and ductility, etc. Theeffecting mechanism of the infill wall and the cast-in-place floor slab on the earthquakedamage modes of the RC frame structure is revealed.2. Based on the pseudo-static tests of RC frame structure with infill panels, and bythe DIANA nonlinear finite element analysis program, the simulation of the interactionlaw between the infill wall and the RC frame is realized. In addition, the interactionmechanism between the infill wall and the RC frame structure is analyzed, and theinfluence laws of the infill wall on the failure and collapse modes of the typicaltwo-span RC frame structure are studied.3. Based on the quasi-static test results of masonry in-filled RC frame models, thetheoretical analyses and nonlinear finite element numerical simulations, the “shortcolumn” failure mechanism of the in-filled RC frame structure is systematically studied.In the analyses, several masonry materials that are commonly used in practicalengineering, including clay bricks, aerated block and autoclaved brick, are selected, andthe influences on the “short column” failure mechanism of the materials, connectionmodes, heights of the infill panels are quantitatively studied, and the discriminateformula and design measures to avoid the damage of short column are proposed.4. This paper put forward an aseismic design measure that can significantlyimprove the overall collapse resistant capacity of the masonry infilled RC frame structure, which is the limited disconnection between the corner of cast-in-place floorslab and beam-column. According to the design specifications, two2x2span, fourlayers,1/5scale masonry in-filled RC frame test models were made. Through theearthquake simulation shaking table test of the two models, the negative impacts of theinfill panels and the floor slab on the failure modes of the typical two-span RC framestructures are verified; through comparative analysis of shaking table tests,pseudo-static tests and actual earthquake damage, the interaction mechanisms of theinfill wall and the cast-in-place floor slab on the earthquake damage mode of RC framestructure are revealed; through contrast tests of failure modes and nonlinear numericalsimulation analyses of the RC frame structure, it is further verified that, the designmethod in the specification by increasing the bending moment amplification factor atthe column end can not effectively achieve the “strong column weak beam” failuremechanism. The limited disconnection measure between the corner of the floor slab andthe beam-column can significantly improve the whole seismic performance of the RCframe structure, and effectively delay the destruction of the frame column.