Research On Design Parameters Of Bursting Crack In Bottom Slab Of PC Box-Girder Bridge

In modern road network traffic engineering, long-span prestressed concrete box-girder bridge has agreat competitive superiority in bridge type selection for its numerous advantages in spanning major riversand deep ravine. Meanwhile there are more and more reports on crack diseases in this type of bridge duringconstruction and operation, and bursting crack in bottom slab is one of the diseases. The occurrence ofbursting crack is fatal for bridge construction quality; sometimes the bridge even needs to be demolishedand reconstructed. The main reason of bursting cracks in bottom slab is that the actual value of prestressingequivalent radical force is slightly larger than the code value, causing the anti-collapse design parameterunreasonable. Due to research lack on this subject, the anti-collapse design and construction of a lot ofbox-girder bridges are based on practical experience of engineering projects. The vague design andconstruction guiding principles have led to engineering accidents happening in different areas of thecountry. To explore more reasonable prevention measures, this paper analyzes the calculation formula forequivalent radical force in box-girder bridges and studies the mechanism of bursting cracks in bottom slabon the basis of referring to a lot of literatures and previous research results. Then the formula forcalculating main anti-collapse design parameter is proposed, and some basic principles of anti-collapsedesign and construction are inducted and summarized.Firstly, a systematic study of equivalent radical force of prestressing tendon in tapered box girder iscarried out. Classic formula for calculating equivalent radical force is derived by force vectors systemequilibrium method. Influences of parabola exponent and exponential term coefficient of taperedbox-girder bridge to the formula are analyzed, and ways of controlling equivalent radical forces byadjusting the two factors are studied. Aiming at the situation that broken line is used to replace parabolicbottom slab of tapered box-girder bridge and that there are inevitable bellows positioning error, influencecoefficients of positioning error and the parabolic fitting with broken line are deducted to modify the classiccalculation formula of equivalent radical force.Secondly, the paper proposes the formula for calculating protective layer thickness of anti-collapsesteel reinforcements and bellows on the background of Huang Jia Long Grand Bridge. By ANSYS spatialfinite element modeling, the influence of bottom slab thickness to stress is analyzed and the formula forcalculating transverse bending moment is modified.Lastly, aiming at the issue of bursting cracks in a bridge in Yiyang, Hunan, the paper analyzes thereason of bursting cracks in this bridge and verifies that the formulas for calculating equivalent radicalforce and anti-collapse design parameters proposed in the paper are appropriate. And optimization analysisis carried out on the bridge. By referring to design drawings of domestic box-girder bridges andengineering experience, some basic principles of anti-collapse design and construction of box-girderbottom slab are proposed as a reference to future anti-collapse design of box-girder bottom slab.