A platform for use between adjacent propulsion rotor airfoils joined to a rotor disk to provide an inner flowpath boundary includes: an axially extending I-beam supporting a radially outer skin having a flowpath surface; the I-beam including an inner I-flange disposed at an inner edge of an axially extending I-web, and an outer I-flange disposed at an outer edge of the I-web; the I-beam including a laterally-extending forward end flange at a forward end of the I-web, and a laterally-extending aft end flange at an aft end of the I-web; and the radially outer skin disposed on top of and joined to the radially outer I-flange such that the forward end flange and the aft end flange abut the outer skin.