So when we think of the tall buildings in Chicago or Boston that lost their windows, we soon realize that there was no mighty earthquake pushing them about. It is nothing more than the wind!

From this picture you can see that there are two components to the wind. The first is the average or mean component. It is pretty steady and always blowing on the building. This is the static load on the building because it really doesn’t change with time. So the structure will move due to this always. As you can see, the mean wind increases with height. That’s why kites can fly because the wind up above is moving faster and blowing harder than down on the ground. The wind also howls a lot on mountaintops but not in valleys. Just think about the wind in your everyday life. It is always there blowing, even if it is just a little bit. The further in the sky you go, the more the wind blows, so the tallest buildings feel the largest wind loads.

However, just like you feel the wind suddenly speed up and then stop in what is called a gust, there are changes in the wind. These are fluctuations. They change with time and are dynamic loads on the structure. They also cause the dynamic motions we are worried about. This is the fluctuating component of the wind, causing the ripples you see in the approaching wind in the picture. As these winds hit the building, they push it and flow around it, creating pressure and suction on the faces of the building. The sucking and pushing of the wind is exactly what causes the building to move. The wind creates pressure all over the surface of the building that pushes it around. The pressures are these little arrows pushing on the building in the figure. Remember, that the fluctuating or rippling part in the wind is changing with every second in time – it is dynamic and is the partner to the building in its dance. With every little gust of the wind, a new dance move is created in this tango in the skies.

But not every dance ends in disaster…so when do things go wrong?