Spatial Behavior Analysis Of Erection Process Of U-shaped Beam For Rail Transit During Construction

A newcomer to the array of types of beam adopted in the construction of viaduct and potentially the mainstream solution to the urban transit system at home and aboard,the U-shaped beam boasts features such as high noise-reducing performance, low construction height, high usage of space and elegant shape. Problems with the traditional U-type beam such as complicated stress condition and fabrication and difficult construction has been limiting its widespread usage. As the technology improves, the construction has seen the transition from in-situ casting to precast fabrication coupled with lifting by twin cranes or the gantry. Yet low torsion strength of the beam due to its thin-walled and open section produces high demand in transportation passage and construction site. To tackle the problem, the new method of "loading the beam at designated position, transporting it on the beam with bridge machine in automation" was successfully adopted in an urban rail project in Shanghai in2011.The method is a successful endeavor in the field. Erecting the T-shaped beam and box beam is a common practice in the railway bridge construction. For the U-shaped beam erection, however, it is an innovation and development with high potential. A through structure with complicated three-directional stress, it hasn’t been maturely studied with sound theory. So far, some efforts have been made yet confined in the field of its plan stress condition research under operational load. Since its stress condition is totally different from under the load produced by the bridge machine to under operational load, the study in the regard and in the safety and usability of the method still has room for improvement.The paper adopts the program of Midas Civil to build the solid model of the U-shaped beam, dividing the dynamic operation of the bridge machine into multiple static sections so that the whole process can be simulated thoroughly, stress distribution situation studied and conducive instruction to the design and construction of the similar beams offered.It is found that:1、Under the working load of the bridge machine, the deformation of the U-shaped beam is mainly seen in the following pattern:in cross direction; tilting towards the inner of the upper flange girth, outward flexion in the bottom of the web; apparent coupling effect of bending, shear and torsion, especially in the end of the30M-span U-shaped beam.2、During the construction process, each characteristic cross-section emerges transverse stress, and The maximum value is2.94Mpa.The stress distribution of the beam is most affected in the middle support of the bridge machine, which shall be closed observed. The whole cross section of the passage slab is under stress in cross section and the stress of various characteristic sections is distributed in clearly varied pattern. The web of the beam shows, to certain degree, the built-in effect to the cross stress in the passage slab.3、The comparison between the plan member structure and space solid model proves the longitudinal normal stress in both conditions is basically same. Therefore, it is feasible to study the longitudinal design of the U-shaped beam with member structure model.4、The cross stress yielded by the support of the machine and load of the beam transporter is mostly local. The longitudinal stress in the passage slab shows clear lagging effect of the negative shear in the cross direction, which is slightly affected by the support of the bridge machine.5、The construction of the beam is a technology in the viaduct construction with high potential. it is recommended in the paper the weight of the bridge machine shall be within330ton for safety purpose.The results of the analysis results show that the U-beam transverse stress and shear lag effect of the rack transport process are visible,so that space entity model for detailed analysis is necessary and feasible.