Optimization of Steel Truss Girders in Pedestrian Bridges - Using Genetic Algorithm

Abstract

The optimization of any structure is an iterative process, where the Structural Engi neer evaluates how certain changes in one or more design variables affect the outcome of the design. With increased number of variables, the design space increases expo nentially and it is not feasible to remodel the structure each time and analyze these combinations to choose the best solutions. One way to overcome this problem is by parametric design, where the structure is programmed and governed by a number of parameters so that any change in these parameters would result in a change in this structure. Although this approach saves time when it comes to the modelingprocess, the designer still needs to run the analysis each time and document theresults to select the best set of parameters that fulfills the goals of the design. To overcome this limitation, the implementation of genetic algorithms allow for a fasterexploration in the design space to find the most optimal solution. In this thesis, the steel truss-girders used in single-span pedestrian bridges are stud ied. A parametric design model is developed to automatically produce these trusses based on a set of parameters that govern the topology of the truss. Furthermore, afinite element analysis is conducted and the necessary design according to Eurocode and the regulations of the Swedish Traffic Administration is done. Finally, a ge netic algorithm optimization tool is used to minimize the weight of the truss while fulfilling all the checks.The study indicates that truss toplogies with bent upper chord have lower weights and this weight difference is increasing with longer spans. Furthermore, it is seen that the diagonals and the upper chord stands for about 70% of the total weight ofthe truss and yet not fully utilized due to their contribution for the capacity against global buckling which is the most critical check in the design. In addition, usinghigher steel grade resulted in lighter trusses. However, this effect decreased with longer spans. Global buckling is more critical for trusses with higher steel grades,because the members are only available in cold-formed cross-sections.