دانلود رایگان مقاله اثر دیافراگم متناقض در دوران تیر اهن بیرونی در طول عرشه ساختمانی اورهنگ

Abstract

The construction of bridge deck overhangs results in unbalanced eccentric loads acting on exterior girders which can cause rotation and increased stresses not accounted for during design. Permanent diaphragms and temporary bracing in bridge exterior girder lines or panels are used to resist these loads and subsequent transverse rotation of the exterior girders. The addition of extra diaphragms in the exterior panels is one potential alternative to temporary bracing which is not always effective. In this paper, a unique steel plate girder bridge in the state of Illinois with extra diaphragms in one exterior bay was instrumented with tilt sensors and strain gages to monitor transverse rotations and strains due to unbalanced loads occurring during construction. Two types of rotations were recorded; maximum and residual rotations. The extra diaphragms were included in the design of this bridge on only one side of the bridge to carry utility lines. The full bridge was modeled using the commercial finite element analysis software ABAQUS and the model was validated using field data. As expected, diaphragm spacing was found to have a high impact on exterior girder rotations that occur during bridge deck construction. The maximum obtained finite element rotation was 0.47° which occurred at mid span and on the bridge deck side that does not have extra diaphragms. Field residual rotations were found higher (approximate 50% on average) than rotation determined from the finite element analysis. These extra stable rotations were seen in the exterior girders and was a result of permanent deformation occurring when the finishing screed passed by the section under consideration.

6. Conclusions

This research focuses on exterior girder rotations in a medium size plate girder bridge with asymmetric end diaphragms due to overhang construction loadings. The following conclusions can be drawn based on the monitored field data and finite element analysis results: Exterior girders at the west end experienced almost a rigid body rotation due to the closely spaced diaphragms but on the other hand the exterior girder at the east end (where fewer diaphragms exist) experienced differential rotation between the web and the bottom flange. Therefore it can be deduced that diaphragm spacing has a significant impact on the behavior of exterior girder rotations during bridge deck construction. i. At sections where continuous diaphragms exist, girder rotations were comparatively smaller with a rigid body rotation (difference in rotation between the flange and the web is not signifi- cant). ii. Although, there is no tilt sensor installed at section S1, the predicted maximum rotation from the finite element analysis was 0.47° which occurred at the mid span of the bridge at the east end. iii. Comparing east and west exterior girders, it was found that the presence of continuous intermediate diaphragms can make a significant change in rotational stiffness and behavior. iv. The maximum measured exterior girder rotation from the finite element analysis is 0.47° which occurred at the web location at d span of the bridge at the east end without a diaphragm and the calculated rotation exceeds the assigned limit rotation (θ). v. Field stable rotations are higher (approximate 50% on average) than rotation determined from the finite element study. These larger stable rotations were seen in the exterior girders as a result of permanent deformation occurring when the finishing screed passed by the section under consideration. vi. Based upon field and finite element analysis, it was found that the additional intermediate diaphragms (at west end) have tremendous ability to prevent exterior girder rotation during overhang deck construction. vii. It was observed in the field that proper tightening of the long and continuous tie bars is difficult because of their interference with other deck reinforcement, and the timber blocking was also frequently improperly shimmed. Improper installation of these elements greatly reduces or eliminates their effectiveness. viii. It was found from the FE parametric studies that fully engaged tie bars and timber blocks work effectively to prevent/decrease exterior girder rotation. In order to consider the feasibility issue for tie bars and timber blocks, further study is needed.