A spar assembly for a rotor blade including a beam structure extending between a closed first end and a second end configured for coupling within a shell of the rotor blade. The beam structure includes a contacting portion extending in a span-wise direction from the second end, a joint portion extending in the span-wise direction from the first end toward the contacting portion, and a transition region between the contacting portion and the joint portion. The contacting portion includes one or more contoured surfaces oriented toward at least one of an internal surface of a pressure or suction side of the rotor blade and configured to follow a contour of one of the pressure or suction side of the rotor blade. The joint portion includes one or more linear surfaces oriented in the same direction as the contoured surface(s). The transition region transitions the contoured surface(s) to the linear surface(s).