| With the advantages of wide application range,good economy and high bearing capacity,socket and disc type M60 steel pipe formwork support has gradually become the main temporary support structure system in engineering construction in the construction industry.However,in recent years,support collapse accidents have occurred from time to time.The main reasons are that the stress of support during construction is complex,the layout of support members and the value of node stiffness are unreasonable,the members have initial defects and transverse loads,which lead to yield failure and buckling instability failure of support members.In order to further improve the construction safety of the turnbuckle support,this paper takes the turnbuckle support node,vertical pole and support frame as the research object,and studies the semi-rigid characteristics and value of the support node,the stable bearing capacity of the vertical pole,the ultimate bearing capacity and stability of the support and the influencing factors by using the methods of experiment,numerical simulation and theoretical research.The main contents are as follows:(1)Collect a large number of support collapse accident cases and analyze their causes,analyze the problems existing in the use of turnbuckle support and corresponding solutions,and summarize the research results and research methods on support bearing capacity and stability at home and abroad by using literature research method.The research results show that the semi-rigid stiffness value of the support joint and the instability critical load of the vertical rod are directly related to the bearing capacity and stability of the support.For the relevant specifications and Research on the semi-rigid characteristics of the socket and socket steel pipe support,the linear value is generally adopted,and 20 ～ 90 k N is obtained Bending stiffness value of coiled joint with different m / rad.The main reasons for the collapse of the support are the local instability failure of the vertical pole and the overall instability failure of the support.The joint stiffness,initial defects,member arrangement,transverse load and nonlinear factors have a great impact on the stability bearing capacity of the support.(2)Summarize the calculation theory and structural stability analysis theory of turnbuckle support at home and abroad,calculate the critical load of vertical pole with common dimensions of 0.5m,1.0m,1.5m and 2.0m and jacking of 0.20 m,0.35 m,0.50 m and 0.65 M in a step by using Euler formula and code method,and verify the critical load of vertical pole calculated by Euler formula and code method by ABAQUS finite element numerical simulation,The calculation results show that the critical load calculated by Chinese code and Japanese code changes little with the length of vertical pole,and the calculation results are safe.The Euler formula does not consider the influence of the semi-rigid characteristics of the joint on the stability of the bar,and its critical load calculation value is generally greater than the calculation result of the code method.For the sake of safety,when the Euler formula is used to calculate the critical load of the bar in the project,the Euler critical load formula with hinged ends should be used.(3)Through the joint bending test,the variation law and value of the stiffness value of the semi-rigid joint of the turnbuckle support are studied.The bending stiffness of the buckle support joint is studied by experimental method.The joint stiffness of the turnbuckle support joint under the action of one cross bar,two cross bars,three cross bars and four cross bars is tested respectively,ABAQUS finite element numerical analysis software is used to simulate the mechanical properties of disc buckle support joints,and the rotation angles of disc buckle joints under different bending moments are obtained.Then,the fitting function of bending moment rotation angle is obtained by mathematical statistical method.The moment rotation curve of the joint with different number and position of cross bars is fitted and compared by using origin mathematical statistical analysis software.The results show that the fitting effect between the two parameter logarithmic model and the joint moment rotation curve is the best,and the fitting result of the trilinear model can meet the accuracy required by the project.Therefore,a three-stage stiffness value convenient for engineering application is proposed: when the joint bending moment is 0 ～200N.m,the stiffness value of semi-rigid joint is 20 k N.m/rad～30k N.m/rad;When the bending moment of the joint is 200 N.m ～ 600 N.m,the stiffness value of semi-rigid joint is 9k N.m/rad～11k N.m/rad;When the bending moment of the joint exceeds 600 N.m,the stiffness value of semi-rigid joint is 2k N.m/rad～6k N.m/rad。(4)Relying on the construction project of a cast-in-situ box girder turnbuckle full support on an expressway ramp,According to the support erection drawings of the project and the actual monitoring values of the support,MIDAS / civil finite element software is used for modeling and calculation to verify the safety of the support construction and the rationality of the joint stiffness obtained from the research.Based on the finite element model of the full support of the supporting project,The influence of the stiffness of the buckle joint on the mechanical properties of the support is analyzed,and the influence of the vertical rod spacing,cross rod step,inclined rod position and jacking extension on the stable bearing capacity of the support is analyzed through the single variable principle.The research shows that the joint stiffness is in line with the safety and rationality of support construction,and the three-stage stiffness value obtained from the research can be applied to practical engineering.The step distance of cross bar and jacking elongation of support have the greatest impact on the stability of support,while the distance of upright bar and the position of inclined bar have little impact on the stability of support.In the construction of turnbuckle support,the stability of the support can be improved by reducing the spacing of vertical rods,the step distance of cross rods,the extension of jacking and increasing the number of inclined rods.(5)The vertical pole within one step is simplified into a simply supported beam mechanical model,and the jacking is simplified into a cantilever beam model.ABAQUS finite element software is used to simulate and calculate the critical load of0.5m,1.0m,1.5m,2.0m vertical pole and 0.20 m,0.35 m,0.50 m and 0.65 M jacking with transverse load.On the basis of finite element numerical simulation,mathematical statistics method is used,The mathematical expression of critical load transverse load of vertical pole is fitted by three parameter exponential model,and the relationship between critical load and transverse load of support is deduced combined with Euler critical load formula.At the same time,the influence of the length of the pole on the critical load of the pole is also studied.Based on the relationship between the critical load of the pole and the transverse load,the relationship between the critical load of the pole and the geometric initial defect is derived.The research results show that the relationship between the instability of the support pole when the transverse load is greater than 4kn and the length of the pole is very small,but the pole is more prone to buckling instability under the influence of the transverse load,The formula of critical load transverse load of upright and jacking can be extended to the calculation of critical load of general compression bar under transverse load. |
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