Study On Stress Mechanism Of Box-girder Web Under The Action Of Vertical Prestressing

In this paper，the current web cracking situation of Prestressed Concrete (PC) box-girderbridge is reviewed, the characteristics and the causes of web cracks are analyzed, the researchprogress of the field is described, and the deficiency of present research is pointed out.Emphasis is put on analyzing complex mechanical characteristics of PC box girder webs anddiscussing the influence of box girder shear capacity under vertical prestress. Based on testingand analyzing vertical prestress loss, putting forward specific measures to overcome theprestress loss, studying redistribution of web stress under concrete shrinkage and creep effect,works are completed detailedly as following5aspects:(1) The web stress state under vertical prestress is analyzed, the influence mechanism ofweb tensile stress and shear stress under vertical prestress is confirmed, and the method ofdetermining vertical prestress value is sought. The diffusion effect of web stress under verticalprestress is analyzed, the stress diffusion angle and vertical prestressed reinforcement spacingare reasonably calculated. As for the principle of setting vertical prestressed reinforcementhorizontally, method that considering transverse framework effect is taken to calculatetransverse offset distance of vertical prestressed reinforcement. Analyzing the sensitivity ofvertical prestress and bending prestressed tendon of web respectively, the influence ofprestressed effect on web stress state is determined.(2) By analyzing PC box girder web cracks with nonlinear method, describing nonlinearcharacteristics of concrete and steel in finite element model, simulating mechanicalcharacteristics and stress-strain matrix after concrete cracking, nonlinear finite elementsolution computation of PC box-girder web cracking is carried on, and combined withnumerical example, the whole process from stressing to destruction is simulated nonlinearlywith numerical analysis method.(3) The influence factors of PC box girder shear performance are analyzed, shearperformance of PC box-gider with vertical prestresed reinforcement is studyed, comparativeanalysis of vertical prestress differences is done when calculating box-girder shear capacitywith old and new standard respectively, and the influence of different degree vertical prestresson PC box-girder failure pattern and the stress (strain) of tendons, stirrup, longitudinal reinforcement is analyzed.(4) According to the object bridge, testing of vertical prestress loss of PC box girder webis carried out, vertical prestress loss is studied detailedly from such aspects as instant prestressloss when anchoring, time-dependent prestress loss after anchoring and prestress loss causedby other factors, and a regression analysis of time-dependent prestress loss after anchoring isdone, prediction equations are established, according to the characteristics and constructiontechnology of vertical prestressed reinforcements, the specific measures to overcome prestressloss are put forward.(5) By the systematic observation of bridge construction process and structural stress(strain), deflection after the bridge is completed a long time, exact reaction of PC box-girderbridge in natural environment is analyzed under the action of concrete shrinkage and creep,the real data and the calculated value of commonly used concrete shrinkage and creep modelare compared and analyzed, the web stress redistribution of box-girder bridge is analyzedunder the influence of long-term load, concrete shrinkage and creep effect.