Embodiments of reinforced composite structures for aircrafts and methods for making such reinforced composite structures are provided herein. A reinforced composite structure for an aircraft comprises a fiber reinforced composite stringer. The fiber reinforced composite stringer comprises a beam body portion and a runout portion that extends from the beam body portion in a first direction. The beam body and runout portions are defined by projection of a variable I-shaped cross-section in the first direction along at least a portion of an entire combined length of the beam body and runout portions. The variable I-shaped cross-section has a cap section, a foot section, and a web section that extends between the cap and foot sections. The variable I-shaped cross-section is configured such that a height of the web section tapers in the first direction along at least a portion of an entire length of the runout portion.