What Is a Box Girder Bridge?

Box-section beam bridge refers to a beam bridge whose main beam is a thin-walled closed section. Usually a long hollow truss made of steel or concrete is used as the beam. This makes the bridge light and strong. The bridge constructed by this method is called box girder bridge.

In 1950, the world's first box girder bridge with segmented overcast construction and post-tensioned prestressing was completed. The completion of the Balduisntein bridge with a main span of 62m in Germany marked the entry of a prestressed concrete box girder bridge into a large-span period. Prestressed concrete box girder bridges have been widely used all over the world for their good structural performance and beautiful appearance. At present, the main span has reached 301m. However, in recent years, with the increase in span, the problem of the deflection of the main beam has become increasingly prominent, which has seriously affected the continued development of this bridge type. The characteristics of its main beam deflection are as follows:

(1) The long-term growth rate of deflection may accelerate, decrease or maintain the trend of average speed change over time;

(2) The long-term deflection of the structure is much larger than the predicted value of the design calculation. The deflection of some typical long-span prestressed concrete box girder bridges in the world not only shows that the problem of deflection of the main beam is widespread at home and abroad, but also shows from one side that the long-term deflection of long-span prestressed concrete box girder is indeed true. It is because there are defects in the system. The specific factors such as the differences in materials and environment and the differences in construction quality caused by different regions are not the inevitable reasons for downswing.

Analysis of the main reasons:

(1) Shrinkage and creep of concrete (including the effects of shrinkage differences caused by different thicknesses of box girder cross-section members, repeated load effects caused by traffic loads and temperature changes, the effects of construction joints, changes in ambient temperature and humidity, etc.);

(2) Low estimate of long-term prestress loss;

(3) Cracking of concrete;

(4) The unfavorable bridge stress state caused by the construction method (especially the closing method). ^[2]

In terms of measures to deal with the long-term deflection of long-span prestressed concrete box girder bridges, many specific prevention and control methods have indeed emerged at home and abroad, such as external beams, mid-span thrusts, mid-span pre-compression, and main beams across mid-span sections Use high-strength lightweight concrete, steel boxes, etc. However, due to the long-term deflection of long-span prestressed concrete box girder bridges, there are still a lot of uncertainties, especially the lack of support for robust long-term deflection prediction calculation methods, and the accuracy of control measures is difficult to grasp. Many bridges have taken measures Shortly after that, they continued to deflect, and even the unfavorable conditions such as damage to the beam caused by excessive internal forces eventually had to be dismantled, causing great social and economic losses. ^[2]

(1) The design of long-span prestressed concrete box girder bridges should be further improved. The calculation of spatial effects and long-term deformation should be solved. The structural layout should be strengthened to control the stiffness of the structure, especially the prestress. The research of reasonable arrangement consciously sets up anti-deflection bundles which are beneficial to the long-term deflection of the structure.

(2) In terms of materials, the long-term influence of traffic, temperature and other alternating loads on the effective prestress of prestressed beams, concrete elastic form, and creep of concrete should be clarified. Appropriate shrinkage and creep model, and consider the correction of long-term shrinkage and creep by influencing factors such as environmental changes and box beam size effects.

(3) Study the mechanism and consideration of the influence of the strong coupling effect of cracking, prestressing effect and concrete shrinkage and creep on the long-term deflection of the box girder concrete under alternating load.

(4) Standardize the design, construction, and treatment techniques for large-span prestressed box girder, and formulate corresponding specifications or technical guidelines. ^[2]