Question

How do you describe earthquake motion and wind forces?

Answer 1

An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in size from those that are so weak that they cannot be felt to those violent enough to propel objects and people into the air, and wreak destruction across entire cities. Earthquake forces are called lateral forces because their predominant effect is to apply horizontal loads to a building. Although earthquake waves do impart a vertical component of force to buildings, the weight of the building normally provides sufficient resistance.

Wind can be defined simply as air in motion. This motion can be in any direction, but in most cases the horizontal component of wind flow greatly exceeds the flow that occurs vertically. As the wind blows against a building, the resulting force acting on the elevations is called the ‘wind load’. The building’s structural design must absorb wind forces safely and efficiently and transfer them to the foundations in order to avoid structural collapse.

Difference:

They're considered separately in calculations because they are different kinds of forces and require different kinds of structural resistance.

For example, wind loads push against the entire face of a building. Therefore a heavy building will resist wind loads better than a light one.

But an earthquake shakes the building from the bottom. In a tall building, the bottom shifts quickly and becomes out of alignment with the top. As the building straightens itself out, a lot of momentum is generated. Therefore lighter buildings do better against earthquake forces because they generate less momentum. They have less weight swinging back and forth.

Basically, weight is good against wind forces and flexibility is good against earthquake forces. (Some fexibility is good against wind too but not to the extreme needed to resist earthquakes). The building needs to be checked against expected values of both of these forces for the area its located in.

I was in Japan during the big earthquake a few years back. Not a single building in Tokyo collapsed. But if that same earthquake were to hit a major city in almost any other country, I don't think they'd be so lucky.