The first question is to define what you mean by "all those forces."
The primary force that a wing spar is required to absorb is the vertical force of lift (and in some cases the opposite vertical force of meeting the ground when landing, if the gear attach to the spar). In effect, the weight of the aircraft, or the lift needed to counter that weight in order to fly, multiplied by any G forces.
By itself, it does not have to directly withstand, for example, the drag of the wing, which is partially converted to lift (more vertical force) by the airfoil skin surrounding it, and also carried through the whole wing structure by the stringers.
The video above shows the Cessna spar - also built from effectively sheet metal.
The key point is that the Cessna sheet metal is formed into an "I" beam through riveted or folded flanges (the parts the guy is working on). In the direction of the strongest force it must handle - the vertical lift and/or weight - it is effectively no longer thin metal, but metal about 10 inches thick, from top to bottom.
Now, if it were just one sheet of metal, it still might buckle if it twists so that the vertical force is no longer edge to edge across those 10 inches, but begins to be sideways across the much weaker thin dimension. That is where the flanges - the cross-pieces at the top and bottom of the "I" - come into play. They stiffen the total shape to prevent twisting, keeping the main spar plate aligned edge-on with the forces it must carry. Twisting or bending the main spar plate, once the flanges are added, now requires twisting 3-5 inches of "flange" - which takes far more force than twisting the 2-3mm thickness of the plate itself.
Also note the weight-reducing circles cut into the spar. They are not simply punched holes, but also have flared rims, in effect giving each hole an "L" flange around the perimeter. Those "Ls" act as 1/4 of an "I", also adding stiffness to what would be otherwise flexible and weak sheet metal.
It is amazing how much stiffness and strength one can add to even material as flimsy as a sheet of paper - once one starts playing around with right angles to redirect forces and convert them from compression to tension forces, and vice versa.
http://farm8.staticflickr.com/7025/6...8c31835f_z.jpg