The diagram below shows the ridged cross-sections and blade profiles of the bronze dao and the two steel daos exhibited on this webpage. It is obvious that the cross-section of this Han Dynasty bronze dao was influenced and derived from that of a typical Warring States Period bronze jian (as shown).

I speculate (based on the excavated steel daos shown below) that ridged cross-sections on steel daos were developed as early as 3rd Century AD, sometime around the late Han Dynasty to the Three Kingdoms Period. The benefits of a ridged cross-section for bronze and steel daos were that daos equipped with this feature exhibited a greater cross-sectional area, which led to greater lateral stiffness to resist bending / breakage as well as to provide more bronze / steel material to support the cutting edge -- to prevent chipping or bluntness (loss of sharpness).