Abstract
This paper applies structural optimization to design an effective frame layout and component sizes of bus superstructure under strength requirements of bending, torsion, and natural frequency of the bus body with considerations of rollover safety according to European regulation, ECE-R66. The bus configuration is first determined by iterative topology optimizations for 4 geometric design variables, i.e., pillar, side, roof and floor structures, via finite element analysis. The objective of the optimization problem is to minimize the bus body weight subjected to the predefined upper bound displacement constraints of structural parts under each loading condition. The obtained bus frame design from topology optimization meets the required stiffness while its structural weight is 2.5% reduced from that of the baseline model. Size optimization is employed to adapt thicknesses of the bus structure so as to account for bus crashworthiness under dynamic rollover. To accelerate the optimization process, all constraints are applied based on equivalent static linear analyses with an adjustment accounting for the effects from material and geometric nonlinearity under rollover condition. The final design of bus structure possesses all strength requirements including rollover safety while the weight of bus structure is minimized. Diagonal members of roof and floor structures are proved to significantly affect bending and torsion stiffness with relatively small weight penalties while redistribution of structural mass provides bus design with improved energy absorption performance under bus rollover.
