Study On Static Performance Of Tibetan-style Stone Masonry

Tibetan-style stone masonry is an important part of the soil-stone-timber structure which is widely used in traditional architectures in Tibet.Tibetan-style stone masonry bears vertical and horizontal loads together with the wood frames,and is a factor directly related to structural safety.Traditional Tibetan architecture with stone masonry as the main bearing member contains not only many world-famous heritage buildings,but also a large number of ordinary civil buildings.Tibetan-style stone masonry is different from sawing or rubble stone masonry in terms of material composition and structure distribution.At present,the research on the performance of Tibetan-style stone masonry is in a blank state basically,the causes and danger level of the damages such as cracks in Tibetan-style stone masonry in service are not clear,compressive and shear properties of Tibetan-style stone masonry lacks sufficient understanding,at the same time,due to the unique structural characteristics of Tibetan-style stone masonry,there is currently no suitable structural calculation model and calculation method.Therefore,a systematic study on the mechanical properties of Tibetan-style stone masonry can lay a good foundation for in-depth study of the performance of Tibetan building structures and provide a scientific basis for structural safety assessment.In this paper,Tibetan-style stone masonry is taken as the research object.The combination of experimental research and theoretical analysis is used to study the compressive performance,shear resistance,equivalent mechanical parameters and structural evaluation methods of Tibetan-style stone masonry.The main research work completed is as follows.(1)Through the investigation and summary,the masonry process of Tibetan-style stone masonry was summarized,the features and structural characteristics of Tibetan-style stone masonry were analyzed according to the masonry process and the masonry morphology after the masonry was completed.Compared with normal stone masonry,Tibetan-style stone masonry has some unique features in the structure,which will obviously affect the structural performance of the wall.In plane,block stone layer and flat stone layer are alternately stacked,which results the feature of quasi-periodic and weak orthotropic.Each layer of stone is laid flat and the height of each stone in each layer is basically the same,the vertical load is transferred between the layers close to the layer-by-layer accumulation with uniform load.The tall walls and the walls that are built on the hills are often thickened at the bottom and gradually adducted upwards,which is good for overall stability.Locking between stones and the adhesion of the earthen mortar make good integrity performance.Structural feature analysis lays the foundation for subsequent experimental design,structural performance analysis,and model simplification of numerical equivalent analysis.The common damage types of Tibetan-style stone masonry are summarized,which provides a reference for subsequent damage identification and structural safety assessment.(2)The compressive properties of Tibetan-style stone masonry were studied through experiments.Firstly,the stone compressive strength test and the compressive test of the earthen mortar which is unique to the Tibetan-style stone masonry was carried out.Experimental results of earthen mortar showed that,the curing earthen mortar made by soil and water have low strength and strong compressibility,and the initial moisture content between plasticity and liquid limit has little effect on the strength of the mortar.In addition,the bed mortar layer filled between the stones is under compacting and spread the force to the stone during the whole process of compression.Considering the structural characteristics and random distribution characteristics of Tibetan-style stone masonry which bring difficulties in simulating real masonry,Three different stone masonry specimens were designed and subjected to axial compression tests.The crack and deformation characteristics,the failure modes,and the complete stress-strain curves were obtained by scaling prism test,the compressive constitutive relationship before cracking of the Tibetan-style stone masonry is proposed.The equations can be expressed by a two-segment curve with quadratic polynomials.Full scale prism test verified the applicability of constitutive relationship,and the failure modes of Tibetan-style stone masonry with vertical joints and the ultimate compressive strength values for reference were obtained.The elastic modulus reference interval of Tibetan-style stone masonry was obtained by the full-scale wall compression test under the real environment,meanwhile some valuable damage such as cracking were observed.(3)The shear properties of Tibetan-style stone masonry were studied through experiments.Firstly,double shear experiment along horizonal bed joints were carried out,Through the test,the shear strength along horizonal bed joints was obtained,meanwhile the failure mechanism of bed joints were analyzed.This part of the research is the basic work of research on shear and earthquake resistance.The results show that earthen mortar have low cohesion and resistance to shear mainly depends on the friction associated with normal stress.The bed mortar's shear strength can be calculated using expressions in the form of Mohr-Coulomb type criterion.The experimental object of this paper is a Tibetan-style stone masonry with typical interface conditions,the cohesion and friction coefficient are lower than the stone masonry with hydraulic mortar.Secondly,methods to estimate shear strength of Tibetan-style stone masonry through shear strength of bed joint were studied through theoretical model proposed by Mann-Muller and Calderini,relationship between the bed mortar's shear strength parameters and the masonry's was analyzed,and experimental data of other researchers were used to verify the results.This relationship can be applied to the estimation of the shear capacity of Tibetan-style stone masonry when the relevant research conditions are scarce.(4)In order to be able to use the homogenized numerical model to calculate the overall structure of Tibetan-style stone masonry,the numerical calculation method of equivalent mechanical parameters of masonry was studied.The theoretical basis of this method is the stratified homogenization theory developed by the conservation of strain energy and the co-deformation conditions of different materials.Using this method,a masonry structure composed of a plurality of materials can be equivalent to a structure composed of an anisotropic homogenized material.In this paper,a masonry with multiple representative layers was taken as the research object,and a numerical calculation method based on homogenization theory was proposed,which is not only applicable to Tibetan-style stone masonry,but also applicable to other masonry structures.The calculation method of equivalent mechanical properties was verified by the testing results of the Tibetan-style stone masonry and the experimental data of other scholars.The equivalent elastic modulus of Tibetan-style stone masonry with typical stone and mortar layer dimensions was calculated.(5)The structural safety assessment methods of Tibetan-style stone masonry was studied using the fuzzy safety evaluation and analytic hierarchy process.Firstly,combined with the unique characteristics of Tibetan-style stone masonry and the mechanism of damage evolution,a fuzzy evaluation method for the safety of Tibetan-style stone masonry components was proposed.The method was verified by laboratory test results.Secondly,hierarchical classification of multi-level safety assessment of Tibetan buildings,minimum requirements for structural systems and construction,and process for comprehensive assessment of structural reliability were proposed.This part of the work provides references for the evaluation of Tibetan architecture.