Inside the wing is an I- beam. Spars (metal rods) are welded to the I-beam. Stiffened plates are put over this frame to finish the wing. The body (fuselage) and engines are covered with stiffened shells.

How much stress or load can an airplane take before a part bends or breaks? Engineers need to know several things before they know how much stress a part can stand.

One way to understand this is to think of a thin rod standing on end on a solid surface. As more and more weight is placed on top of the rod, it will bend or buckle. To compute when this will happen, the engineer must know the strength of the material (its elasticity), the length, shape and diameter of the rod.

If a shell or plate is not stiffened it will bend, under stress, sooner. Stiffened plates and shells can take much more stress before bending.