The Theoretical And Experimental Research On Shear Lag Effect Of Box Girder Under The Pressing Force

Modern PC bridge is a symbol of the technology of PC. Due to its advantages, PC box girder bridge, is widely used, has made great achievement in the field of transportation construction in China.Our study was funded by the National Natural Science Foundation一Study on the shear lag effect of PC box girder with considering prestressed effect(51,208,242).Aim to PC box girder particularly, through theoretical and experimental research model we investigated the effect of prestressing on the distribution of box girder shear lag. The main contents and results are as follows:(1) A shear lag warping displacement function is derived. The prestressed beam of PC box girder’ eccentric anchor on the ends of box grider could produces a longitudinal axial force and a central bending moment on the beam. According to the theory, our study proposes a longitudinal warp position function, reflecting the shear deformation’s difference PC box girder’s flanges under a central bending moment to establish a differential equation concerning a shear lag effect of PC box girder, combined typical boundary condition, get a stress answer. Based on the theoretical analysis, we studied the longitudinal prestressing force’s distribution of box girder and verified Analytical solutions. It is in a good agreement with finite element simulation, which means a proper longitudinal warp position function was selected and related formula is feasible with its high accuracy.(2) The mechanical mechanism of the axial force without shear flow is analyzed.The form of prestressed beam varies depending on the stress distribution including curved tendon, curve strand and curve straight. The first two would produce shear flow in the upper and lower flanges, and then the shear lag effect, less complicated than the last form does. Axial force of curve straight witnesses local stress concentration in the vicinity of action point during transmission into the beam.After some distance, prestressed force will be in the state of plane stress without shear flow. So the axial force does not produce shear lag effect.Bending moment of eccentric anchorage is equivalent to the concentration of bending moment on beam, which could cause an asymmetrical distribution of prestressed force in a limited cross-sectional range of the beam end.(3) Article simulated reduced scale plexiglass box girder experiment. To get the distribution of internal stress of PC box girder,we carried a Plexiglas-simulated box beam research. The internal stress of PC box girder varies depending on the action point of the axial force. From beam end to Le, an obvious shear lag effect(Le is approximately equal to the transverse distance between two webs). Closer to the end of beam, more obvious the effect of shear lag. From Le to span, shear lag effect does not exist: effective value of longitudinal stress accords with the solution of primary beam stress.(4) The experiment of single box single room box girder under the action of pre stress was carried out.When the single box single room box girder is subjected to the action of the eccentric symmetrical axial force. The shear lag effect mainly occurs in the range of thebridge width at the two ends of the beam. The longitudinal stress of the cross section is more violent. In the width of the bridge, the stress value is close to the stress value of the elementary beam theory. The L/4 cross section has weak shear lag effect.The shear lag effect is not produced in the span. When the prestressed reinforcement is placed in the roof, the shear lag effect is obvious, and the shear lag effect is weaker than that of the top plate. When the prestressed reinforcement is placed in the bottom plate, the shear lag effect is significantly more than that of the top plate. The shear lag effect of box girder is obviously different when the position of prestressed beam is changed, the difference is large, and there is a negative shear lag phenomenon, the stress peak position of each section is also changed.(5) The experiment of single box double room box girder under the action of pre stress was carried out.The structure of Single box double room box girder is more complicated than the single box girder structure, the box girder mechanics space effect is more obvious. Mechanical behavior has new characteristics. The range of shear lag effect of double chamber box girder is wider than that of single room. L/8 cross section has shear lag effect. L/4 cross section shear lag effect is still exist. There is no shear lag effect in the span of the bridge. When the steel bar is in the top of the roof, the shear lag effect of the roof is obvious. The stress on the bottom plate is not even in the top of the roof. When the bar at the bottom, the shear lag effect of bottom effect is obviously than on the roof.When the cloth beam position is changed, the shear lag effect of box girder section changed. Overall, the stress of the bridge is basically unchanged. The shear lag coefficient of 1/4 cross section changes with the change of the position of the point of action. L/8 cross section of shear lag coefficient changes with the position change more than L/4 across more intense. And the peak stress of lateral position with cloth beam position.(6) The effect of the concentration of the prestressed beam on the shear lag effect of box girder is analyzed. When the steel beam is four parts symmetrically arranged in the beam, Regardless of the top or bottom, the effect of shear lag effect are weaker than two portions of tendons, the distribution of the stress of each section is more uniform. As the shear lag effect is severe L/8 section as an example,the shear lag coefficient is 1.34, reduced by 25%.