On most days, the wind blows on the building and we hardly notice the building dancing, so when does the wind become a problem? The only time that this dancing can hurt you or I is when the wind and its gusts are moving at the same pace or natural frequency as the building. Since the fluctuations in the wind change with time, if they are changing at the same pace that the building is moving at, then a very dangerous thing happens – resonance! Resonance describes the situation where the frequency of the wind matches the frequency of the building. When this happens, the building will move at great levels, even dangerous levels. It can move so much that it is destroyed!

Have you ever seen an opera singer even in a cartoon hit a high note and shatter glass? Her voice hit the same frequency as the glass’s natural frequency. This is an example of resonance and will cause the glass to move so quickly and so much that it pulls apart and shatters. The same can and has happened to real structures.

Here are photographs of the Tacoma Narrows Bridge in Washington State. One day in 1940 this bridge was excited by wind blowing at just the right frequency, causing resonance. It caused the bridge to dance excitedly, well beyond what it was designed for. Ultimately, it failed. It was a tragic reminder to its engineers that dynamic motions are something that must be reckoned with.

This was one of the first times that the dynamic motions of buildings were a problem. In the 1800’s, most buildings and bridges were massive structures made of stone and concrete. They were very heavy and stiff. As result, they were very difficult to move and didn’t dance much. But in the beginning of the 1900’s, engineers learned they could build their buildings and bridges with a new, lightweight material – steel.

With new strong and light steel, buildings didn’t have to have thick walls to carry their weight. Instead, they could build simple steel skeletons and then simply cover them with glass skin. These new buildings were strong and light. As a result, buildings could be made much taller than before and the skyscraper was born. Unfortunately, steel is very light and not as stiff as stone or concrete. As a result, these new buildings were very flexible and could be forced to move and dance more easily, not to mention the fact that they were taller and thus received greater wind loads.

Even when resonance occurs, the results aren’t as disastrous as the Tacoma Narrows Bridge. Often, windows pop out, walls crack or elevators cannot function. On some days, the Sears Tower moves so much that they have to close down the elevators because they get stuck as the shafts twist in this dance. Excessive motion will cause failure of the building or bridge. Another interesting result of the wind and building’s dance is the feeling its occupants have. The twisting and dancing often leaves people living inside to feel as if they are on a ship that is swaying back and forth. The result is seasickness on land! People living and working in tall buildings often feel nausea and dizziness due to these motions.

So while skyscrapers are an exciting part of our culture, they do have many unique problems. Engineers are usually successful in making sure they do not collapse, however the new challenge is making sure that small failures to the windows, walls and machinery do not occur. It is also important to limit the motions of the buildings so that people do not become ill and move out of their expensive condos and offices. Owners do not want to lose money and thus hire engineers to propose creative solutions…