The Research On Flexural Behavior And Design Method Of Curved Bottom Concrete Box Girder

Curved bottom section(commonly known as fish-bellied section)box girder as a modern urban concrete box girder bridge commonly used in a structural form,with beautiful shapes,large clearance under the bridge,good bending and torsional properties and many other advantages,can effectively solve the excessive occupation of the space under large-scale viaducts and has a broad application prospects in the future bridge engineering field.At present,although domestic and foreign scholars have carried out many researches on the design scheme and construction technology of the curved bottom concrete box girders,there are few studies on the mechanical performance of the cross section.Based on this situation,this thesis studies the spatially mechanical properties and design methods of the curved bottom concrete box girders with the foundation of Wusihu bridge project,which is located in Ezhou City,Hubei Province.The main contents and conclusions are summarized as follows:(1)Based on the finite element method,the shear lag effect of the curved bottom continuous box girder is analyzed and compared with the corresponding ordinary box girder.The applicability of the current rules on the effective distribution width of the flange for the curved bottom section box girder is discussed.The results show that:under the action of concentrated load and uniform load,the shear lag coefficient of the side-web at the section of the mid-support point of the curved bottom girder is 35%larger than that of the ordinary box girder.However,at the other position,the shear lag coefficients of the two kinds of box girder are similar.In particularly,the maximum shear lag coefficient of the curved bottom section is 8.9%higher at the mid-span section,and reduced by 3.4%at the mid-support point section than that of the ordinary box girder.For the curved bottom section,the top flange has a lower effective width coefficient than the bottom flange,so it can take the top flange coefficient conservatively for design purpose.Compared with the calculation results of the ordinary box section based on the code method,the finite element analysis results of the curved bottom section show that the flange effective width coefficient is decreased by 23.4%?27.7%at the top flange of the mid-span segments,while it is increased by 0.2%?17.2%at the top flange of the fulcrum section.It can be concluded that the calculation method according to the current specification is not applicable for continuous box beams with curved bottom flange.The analytic expression of the calculation for the flange effective width coefficient of the middle section is proposed for convenience of application.And the coefficient of the fulcrum section can be determined according to the specification in conservatively.(2)The grid frame method is adopted to model the curved bottom concrete simply-supported box girder,of which the eccentric load effect is analyzed.The eccentric load coefficient can be calculated in a simplified formula according to the actual lateral load distribution factor of each girder.The applicability of the common approximate calculation method for eccentric load coefficient is discussed.Main conclusions are as follows:while the empirical coefficient method underestimates the result,the eccentric pressure method and the modified eccentric pressure method overestimates the coefficient.So it is suggested that the eccentric pressure method and the modified eccentric pressure method can be used to calculate eccentric load coefficient of the box girder with curved bottom in conservatively.Based on the finite element method,it is suggested that the eccentric load coefficient of the curved bottom box girder is 1.2.(3)The nonlinear procedure for analyzing the whole process of the prestressed curved bottom box girder is compiled.An example of a destructive test beam is used to verify the correctness of the program.A simple method,which converts the curved bottom section into an I-shaped section,is proposed to calculate the flexural capacity of the box girders.According to the ratio of each I-beam's contribution to the cross section flexural capacity of the beam,the thesis gives the suggestions for the design of the curved bottom box girders.