structure dynamics 1

[flyby911]

hello fellow engineers
i've been reading in the building dynamics field and i think i'm missing something

1-i understand the physical definition of the natural frequency but (is it the reciprocal of the periods at which the different mode shapes take place)?

2-due to seismic action does the structure vibrates by the same frequency of the ground vibration frequency or structures respond by different frequency of the ground frequency

3-if (NO.1) is right , how come we enter the response spectrum curve with the modal periods to get the ground acceleration value and then the lateral force? (that's a hazardous case as i understand)

 

[GregLocock]

Not if it is flexing along its length.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[hippo11]

interesting comment by flyby about an antenna having only one DOF therefore only one mode...we structural engineers tend to visualize structures as "stick" computer models with discretized "nodes" at joints each having 6 DOF (3 translating 3 rotating)...however in reality remember that a structure such as a simple antenna has infinite DOFs...all along the length of the antenna...therefore many modes.

 

[271828]

flyby, that would be true if all the mass was concentrated at the end.

Try this experiment. Strike a car antenna about halfway up. Flip it with your finger or whatever. Watch the response closely for that first second or so and you'll see that the displaced shape doesn't look like a cantilever beam in single curvature. It'll be some weird multi-curvature shape, a superposition of several natural modes. These other modes damp out earlier and it'll vibrate in the single curvature mode longer which I think is the interesting part.