Cracking Assessment of Concrete Slab Frame Bridges Exposed to Thermally Induced Restraint Forces
The usage of linear 3D FEA is widespread within the bridge design community, and although this tool provides substantial beneﬁts in the design process, there are certain practical issues related to the application of this analysis tool. A situation in which such an issue prevails is when linear 3D FEA is used to analyze restraint forces due to thermal shrinkage or expansion in concrete slab frame bridges. Eﬀects related to restraint forces in concrete are diﬃcult to model and predict as these forces diﬀer signiﬁcantly in nature from external loads, and dealing with them in practical design situations is complicated.
In this thesis, cracking due to restraint forces in concrete slab frame bridges was investigated using 3D non-linear FE-analyses in the software package ATENA 3D. Using volumetric ﬁnite elements, attempts were made to realistically capture the load response and cracking behavior of concrete slab frame bridges subjected to restraint forces induced by temperature diﬀerences among members.
The initial parts of this thesis aims to ﬁnd appropriate modelling techniques and material models for the prediction of cracks due to restraint forces in base restrained walls using a previously reported experimental research project as reference. Comparative simulations were performed, using crack widths and crack patterns as comparate. Overall good correspondence was obtained with an exception of deviation in cracks formed at locations near the restrained corners where crack widths were overestimated in the simulations. The technique used to model the restrained boundary proved to be highly inﬂuential in the context of obtaining realistic results. Subsequent to the comparative study, a parametric study was performed where the correlation between crack widths and selected attributes was investigated. The parameter which exhibited the most distinct inﬂuential eﬀect on the results was the length-to-height (L/H) ratio of the wall.
The applicability of a crack control approach intended for crack width estimation in liquid retaining and containment structures made of concrete exposed to thermally induced restraint forces, given in EN 1992-3, was then evaluated for use in design of concrete frame bridges. This approach proved to return conservative results for walls with low L/H-ratios when compared to results produced in the numerical simulations.
Finally, a NLFE (non-linear ﬁnite element) model of a concrete slab frame bridge designed by the consultancy company Tyréns AB was composed and tested. Diﬀerent procedures of applying temperature diﬀerences between front wall and bridge deck were evaluated. This study indicated that the width of cracks induced by lateral restraint forces decreased when temperature was modelled applying a discrete thermal gradient to members in contact with back ﬁlling material compared to using a uniform temperature in these members when temperature diﬀerences between bridge deck and wall was simulated. The interacting eﬀects of permanent external load eﬀects and restraint forces were also investigated. Crack widths from NLFE simulations were then compared with corresponding results calculated using linear FEA results as input for design equations given in EN 1992-1-1. The latter resulted in estimated cracks more than 10 times wider than that obtained in the simulations, while crack widths predicted using the EN 1992-3 approach showed better correspondence to the NLFE results.
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