Study On Shear Behavior And Seismic Shaear Failure Mechanism Of Masonry Of Ancient Pagoda

Masonry pagodas are excellent cultural buildings of Chinese civilization,with various structural forms and distinctive styles.However,masonry pagodas have a large self-weight and height-to-width ratio,and masonry materials are brittle and easily damaged by earthquakes,leading to structural damage or even collapse.Damage to mortar joints is a typical pattern of masonry damage in a structure due to the low shear strength of mortar joints in the structure.To scientifically assess the seismic capacity of existing ancient pagodas,shear mechanical properties and damage mechanisms need to be studied.The effect of different factors on the shear properties of mortar was investigated by performing shear tests on specimens of masonry glutinous mortar.The interfacial shear effect between the glutinous mortar and the bulk was analyzed by performing shear performance tests and numerical simulations on ancient pagoda specimens,and the Cohesive Interface property was introduced to model the bond-slip behaviour between the mortar joints.Finally,the analysis was extended to ancient pagoda structures,where crack patterns and shear damage patterns at mortar joints of ancient pagoda substructures were investigated.The details and conclusions are as follows:(1)Analysis of the shear performance of the masonry mortarCombined with relevant literature,the effects of water-to-cement ratio,glutinous rice powder content,additives and other factors were considered.The mortar was designed to approximate the ratio of the glutinous rice mortar used in the construction of ancient pagodas and specimens of the mortar were made for shear testing.The shear strength curves for each specimen were plotted to analyze the change in shear strength of the mortar for different matching ratios.Subsequently,a microscopic electron microscopy scan was performed to observe the properties of the damaged surface in order to analyze the microscopic mechanisms affecting the shear strength of the mortar.The results show that the glutinous lime mortar is severely damaged by shear and mainly undergoes compressive-shear compound damage.Repeated shear forces in the transverse direction result in a large cumulative strain,and the glutinous lime mortar exhibits stress decay after the peak,with a material brittle index of 0.626.The Mohr-Coulomb intensity curve of the mortar with vertical pressure was obtained by fitting and using an exponential distribution function to describe the post-peak strain softening deformation properties of the mortar.(2)Analysis of the shear mechanism of the masonry mortar of the ancient pagodaThe additive nano-silica effectively strengthens the granular cohesion between the internal structures of the mortar,increasing its shear strength to 15-20%with a brittleness index of 0.502,and its additive content of 1-2%improves the best results.The pulling force and frictional resistance of fibers effectively improved the shear strength of mortar to 60%,and the mesh cage formed by the fiber surroundings provided the skeleton role for the mortar,but the excessive content would weaken its shear resistance,where 0.5%fibers improved the strength to 40-60%and the brittleness index was 0.282.The mortar mixed with sand or earth aggregate has a larger capacity than pure lime mortar at the same volume,and the friction and mechanical bite between the aggregates provide the main shear support for the mortar,with an average brittleness index of 0.2～0.5,so its plasticity is better than that of lime mortar,but its shear strength is slightly weaker.(3)Analysis of the shear performance of the masonry sub-specimen of the ancient pagodaThree sets of masonry sub-specimens of ancient pagodas were designed and produced for shear testing.The analysis of the damage phenomenon is dominated by the destruction of the glutinous rice mortar first,and the bond-slip between the interfaces reaches a critical value after the mortar debonding damage due to the enhancement of the compressive shear effect.The stress-strain curve variation characteristics were analyzed and the shear strength equation was calculated and modified for pagoda damage along the mortar joints.The cohesive interface attribute is introduced to simulate the bonding and sliding action between the cracks of the ancient pagoda,and compared with the test results,the interface shear behaviour of masonry is analyzed,the accuracy of the numerical simulation is verified,and the damage parameters of masonry are obtained.(4)Study on shear failure mechanism of ancient pagodaWith reference to the study of the typical earthquake damage of ancient pagoda and its extension to the earthquake damage of its substructure,the compressive shear failure analysis of the subunit structure of the ancient pagoda under different working conditions is carried out to determine the cracking weak area and damaged location of the ancient pagoda.The results show that:different vertical pressures and opening forms correspond to different forms of structural damage.When the compression-shear ratio is between 1 and 3,shear slips between mortar joints,mainly horizontal cracks,are accompanied by several stepped oblique cracks.When the compression-shear ratio is between 3 and 7,the vertical pressure increases the shear strength of the structure and the damage gradually extends diagonally from the top to the bottom;when the compression-shear ratio>10,the shear strength no longer increases with the vertical pressure,the vertical cracks cracking is serious,some blocks split,and oblique compression damage occurs.After the opening of the structure,the damage is mainly concentrated in the corner and middle opening areas of the wall,with weak areas shifting towards the opening and spreading to the surrounding areas,where the damage is more severe.