What two forces does every building fight: squeezing and stretching?

Nearly every part of a building is being squeezed, stretched, or, where it bends, both at once. Engineers call the squeeze compression, a pushing force that tries to make an object shorter, and the stretch tension, a pulling force that tries to pull it apart. A rope holding up a swing is in tension. A stone pillar under a roof is in compression. Almost everything a structure does comes down to carrying those two forces safely to the ground.

Most materials are far better at one than the other, and that decides what you can build with them. Concrete, for one, has relatively low tensile strength, so it is a poor choice wherever something is being pulled apart. Ropes and chains are built for exactly that pulling, and they cannot push at all. Reinforced concrete strikes a bargain between the two: steel bars with higher tensile strength take the stretch, and the concrete does the rest.

Bending mixes the two. Put a plank across a gap and stand on it. It sags into a slight arc, and the top edge is compressed while the bottom edge is stretched. A truss, the triangulated frame under a bridge or a roof, spreads this out: its top beams are typically in compression and its bottom beams typically in tension. The triangle matters because it is the simplest shape that cannot change form when its side lengths are fixed. The same trick holds up roof frames and, in miniature, the frames of bicycles.

Squeezing has a nasty surprise, though. A short, stubby post fails by being crushed, but a long thin one can suddenly bow sideways, called buckling, long before the material itself gives way. Tension has no such problem: it tends to pull small sideways wobbles back into line. That one difference is why slender things like cables are so efficient, and why so many buildings are really a clever argument over where to put the compression and where to put the tension.