Experimental Study On Transverse Internal Force Of Multi-box Girder

Multi-box small box girder bridge is prone to longitudinal cracks,mainly concentrated in the upper edge of the box girder flange and web junction,the lower edge of the flange plate at the small box girder connection,the middle of the bottom plate of the box girder,etc.Sometimes there are cracks in the middle and lower edge of the top plate of the box girder,but they are not easy to be found due to their confinement in the box.These problems belong to the transverse force problem of small box girder bridge,most of the existing bridge design analyses use rigid joint beam method or lane plate simplification method for analysis and calculation,and these methods often do not take into account the spatial effect of the bridge,resulting in some shortcomings.In this paper,based on a prefabricated 30 m small box girder bridge as the background,a Plexiglas single girder and six-girder small box girder(none,one,three)internal crossbeam model was made,and tensile tests of model materials and single-point precise lateral movement loading tests of typical cross sections of single or multiple girders were conducted.By comparing the test with the analysis results of various models,the author studied the effects of Poisson’s ratio parameter changes on the force of small box girder,the effects of load action position and inner crossbeam settings on the transverse internal force of box girder,and the applicability of various analysis models to the analysis of structural force performance of small box girder.The main contents and research conclusions are as follows.(1)Four methods used to analyze the transverse internal force analysis of small box girders are described,with the focus on the rigid joint beam method.Based on the interlaminar block slab element theory of Mindlin thin plate theory,the author compiled relevant analysis programs in Matlab environment,established corresponding mechanical analysis models,and by using Midas Civil and Midas FEA,established the three-dimensional spatial dense beam lattice model and solid unit model.(2)The material properties of the Plexiglas material are very different in itself,and the elastic modulus and Poisson’s ratio of the material have a very obvious influence on the structural internal force analysis,so it is helpful for ensuring the accuracy of the test results to conduct as many tests as possible on the mechanical properties of the model material before the structural model test.(3)Under the action of large concentrated load,the bottom and top plates of the small box girder have large longitudinal tensile and compressive stresses and obvious shear hysteresis effect,while the top plate shows large transverse tensile stresses due to the influence of Poisson’s ratio of material,and the bottom plate has small transverse tensile stresses under the action of eccentric load,while other cases show mostly compressive stresses,which are slightly different from the results of solid finite element analysis.The actual projects should attach importance to the analysis of the transverse internal force of the top plate,which requires engineers’ ample attention.(4)For multi-box small box girder structure,the setting of the inner crossbeam can improve the overall force of each main beam.The positive stress and vertical deflection of the main beam is reduced to a certain extent,the transverse force of the box girder flange plate is significantly reduced,and the overall transverse force of the bridge becomes more uniform.In actual projects,it is advisable to set as many inner crossbeams as possible if conditions permit.(5)Analysis of the transverse force at the root of the flange of multi-box box girder can be used near the outer box girder position,while the analysis of the transverse force at the flange joint of box girder should be selected as close to the middle box girder position joint as possible.(6)Rigid beam method can be used to analyze the longitudinal force of the main beam of multi-box type small box girder and the transverse force at the flange joint when there is no inner cross beam,and it is advisable to use it carefully for the transverse force with multiple inner cross beams.Three-dimensional solid finite element can accurately describe the spatial force distribution of multi-box type box girder structure,which has great superiority for local analysis,but has certain limitations for engineering design analysis because it is time-consuming and relatively low in calculation efficiency.Space dense beam lattice can efficiently analyze the longitudinal force of the main beam of multi-box small box girder,and the correct setting of transverse virtual beam lattice spacing to analyze the transverse force of the flange can achieve the effect of 3D solid finite element,and the analysis results are more suitable for engineering design.The laminated slab element can analyze the spatial force analysis of multi-box box girder concisely and quickly,but the method of this paper has some limitations for the prestressed concrete box girder structure.