Abstract: A method to optimize the topology of grillage structures under multiple load cases with stress constraints is presented. The optimal structures are grillage-like continua/stiffened-plates containing infinite number of beams/ribs of infinitesimal spacing. The fiber-reinforced orthotropic composite is employed as the material model to simulate the constitutive relation of the grillage-like continua/stiffened-raft. The beams/ribs densities and orientations at the nodes are taken as design variables. The material distribution for each single load case is optimized by a fully-stressed criterion based on finite element analysis. The directional stiffness of the optimal structures in multiple load cases approach to the biggest of the stiffness defined for each single load case. The optimal continuous material distributive field is achieved after several iterations. Three examples are presented to demonstrate the effectiveness of the proposed approach. The beams/ribs densities and orientations distribution at the nodes are given finally.