Swaying Skyscrapers

[Mr Downtown]

Something I've never gotten a good answer on is the period of structural drift in skyscrapers. If you have 100 cm of movement, how quickly does that take place? If it's over a period of a full minute, it doesn't seem like it would be that noticeable.

[Nowhereman1280]

Wouldn't the period depend on the forces at work? For example, one would expect the period generated by an earthquake to be much shorter than one generated by wind forces. That's why you can see the buildings in the video moving when they probably sway nearly as much in a high wind event, but you can't see it because they are moving very gently back and forth?

I don't remember much from my physics courses at this point though, interesting topic.

[THE BIG APPLE]

Tall buildings actually sway all the time. While I wouldn't say 24/7, I would say a chunk of a day. On windy days, it's constant start-stop. It's similar to putting a thin, long stick vertical for a long period of time on the ground. It WILL move, whether noticeable or not. It will move but not fall (unless you move it, or exert enough force).

[scalziand]

Originally Posted by Mr Downtown

Something I've never gotten a good answer on is the period of structural drift in skyscrapers. If you have 100 cm of movement, how quickly does that take place? If it's over a period of a full minute, it doesn't seem like it would be that noticeable.

Originally Posted by Nowhereman1280

Wouldn't the period depend on the forces at work? For example, one would expect the period generated by an earthquake to be much shorter than one generated by wind forces. That's why you can see the buildings in the video moving when they probably sway nearly as much in a high wind event, but you can't see it because they are moving very gently back and forth?

I don't remember much from my physics courses at this point though, interesting topic.

Generally speaking, the period depends on the stiffness(k) and mass(m) of the structure. The period will be longer with a higher mass and a less stiff structure. The period will be shorter with a lower mass and a stiffer structure.


wiki

With that said, occupancy comfort is more determined by the peak acceleration during the sway, which is inversly related to the period.