Collapse Mechanism Of Multi-Story Masonry Structure With First Story Used As Shops

Most of the death toll in an earthquake comes from buildings collapse.Multi-story masonry buildings with first story used as shops were very popular inareas affected by Wenchuan earthquake. In down town of Beichuan county,morethan80%of this kind of buildings ruined. Multi-story building with first storyused as shops is also very common in the small and middle towns in south half ofChina. According to statistics, there are about twenty million such buildings,which hosting one hundred million people. It is very necessary to study thedesign method of this kind building and improve the capability of structuralcollapse resistance.Staff apartment block of Beichuan Telecommunication Bureau is a typicalmulti-story building with first story used as shops, which just suffered from amoderate damage in the earthquake. According to analysis of this building, theaseismic structural measures were proposed, and they were proved effective toimprove the capability of structural collapse resistance based on shaking tablecollapse tests. In order to use the measures proposed in seismic strengthening ofexisting building, the lateral stiffness of masonry wall and the calculation methodof ductility need to be built. The pseudo static tests of masonry walls were carriedout for this purpose. The conclusions can be summarized as:1) According to fieldwork, the damage state of staff apartment block wascollected, modal test to the building was carried out based on ambient vibrationsurvey and the natural frequencies of it were gotten. The most prominent designfeatures of the structure are the set up of wing wall and enhanced structuralcolumn through layers beside the door in the longitudinal wall of bottom floor.The structural measures effectively increase the lateral stiffness of longitudinalwall facing street in the first floor, reduce the stiffness difference between thefront and back wall in the bottom floor, make seismic shear force distributinguniformly in each wall. Thereby it reduces the torsional effect of structure, andimproves the capability of structural collapse resistance.2) Based on the first strength theory, the formula is proposed to calculate thestory drift limits when masonry structure cracks. It is known by the formula thatthe stiffness of the masonry wall is larger, the ductility of it is worse. Based oncompare of many constitutive relations of masonry materials, one model isrecommended to simulate the masonry of clay bricks, and it is used to do thenonlinear seismic response analysis of staff apartment block. Based oncalculation results, the elastic-plastic story drift limit of masonry structure isproposed as1/160, which is equal to the pseudo static test results of masonrywalls.3) According to the device for prestress loading, the shaking table test wascarried out in which the compression ratio of model was equal to that ofprototype structure. Test results show that the prestress load device is feasible andeffective to shaking table test, and the device provides a new way for shaking table test to load artificial quality of model.4) The failure mode of masonry structure was obtained according tocomparative shaking table collapse tests of four single-layer masonry structures,The shaking table tests prove that the main factor leading to masonry structuredamaging or collapsing is the great disharmony among the lateral stiffness ofeach longitudinal (or transverse) wall. Seismic shear force walls bear is disparitybecause of the different stiffness, so that walls are damaged one by one whenearthquake occurs, and finally the building collapses. Test results show that wingwall and enhanced structural column can improve the collapse resistancecapability of masonry structure significantly.5) In order to provide theory to stiffness design method of masonry structure,the defects of existing methods for lateral stiffness of masonry wall were pointedout according to analysis, and the formula considering the ring beam andstructural column for calculating lateral stiffness of wall was proposed, and it isproved reasonable by the pseudo static tests of12masonry walls.