Research On Shear Capacity Of Prestressed Concrete Simply Supported T Beam In Heavy-haul Railway

With the increasing axle load and traffic volume of existing railway heavy haul transportation,higher requirements are put forward for the safety of bridge structure.This has higher requirements for ensuring the safe operation of existing heavy haul railways and improving the normal service life of railway bridges.Daqin heavy haul railway is an important channel for coal transportation in western China.In recent years,line maintenance personnel and experts and scholars have found that there is obvious oblique crack at the end of prestressed concrete beam in the field inspection and detection of heavy haul railway.Among them,the oblique crack at the end of 32 m post-tensioned prestressed concrete beam is the most obvious and the disease characteristics are relatively complex.The occurrence of oblique cracks often presents brittle failure characteristics.The stirrup stress at the crack increases or even reaches yield eventually.The cracking of concrete will also lead to water seepage and affect the durability of the beam.The further development of oblique cracks may lead to the gradual decline of the bearing capacity of the beam,and ultimately affect the normal service life of the bridge structure.In view of this situation,this paper takes the 32 m post-tensioned prestressed concrete simply supported T beam with typical inclined crack disease in Daqin Railway as the research object,through literature research,theoretical calculation,field dynamic and static load test,long-term operation monitoring and other research methods,the shear performance of prestressed concrete simply supported T beam under heavy haul transportation is studied.The main research contents are as follows :(1)Literature investigation.The research process of domestic and foreign scholars on the shear performance of concrete beams is combed,and the analysis theory and influencing factors of the shear capacity of concrete beams are summarized.The representative calculation formulas of shear capacity at home and abroad are listed and the main factors considered are analyzed and compared.The literature research on the crack resistance of concrete beams and the fatigue tensile strength of concrete was carried out,and the research on the crack resistance of normal and oblique sections of beams and the value of the reduction coefficient of the fatigue tensile strength of concrete by experts and scholars at home and abroad through theoretical analysis and fatigue load test were summarized.(2)Checking analysis.The calculation results of 32 m prestressed concrete simply supported T beam show that the normal stress and shear stress at each calculation position of the beam are not overrun,and the bending performance of the beam is good.By calculating the main tensile stress and the main tensile stress angle of the beam,it is shown that the main tensile stress of the straight curve beam is the largest at L / 8 from the end of the beam in the calculation section,and slightly exceeds the standard limit,which is the most likely position for oblique cracks.According to the direction angle,the direction of oblique cracks is roughly determined,which is close to the actual direction of oblique cracks.The main tensile stress near the end of the beam is the main factor to control the oblique crack.Under the action of long-term repeated load,the tensile strength of concrete will be reduced.Combined with the previous studies on the tensile fatigue strength of concrete by experts and scholars and the experimental results of the fatigue loading test of the indoor model beam,the axial ultimate tensile strength of concrete with the fatigue tensile strength of 0.55 times is taken and compared with the calculated tensile principal stress at the beam end.It is found that the principal stress at the beam end exceeds the fatigue tensile strength of concrete,and the possibility of concrete cracking is great.The reason of concrete inclined cracks can be summarized as the main tensile stress of beam end web exceeds the limit value of concrete fatigue tensile strength,which leads to cracking under long-term repeated load.(3)Field test.Two prestressed concrete simply supported T-beams with 16 m and32 m spans were selected for dynamic and static load tests.It was found that the deflection of the beam and the concrete stress at the lower edge of the mid-span section were lower than the standard limit under static load and train load.The vertical stiffness of the beam meets the requirements and the structure is in elastic working state.In the dynamic and static load test,the crack propagation of the beam generally presents a regular distribution of small upper part and large lower part,and the cross-crack strain is significantly greater than the main tensile strain of the non-cross-crack measuring point at the adjacent position under load.The crack propagation in the outside of the web is greater than that in the inside,and the strain ratio of the corresponding measuring point is 1.4 ～ 1.6.The reason may be that when the train axle acts on the T beam,the load is not at the position of the symmetrical axis of the T beam,but in the transverse direction of the bridge.The crack propagation of heavy train is greater than that of light train,which is due to the load of heavy train is significantly greater than that of light train.There is a positive correlation between the crack propagation and the size of live load.(4)Long-term monitoring.The long-term monitoring of the same beam span on different bridges shows that under the action of train operation,the left beam is greater than the right beam,and the outer web is greater than the inner web.The comparative analysis of straight curve beam shows that the eccentric effect of line bridge has obvious influence on crack propagation under load,and the stress condition of inclined section is different.The crack propagation of heavy vehicle line is greater than that of light vehicle line,and there is a positive correlation between the expansion of oblique cracks at the beam end and the live load.At the same time,the cracking span and the uncracking span of the same beam type of the same bridge are selected for comparative test,and the corresponding measuring points at the position of the beam end web are selected.It is found that the strain of the cracked beam is twice that of the uncracking corresponding position,and the crack propagation is obvious.At the same time,the tensile stress increment of uncracked beam converted concrete is 2.94 MPa.Main conclusion: The existing heavy haul railway has only no excessive principal tensile stress under the original design load,and only a small amount of calculation positions are slightly excessive under the existing load,indicating that the oblique section crack resistance of the original design bridge is good.In view of the phenomenon of oblique cracks in the field,combined with the train operation of Daqin Railway,the tensile strength of concrete under the action of constant amplitude repeated load needs to be further reduced.It is found that the main tensile stress of the beam is significantly exceeded under the limit value of fatigue tensile strength,indicating that the reason for the occurrence of oblique cracks is that the main tensile stress at the oblique section of the beam exceeds the fatigue tensile strength of concrete under long-term fatigue load,resulting in concrete cracking.Through the dynamic and static load test and long-term monitoring system to test the mechanical properties of the bridge,it is found that the overall bending performance of the bridge with oblique cracks is better,and the mid-span deflection,amplitude and bearing displacement of the beam meet the specification limits and verification requirements.However,the crack propagation of the oblique section of the beam is obvious,and the stress of the stirrup at the position of the crack increases.It is suggested to take corresponding reinforcement measures to strengthen and transform the shear performance of the oblique section.