Design bending moment in concrete box structures : a comparative analysis between shell and soild element models

Abstract

Concrete box structures are mainly used in railway and highway projects as culverts or frame bridges. Structural engineers prefer to do finite element method (FEM) analysis using shell or frame element models to obtain bending moment diagram (BMD) for design. Structures are modelled center to center supports in shell element models. BMD of general shell element models are continuous throughout the center to center spans and maximum support moment occurs at center of supports. ACI design practice recommends selecting bending moment at face of the support as design value, but BS design practice is different and obtains bending moment value at center of the support for designs. Some literature suggests tedious bending moment correction according to the stiffness of the members of the joint. In general shell element modeling, inside rigidity of supports is not considered. However general shell element models can be modified at support region to represent the rigidity of the support area. Previously tested concrete box structure was modelled using general shell, modified shell and solid elements. Results of solid element model are much closer to experimental results at supports and spans than other models. This result validated that solid element of box culvert can be used as a base for comparison of general and modified shell models. In this study, BMD of general shell, modified shell and solid element models relevant to concrete box structures were compared to load combinations relevant to Sri Lankan Railways. The results show that BMD of solid and modified shell elements are much more similar than the general shell models. Support design bending moment can be obtained from modified shell models with reliability without confusion of center or face value to select for the design as for the general shell element models.