Study On Flexural Properties Of Super-wide Separated Concrete Side-boxes Girder

At present, the cracking phenomenon of box girder was very common on span prestressed concrete box beam bridge; it had seriously affected the structural safety and durability. The thermal effects, prestress losses, and shear lag effect were important reasons for this cracking. According to the project"Flexural Properties and Reasonable Design of Separated Concrete Side-boxes Girder"(subitem of"On the Key Technologies of Main Bridge of Jing-yue Changjiang Highway Bridge"), which was sponsored by Ministry of Communications, this paper did a research on flexural properties of superwide separated concrete edge-box girder,three main aspects are as follows:1.The thermal effectsThe hydration heat of the concrete side-boxes girder in the center guide nozzle was tested, based on our experiments the heat of concrete hydration temperature of the studied beam, reached the maximum values 70.2℃, and the maximum temperature difference of interal and external was 39.3℃.So this paper propose relevant measures to be taken to avoid the early temperature cracks of following beams.The temperature gradient model was proposed,which suits for the climate characteristics of the bridge's site. Load effects of this temperature on the girder were calculated and agreed well with measured values, it showed that the nonlinear temperature gradient model was reasonable. On the analysis of the case, the status was construction or operational, the concrete girder, with the positive temperature gradient, the upper edge of the transverse tensile stress was 1.16MPa, and with the negative temperature gradient, resulting in tensile stress on the upper edge of 2.38MPa, lower edge of the tensile stress was 1.14MPa.So design and construction should be fully taken into account.2. The prestress lossesA detailed experimental research plan, which encompassed the field measurement and associated analysis, was developed for evaluating the factors on the prestress losses in this concrete box girder bridges. The results showed that: the total loss tested of one side tensed of the horizontal tendons is about 21.0% of the controlled stress, when the mass loss of anchoring 65% of the total loss, and the loss friction caused was the largest proportion. Prestressed vertical tendons total loss tested is about 23.0% of the controlled stress when the mass loss of anchoring 76.8% of the total loss.3. The shear lag effectSpace finite element model was established to analyze the shear lag effect of the side-box girder about the operational state of the bridge. Compared with the measured values, the model is feasible. Near the tower and the box girder's shear lag effect was more obvious, leaving this place the effect decreases rapidly. The shear lag acted as positive in both cross-section's upper and lower of the girder in axial compression, and the shear lag coefficient of the upper edgeλ= 1.04 ~ 1.13, and the lower edgeλ= 1.03 ~ 1.10. In the state of bending force , the upper edge of the girder acted as positive shear lag,λ= 1.04 ~ 1.09; the lower edge should divided into two parts, the site of inclined web acted as negative shear lag,λ= 0.76 ~ 0.89 ,and the site of vertical web acted as the shear lag is positive,λ= 1.26 ~ 1.65. The levels of shear lag reduced with the increase of stress level.