Abstract: A method for making grid-stiffened structures as provided. The method includes providing a base tool having a plurality of grooves and an expansion tool having a plurality of compaction grooves therein for fitting into a plurality of the base tool grooves so as to define compaction grooves therein. A plurality of prepreg tows are then wound into the compaction grooves. A skin is then placed over the so-wound tows, the expansion tool and the base tool to define an assembly. The assembly is then bagged and autoclaved so that the expansion tool expands to provide lateral pressure to compact the prepreg tows in the compaction grooves to shape same into ribs to form a rib-stiffened structure. An analytical thermal model is developed to predict the behavior of the expansion tool.