Study On Crack Control Measurements For The Cross Diaphragms Of Long-span PC-box Girder Bridge

The diaphragm is an important component of the large-span PC-box girderbridge. It serves as a connecting link between the support and super structure. Itperformance good for passing various force from up to down, and also playsimportant role in maintaining the bridge’s structural behavior.To ensure the integrity of the diaphragm is good for making full use of its ownfunction, but it faces danger of cracking under kinds of loads from being poured tothe whole service stage. Many engineering practice shows that the crackingphenomena of the diaphragm is common. The reasons to lead the diaphragm tocrack are complex, it causes trouble to our design as well as construction. So thisarticle’s center task is to seek the crack control measurement. With this purpose,this paper has done some research results including the following aspects:Calculates the non uniform temperature loads based on the heat of hydration,and the shrinkage stress based on the moisture diffusion. The analysis shows thattensile stress occurs on the concrete surface at temperature rising-stage, but into theinner face at temperature drop-stage. The latter one is much more of danger ofcracking. After dismantling formwork, the shrinkage stress is very obvious, theregion of tensile state tends to be deeper with the dry time increasing.Under the construct loads, the diaphragm’s boundary occurs displacement, thismakes the diaphragm occur displacement and undertake loads at the same time.There exists obvious difference between1thick diaphragm3chip diaphragms. Theformer also occurs bending deformation, thus making the diaphragm generatecrosswise stress and vertical stress on whole diaphragm; the latter occurs warpingdeformation, thus making the diaphragm’s surface generate partial tensile stress.This article gives suggestions on construction and design measurements basedon the analysis results. The stress concentration is obvious on the hole boundary,this paper gives the ordinary steel bar’s layout scheme for partial reinforcing thehole boundary. From the view of reducing displacement of diaphragm’s boundary,that adding the areas of the box’s section or adding ordinary steel bar’s ratio ofreinforcement can improves the whole stress distribution. This paper discusses theprestressed steel strand’s layout scheme; it shows that this works well in preventingconcrete cracks. It offers pressure stress and neutralizes tensile stress produced under construction loads. Both vertical and crosswise steel strand are prestressed atthe same time can this method works efficiently. It is not reasonable for ignoringthe vertical steel strand. This paper also compares the difference between curvesteel strand layout and linear steel strand layout. The result shows that the curvesteel strand layout is less effective than that liner ones. There occurs tensile stresson the top of curve steel strand’s boundary of the concrete. So it’s better use linersteel strand layout.