Research On Shear Lag Effect Of Box Girders Caused By Prestressing Force

With the push of modern traffic demand and the idea of bridge design and the combination of box section beam and prestressed technology, the structure of box section girder trend to be thin wall and long cantilever. The prestressed box girder has become a main trend in bridge construction. This paper was supported by the National Natural Science Foundation-“Shear Lag behavior of Pc box girders under prestressed effect”. For the prestressed concrete box girder, through theoretical research combined with numerical simulation, systemly to studied the key issues in theory analysis of shear lag effect when the box girder under prestressing effect. The main research contents and results are as follows:(1) In order to answer the axial force was caused shear lag effect, respectively for the single room box girder without cantilever plate and include cantilever plate, used shell finite element analyzed the longitudinal stress when axial force was dispersed in different parts of the box girder, introduction of the concept of the axial force of the shear lag, studied the axial force of shear lag of simply supported, cantilever and continuous beams under different conditions, the the results showed that the beam generally wouldn’t produce shear lag effect when the axial force in box girder, only near the site of the axial force acting point existed local stress concentration area, the area was about 1 time the length of the beam width, its value was equal in the length of stresses non-uniform transmitted when generally components to withstand the partial pressure.(2) The box girder’s prestressed steel on the beam end is usually eccentric anchorage but not anchored in centroid axis position, the equivalent prestress including axial pressure of role in the centroidal axis and concentrated bending moments at the beam end. In this paper, separately calculated the prestress equivalent load of axial force and bending moment when the twin-cell box girder’s prestress wire eccentric anchoraged. When calculate the shear lag effect of box girder under centralized bending moment, proposed under the effect of concentrated bending moments can reflect box girder’s shear lag warping displacement mode was different between the wing plate, established the control differential equation of shear lag effect on the box under the action of concentrated bending moment and got the analytical solution. Systematic studied the distribution regularity of the shear lag effect of box girder when the concentrated bending moments at different position. Compared with the analytical solution and ANSYS finite element numerical solution, the results of basic rule and numerical analysis results were basically in the same, instructioned that this paper’s warping displacement function was reasonable, the control differential equation and analytical solution formula was correct and the calculation accuracy was well.(3) Focus on the simply supported twin-cell box girder only under the effect of concentrated bending moments,analyzed the shear lag effect. In parts of the concentrated bending moments left and right sides of the l/4 scope was bigger, the shear lag coefficient in concentrated bending moments to obtain the maximum value, when bending moment only effect on across at the l/4 site, near the part of bending moments position at the junction of roof and web considering the shear lag effect of stresses reached 2.24 times of the primary beam’s theoretical value, while the rest part was small. The shear lag coefficients were differences between webs, so it is necessary to consider the differences of wing plate stress between different webs in structure design.(4) Focus on the prestessed steel eccentric anchorage, when the horizontal arrangement of steel beam in different ways, researched the influence of box girder’s shear lag effect under different shear lag load effect models, used ANSYS finite element software established the simply supported beam and cantilever beam and continuous beams’ prestress wires respectively in three web average and on both sides of web and only middle web, got the shear lag effect of typical cross section and along the span direction. The results showed that different eccentric beam had different distribution of shear lag effect and the differences was relatively large. Combination of three kinds of shear lag coefficient distribution at typical cross-section, analyzed the top and bottom plates’ shear lag coefficient, when the middle web distribution, the cross-section of the roof and floor’s shear lag coefficient changes greatly, roof’s shear lag coefficient near to web was 1.22 ranged to the brink of flange near’s 0.531, the shear lag effect was obvious. In the case of three wed uniform distribution, the shear lag effect of cross-section was relatively small differences. At the same time, the different beam arrangement on the box beam shear lag effect under different boundary conditions were different, the biggest influence on continuous beam, then simply supported beam, cantilever beam impact was smaller.(5) Focus on statically indeterminate prestressed box girder structure, analyzed the prestressed internal force impact on box girders’ shear lag effect in theory. In theory, the values of additional bending moment caused by shear lag effect on prestressed anchor end part and prestressed initial bending moment value with the same sign, and the numerical was only related to the geometric characteristics of box girder’s cross-section. In the middle supported cross-section’s additional bending moment was related to box girder’s cross-section geometric parameters and the ratio of wide and span. Combined with relevant examples of the finite element analysis results, the theory value was conformed to the the results of numerical analysis.(6) Focus on the actual prestressed continuous beam’s line layout design was generally non consistent way in beam mode. These fabrics beam mode will produce secondary internal force in the beams, combined with the two span continuous box girder of prestressed concrete beams in different shape steel layout, used the ANSYS finite element software to established the shell model, researched the shear lag effect of box girder while straight line fabrics bunch, polyline fabrics bunch and parabola fabrics bunch. Fabrics bunch linear change should have a certain effect on the support section’s shear lag effect, when straight line and parabolic fabrics bunch, the difference of shear lag coefficient near the edge of web at middle supported section was reached 23.68%, the left cross section near the edge of the web was reached 33.45%, distribution of shear lag effect between the web and the roof along the span direction were basically the same, but the shear lag coefficient at middle supported section near the edge of the web was reached 23.68%. From the overall perspective, the shear lag coefficient variation range was basicly near the L/4 parts of support position, the influence in the other distant regions was very small.