Structural Modeling for Preliminary Design of Steel-Concrete Composite Bridges

Abstract

The preliminary design of steel-concrete composite bridges must be completed in a limited time frame, often representing a small fraction of the time allocated to the final design. This requires fast and accurate methods for estimating load effects, member di mensions, and material quantities. At the same time, preliminary estimates must remain consistent with the final design while being competitive in bidding. This study evaluates modeling approaches for determining load effects using Brigade/ Plus and investigates methods for preliminary cross-section design. A literature study was conducted to identify relevant modeling approaches, followed by a case study of a steel-concrete composite bridge in Akalla. Three main modeling approaches were examined: a shell model, a grillage model, and three-beam-element-based models. The approaches were assessed with respect to accuracy, computational efficiency, and suit ability for preliminary design. The results show that a transverse beam model with vertical springs in combination with a longitudinal beam model is the most suitable modeling approach for preliminary design. Although conservative, the model provides a favorable balance between accu racy, simplicity, and computational efficiency. The study further demonstrates that the model could be improved by considering the spread of the load through the concrete slab using distribution angles. By updating the model by incorporating the resulting effective widths, material consumption was reduced by up to 17% compared to the original model. Furthermore, adjusting the slab strip width proved more effective than modifying the spring stiffness alone. The greatest improvements were observed in cases where fatigue governed the design. The proposed modeling approach provides a practical workflow for preliminary design, enabling more accurate estimations of material quantities during the limited time frame. Keywords: Steel-concrete composite bridges, preliminary design, load distribution fac tors, transverse beam model, structural modeling