"It suggests that large airplanes have some sort of a solid, massive frame that runs along the fuselage. This frame is needed to provide secure connection with wings and the tail sections with the engines. It holds the whole structure together and provides safety to passangers and crew (for example, seats may be securely fastened to the frame through the floor). If this is true, the "frame" itself must be a rather large and sturdy part of the structure."
Bielec, the professionals aren't going to answer you because your concept of aircraft construction is ridiculous, not worth their time.
But let me just say that no, there is no "solid, massive frame" in an aircraft, any more than there is in an eggshell. These are not pickup trucks. Structural strength in an airplane comes from the structure as a totality, not from "a frame."