Spar (aeronautics) - Wikipedia
Some of the forces acting on a wing spar are: Upward bending loads resulting from the wing lift force that supports the fuselage in flight. These forces are often offset by carrying fuel in the wings or employing wing-tip-mounted fuel tanks; the Cessna 310 is an example of this design feature.; Downward bending loads while stationary on the ground due to the weight of the structure, fuel ...