Compression and tension stresses are not the same, and need different structures to carry them.
A typical spar has upper and lower booms and a shear web (or other shear structure). Under positive G the upper boom is loaded in compression while the lower boom is loaded in tension, and the web is loaded in shear.
To react the tension load the lower spar only needs tensile strength. The application of a tensile load inherently stabilises it.
To react the shear loads the web just needs to be joined to the upper and lower booms (either continuously or at short intervals) and it has to have sufficient shear strength.
To react the compression loads the upper boom has to have both sufficient compressive strength AND sufficient stiffness to ensure that it dows not buckle under the applied load, because compressive loads are inherently [i]de[/e]stabilising.
So whilst upper and lower booms will be handling almost identical maginitudes of stress, the difference between the compressive and tensile stress means that the upper boom needs to be much STIFFER than the lower one. This can be achieved with additional material, additional bracing or the same material formed into a more buckle-resistant shape. But the point is that the upper and lower booms will be different if you are looking for the lightest structure solution for an asymmetric loading requirement.
Obviuously if you're looking to design a wing for an SU-26 or similar with *symmetrical* load cases then you would design it with the required stiffness in both upper and lower spar elements - which would be heavier, of course.
Last edited by PDR1; 5th Dec 2016 at 21:42.
Reason: Keyboard can't spell. My gamekeeper will give it a sound horsewhipping later