Research On Spatial Grid Simulation Method Of Multi-Span Continuous Rigid Frame Bridge Considering The Local Lower Concrete Grade Of Box Girder

It is difficult to cantilever cast the multi-span continuous rigid frame bridge in the course of construction,and it is inevitable that the strength grade of concrete can not meet the design requirement in the top plate,bottom plate or web plate of some sections in the process of casting.It will bring great economic loss and great potential safety hazard to demolish the cantilever section for pouring again.As the strength grade of local concrete of the main girder can not meet the design requirements,the construction unit needs to carry out the safety assessment of the whole bridge,and when the safety of the structure is not greatly affected,the reinforcement method can be used to solve the problem,however,the single beam model can not be used to evaluate the safety of the structure.Based on a five span continuous rigid frame bridge,this thesis establishes a multi span continuous rigid frame bridge spatial grid model,which is compared with the common single beam model for calculation and analysis.For the first time,the spatial grid model is used to solve the stress analysis of the bridge structure when the local concrete strength of the top plate,bottom plate or web does not meet the design grade requirements.The main research contents and achievements of this thesis are as follows:(1)The correctness of the spatial grid method for box girder bridge calculation is verified.The spatial grid method is seldom used in single-box concrete structure,and it has never been used to calculate and analyze the insufficient local strength grade of main beam concrete.Therefore,it is necessary to verify the correctness of the application of the spatial grid method in box girder bridges,compared with the single beam model established by Midas/Civil and the solid element model established by MIDAS FEA,the virtual box girder bridge is calculated and analyzed.The results show that the precision of the spatial grid model is higher than that of the single beam model,which verifies the correctness and applicability of the spatial grid model.(2)Research on advantages and disadvantages of spatial grid method and single beam model in multi-span continuous rigid frame bridge analysis.A five-span continuous rigid frame bridge spatial grid model is established by Midas/Civil,and the transverse effect of the box girder bridge is calculated and analyzed,and the five-span continuous rigid frame bridge space grid model is compared with the single beam model.The results show that the transverse effect should be considered when the spatial grid model is used to calculate and analyze the bridge structure;Under the combined effect,the difference between the calculated results of the spatial grid model and the single beam model is mainly caused by the dead load and the concrete shrinkage and creep;The spatial grid model can reflect the shear lag effect of box girder structure just like the solid element model,while the single beam model can not reflect this effect;The spatial grid model can be further used for lateral calculation and analysis.(3)This thesis proposes a solution to the self-defect of spatial grid method for the first time.The PSC verification of the spatial grid model of the five-span continuous rigid frame bridge is carried out.The results show that the spatial grid model can not be directly used in the PSC checking calculation in the design process because some beam elements are not distributed to the prestressed steel beams,the internal forces of each section can be converted to the full section after the spatial mesh model is divided,and then the full section can be checked.(4)In this thesis,the spatial grid method is firstly used to solve the problem of insufficient local concrete strength of the main girder of continuous rigid frame bridge.The single beam model can not be used to analyze this phenomenon,and the spatial grid model is used to reveal the effect of this defect on the structural stress state.The results show that the compressive stress of the concrete in the weak area decreases and that of the concrete in the adjacent area increases when the local concrete strength of the main beam is insufficient.