Study On Bearing Mechanism Of Concrete Filled Steel Tube Support In Highway Tunnel

Since the 21st century,with the rapid development of domestic economy,the acceleration of urbanization and the increasing demand for traffic volume,it has become an inevitable realistic demand to build more lanes of highway facilities.However,because of the large span of super-large section flat highway tunnels,the stress of supporting structures will become more complex.Especially in soft,fractured and expansive soft surrounding rock strata,as well as large deformation tunnel with high geostress,the bearing capacity of supporting structure is put forward higher requirements.Moreover,highway tunnels in China are basically constructed by the New Austrian Tunneling Method.There are many problems in the actual construction and supporting effect of grille and rib.In order to deal with the more complex super-large cross-section highway tunnels in the stratum,it is necessary to seek an arch support structure with superior performance.Concrete filled steel tube(CFST)is a kind of steel-concrete composite structure,which is made by filling concrete in steel tube.The core concrete is restrained by the hoop of steel tube and the steel tube is supported by the inner part of the core concrete.CFST has the advantages of high bearing capacity,good plasticity and toughness,and good economic benefit.Moreover,the structure has been used in high-rise buildings,bridges,underground structures and ports.Engineering has been widely cited and great benefits have been achieved.This paper summarizes the research results of CFST in model test and numerical value.Taking compression-bending model test of CFST as the breakthrough point,and deepening the model test conclusion by numerical simulation,the influencing factors of compression-bending bearing capacity CFST are analyzed,and the deformation and failure characteristics of CFST are summarized.Finally,the bearing characteristics of CFST support structure are explored by numerical simulation,and compared with I-beem support structure.The main conclusions are as follows:(1)The diameter,wall thickness and steel content of CFST can not accurately represent the bearing capacity of CFST.The contribution of steel tube to the bearing capacity of CFST is the largest.The geometric parameters of steel tube can be used to characterize the bearing capacity of CFST,and the flexural section coefficient of steel tube is more reasonable.(2)Core concrete is mainly subjected to pressure,and its mechanical property shows elastic-plastic characteristics obviously.In the elastic stage,the compressive stress of core concrete shows a general linear relationship with the load;in the plastic stage,the compressive stress of core concrete increases significantly,and the smaller load increment can cause greater stress increase;in the failure stage,the compressive stress of core concrete is still increasing rapidly,indicating that although the core concrete has been destroyed,it can continue to bear due to the restraint of steel tube.(3)The mechanical property of steel tube also shows elastic-plastic characteristics obviously.The stress process at the bending part can be summarized as compressive yield and tensile failure,which can be divided into three stages.In the elastic stage,the stress increases linearly with the load,a the steel tube is only subjected to compressive stress,and the tensile stress can be neglected;in the compressive yield stage,the minimum principal stress remains unchanged,and the steel tube will undergo large deformation,resulting in stress redistribution of the steel tube;in the plastic stage,the tensile stress and compressive stress of the steel tube will increase sharply,but the compressive stress will not exceed the ultimate strength,while the tensile stress will increase until the ultimate strength is reached and the member will ultimately fail.(4)When the CFST is subjected to axial compression,it yields as a whole,and bending failure is caused by the difference of deformation.When the CFST is subjected to eccentric load,firstly it yields in the central region of the eccentric compression side,then the yield area expands and the deformation superimposes,which ultimately leads to the bending failure of the CFST.(5)In the elastic stage,changing the form of arch support can not play a significant supporting effect,shotcrete is still the main bearing body,but the use of CFST support can effectively reduce the plastic damage area of shotcrete,which is of great significance to maintain the normal and long-term use of shotcrete.(6)With the increase of diameter,the growth rate of compressive stress of thin-walled CFST decreases obviously.It shows that the stress of steel tube can be controlled accordingly,and the elastic stage of steel tube can be prolonged.