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Beitrag zur rechnerischen Überprüfung von Betonhohlkastenbrücken
Citation Link: https://doi.org/10.15480/882.1202
Other Titles
Contribution to computational assessment of concrete hollow box girder bridges
Publikationstyp
Doctoral Thesis
Publikationsdatum
2014
Sprache
German
Author
Advisor
Title Granting Institution
Technische Universität Hamburg
Place of Title Granting Institution
Hamburg
Examination Date
2014-04-25
Institut
The structural assessment of existing bridges is gaining increasing importance within civil engineering, as many reinforced concrete bridges that were built 40 to 60 years ago show deficiencies regarding serviceability as well as load-bearing capacity. These deficiencies are due to poor detailing, increasing traffic loads as well as structural damage caused by vehicles, environmental influences and material fatigue. To ensure that the bridges remain viable, it is necessary to carry out structural assessments using state-of-the-art methods and to estimate the structural performance under current and future traffic loads. When the structural capacity of these bridges is assessed using widely used, simple static models, the bridges often do not meet the requirements of the current design codes. This is due to the fact that the design actions determined from simple models are greater than the capacities calculated according to current design codes.
The shear capacities of concrete box girder bridges in particular are often insufficient, because previous design codes stipulated lower design loads and allowed lower shear reinforcement ratios in the webs. As current design codes demand larger shear resistances for webs and deck slabs, elaborate and expensive strengthening measures and even partial or complete bridge renewals can become necessary.
Using more detailed analyses of the design loads and the structural behaviour of the bridges, more realistic design actions and structural resistances can be obtained. It is hence possible to gain a more complete understanding of the flow of forces and the structural resistances of the bridges. Therefore, this dissertation is concerned with determining the flow of forces in single- and multi-cell concrete box girder bridges. The goal is to show that the existing bridges meet the requirements stipulated in the current design codes, so that costly rehabilitation measures can be avoided.
The focus of this dissertation is on determining the shear forces in the critical sections: in the webs near the supports as well as in the areas of the deck slab close to the webs. Experience shows that existing bridges generally have sufficient bending capacities, as they usually contain a sufficient amount of flexural reinforcement and are highly prestressed.
The shear forces are determined according to the traffic load model described in DIN Report 101 [35], using spatial finite element calculations with shell elements and linear-elastic material laws. The comparison of these FE results with the results obtained from a two-dimensional analysis of framed structures shows distinct differences between the two methods. The analysis of framed structures yields non-conservative results for multi-cell sections, as the shear force is assumed to be evenly distributed among the webs as the bridge bends. The 3D FE calculations, however, indicate that the shear forces flow mainly through the webs closest to the load.
Subsequently, the shear forces are determined from spatial FE models with non-linear material laws for reinforced concrete and compared with the results from the previous, linear-elastic analyses. Compared with the linear-elastic approach, the non-linear calculations yield lower shear forces in the critical web sections near the central support and higher shear forces in the critical sections near the end supports. An explanation for the different results obtained from the non-linear calculations is that the areas of the deck slab close to the webs aid in resisting the shear forces in the direction of the bridge axis. On the other hand, the load paths within the bridges change significantly, causing a decrease in the inclination of the compression fields and a hence a reduction of stress in the webs. Further investigations show that due to the redistribution of the longitudinal shear forces from the webs to the slabs the existing shear resistance in the slabs is not exceeded.
The effective width of the deck slab overhang for point loads is also determined in this document. It is shown that the effective width is dependent on the slab geometry as well as on the location of the point load on the deck slab overhangs. The calculated effective widths are smaller for sections close to the diaphragms than elsewhere in the span. Diagrams based on these calculations are presented; they can be used to determine the effective slab widths for sections close to the diaphragms.
The shear capacities of concrete box girder bridges in particular are often insufficient, because previous design codes stipulated lower design loads and allowed lower shear reinforcement ratios in the webs. As current design codes demand larger shear resistances for webs and deck slabs, elaborate and expensive strengthening measures and even partial or complete bridge renewals can become necessary.
Using more detailed analyses of the design loads and the structural behaviour of the bridges, more realistic design actions and structural resistances can be obtained. It is hence possible to gain a more complete understanding of the flow of forces and the structural resistances of the bridges. Therefore, this dissertation is concerned with determining the flow of forces in single- and multi-cell concrete box girder bridges. The goal is to show that the existing bridges meet the requirements stipulated in the current design codes, so that costly rehabilitation measures can be avoided.
The focus of this dissertation is on determining the shear forces in the critical sections: in the webs near the supports as well as in the areas of the deck slab close to the webs. Experience shows that existing bridges generally have sufficient bending capacities, as they usually contain a sufficient amount of flexural reinforcement and are highly prestressed.
The shear forces are determined according to the traffic load model described in DIN Report 101 [35], using spatial finite element calculations with shell elements and linear-elastic material laws. The comparison of these FE results with the results obtained from a two-dimensional analysis of framed structures shows distinct differences between the two methods. The analysis of framed structures yields non-conservative results for multi-cell sections, as the shear force is assumed to be evenly distributed among the webs as the bridge bends. The 3D FE calculations, however, indicate that the shear forces flow mainly through the webs closest to the load.
Subsequently, the shear forces are determined from spatial FE models with non-linear material laws for reinforced concrete and compared with the results from the previous, linear-elastic analyses. Compared with the linear-elastic approach, the non-linear calculations yield lower shear forces in the critical web sections near the central support and higher shear forces in the critical sections near the end supports. An explanation for the different results obtained from the non-linear calculations is that the areas of the deck slab close to the webs aid in resisting the shear forces in the direction of the bridge axis. On the other hand, the load paths within the bridges change significantly, causing a decrease in the inclination of the compression fields and a hence a reduction of stress in the webs. Further investigations show that due to the redistribution of the longitudinal shear forces from the webs to the slabs the existing shear resistance in the slabs is not exceeded.
The effective width of the deck slab overhang for point loads is also determined in this document. It is shown that the effective width is dependent on the slab geometry as well as on the location of the point load on the deck slab overhangs. The calculated effective widths are smaller for sections close to the diaphragms than elsewhere in the span. Diagrams based on these calculations are presented; they can be used to determine the effective slab widths for sections close to the diaphragms.
DDC Class
720: Architektur
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