CAE Based Multi-Level Optimization of a Bus Body Constituted of Aluminum Extrusions

A new body structure of low-floor city bus is proposed in terms of optimization in the multiple levels of the structural system. In the framework of multi-level optimization, a bus body system is viewed as an integration of sub-systems like roof, side structure and front/rear body modules, and each of the sub-systems is seen as an assembly of components. Once the system is decomposed into the lower level sub-systems, the design target is cascaded for the lower level. That is, the design target of the bus body system is cascaded into the design targets of the sub-systems, and the target cascading procedure eventually gives the design targets in the component level. Analytical evaluation and prediction of the target values of each level is provided by computations based on computer aided engineering (CAE) so that an optimized design can be obtained through a number of iterations. Since the mechanical performance to meet the design requirements of the bus body is highly dependent upon clever joints and cautiously engineered extrusion profiles, the proposed design process for the bus body with a new concept is focusing on creating a mechanical configuration of non-welded joints and the section shapes of the profiles. In particular, a shape optimization is performed to obtain the maximum strength capacity of a cant rail section for a given geometric constraint under the extreme load case of bus rollover, and the tool of design for six sigma (DFSS) is applied to determine the design variables of the non-welded joints so that the target stiffness can be reached, in the component level.