Every bridge must carry its own weight as well as the weight of the traffic that crosses it. Engineers these loads in of two forces. pushes material , while pulls it . is strong in compression weak in tension, is why ancient bridges were built as arches.

In a suspension bridge, the road hangs from vertical cables that are attached to two cables. These main cables over tall and are firmly each end. The are tension, the towers are compression. This design bridges to span very long distances.

On November 7, 1940, the Tacoma Narrows Bridge in Washington State twisted itself apart in a moderate wind, only four months after it opened. The bridge’s movement had been so noticeable from the start that local people nicknamed it “Galloping Gertie”.

The bridge was a suspension bridge with an unusually thin, flexible deck. Its designers had focused on making it elegant and economical, and they did not fully understand how wind would interact with such a light structure.

On the day of the collapse, winds of about 65 kilometers per hour caused the deck to twist back and forth. Each twist changed the way the wind struck the bridge, which made the next twist larger. Engineers now call this kind of self-reinforcing motion aeroelastic flutter. After about an hour, the main span broke and fell into the water below. No person was killed, although a dog left in a car was lost.

The disaster transformed bridge engineering. Since then, designers have routinely tested models in wind tunnels and built decks that are stiffer and more aerodynamic.

Some of the oldest bridges still in use are stone arches. Roman builders constructed arch bridges that have carried traffic for nearly two thousand years.

The secret lies in compression. In an arch, each wedge-shaped stone presses against its neighbors, so the weight of the bridge and its load is carried around the curve and down into the ground at each end. Because stone is very strong when squeezed, a well-built arch can stand without any mortar holding the stones together.

Arches do have limits. They push outward at their bases, so they need solid ground or heavy supports, called abutments, to stop them from spreading apart. They also become impractical for very long spans, because the arch would need to be enormously tall or thick.

Modern engineers still use the arch, but often in steel or concrete. These materials allow longer, lighter arches, while the basic principle remains the one the Romans understood.