| Ningbo airport expressway southward line project is an important part of expressway net in the central city of Ningbo.Main line elevated adopts the design of "highway and rail traffic building jointly".The superstructure is urban express road and the substructure is urban rail transit.In order to reduce the design elevation,U-beam as a new structural form is adopted in urban rail transit.It has the advantages of environmentally friendly,being economical and beautiful,safety and high efficiency.However,as typical open thin-walled members,it also has problems such as small lateral bending stiffness and poor crack resistance,which limits its wider spread and use.The maximum design span of prestressed concrete(PC)U-beam in this project is 35 meters,which is the maximum span used in domestic design.To solve the problems caused by thin-walled member,the pre-stress is planned to adopt the new design and technology of combination of pretensioning and posttensioning,which make the mechanical performance of U-beam in this project more complicated and put higher requirement on the level of design and construction.Therefore,in order to ensure the shaping quality of U-beam,test the implementation effect of new design and new process,study and define the cracking load and cracking state of U-beam,this paper mainly carried out the following work:(1)In order to grasp and understand the mechanical characteristics of U-beam,the theoretical analysis is carried out with reference of the design load and value in the code.The internal forces,displacements and stresses of the 35 m U-beam under various loads were analyzed by member model and 3D solid model.By comparing the calculation results,the accuracy of the theoretical model is verified.(2)Based on the pre-calculation analysis results and the static equivalence principle,under the premise of ensuring that the mid-span bending moment of the prefabricated component is equal to the mid-span bending moment caused by the normal operational load,the number,spacing and location of the loading points in cracking load test are determined comprehensively.Through,the cracking load value of U-beam under loading point load is defined.(3)For the load test,there is no supporting loading device.In order to realize loading to cracking state of the full-scale U-beam,a supporting “ground anchor” loading test device that meets the test safety and functional requirements had been developed,and the fracture testing of U-beam in site was successfully carried out.(4)Before the mass production of U-beams,the full-scale cracking load test of 35 m U-beam was carried out in the site for the first beam.The displacement and stress responses of the beam were emphatically tracked and tested.The cracking load,crack distribution and crack propagation situation were determined.(5)According to the measured concrete material parameters and effective prestress values on site to replace the normative values in the original model,the parameter correction and deep analysis are carried out.Comparing the calculation results of measured conditions,the model of value in the code and the corrected model,the calculation results of corrected model are closer to the measured conditions.Based on the comprehensively revised model and measured results,the strength,stiffness and crack resistance of the beam meets the design requirements,and the subsequent U-beams can be prefabricated according to the standards of test beam.(6)Based on the modified calculation model,the cracking failure load and law of crack propagation of the beam body are obtained.Under the cracking failure load,the tensile damage for bottom plate of the beam gradually accumulates into sheet and damage line become more and more.Finally,through-type cracks appear at the bottom of the beam,and most of them are lateral damage cracks. |
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