Time period of structure

[compe_ad]

Hi all! Can you please help me to understand the time period of structure?

I have a structure which has shear walls at all four sides as shown in attached figure and has gravity columns-beams in the central region to support gravity loads. Since the stiffness of the structure is provided only by the shear walls, shouldn't the time period of one wall should be nearly equal to the time period of whole structure. I am using analysis software and performing modal analysis. The time period of whole structure is 1.3 secs and if I model an individual wall separately, then the time period is 0.15 secs. I know time period is dependent on the mass. For model with single wall, I am taking the mass from tributary area. Does this makes sense to you? Can you provide the reasoning?

 

[milkshakelake]

Reference: Dynamics of Structures by Ray W. Clough and Joseph Penzien

I don't actually recommend going through that book. It's not very practical for everyday use. But it's good if you really want to understand the whole theory and you have like...a few agonizing weeks to spare.

Anyway, if you apply the tributary mass and model one wall, you should get the same period. I think there's two reasons it could be different:
1. Program is also analyzing the period in the perpendicular direction for the single shear wall model.
2. The flexible diaphragm itself is increasing the period in the whole model.

 

[rb1957]

do you mean that you are surprised that one wall in isolation is more flexible than a close set of four walls ?

if only because a single wall has a very limited loadpath to ground, has to have a fixed base; but a closed set of 4 walls can resist bending by an in-plane couple (opposite pairs of walls acting together).

There is a forum, now, for student questions ...

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[milkshakelake]

"More flexible" means higher T, not lower T like OP is getting.

rb1957, I think that might be another possibility. Maybe the diaphragm itself is contributing to the rigidity of the four shear walls, if modeled incorrectly.

 

[HTURKAK]

Do you really need analysis software for this case? You may look any structural dynamics book to see the period of inverted pendulum.

T= 2*Π* SQRT((M*L**3)/(3EI) . If the subject 8 walls identical and the mass distribution the same for all , and if the stiffness of beams and diaphragm negligible ,the period will not change.
Moreover, if the individual wall period is 0.15 secs. , time period of whole structure cannot be 1.3 secs .

You may review your model to see the mistakes.



He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[Greenalleycat]

0.15s sounds reasonable for the single wall so I'd expect that to translate pretty closely to the overall system
Are you sure that the period of 1.3s isn't actually the out of plane period of a single wall?

I had this on a building I modelled in ETABS - I was having a pig of a time getting 90% mass participation in my modal analysis, turned out it was because heaps of the lower modes were actually just individual walls flapping around out of plane and getting reported as an actual 'mode'

 

[milkshakelake]

turned out it was because heaps of the lower modes were actually just individual walls flapping around out of plane and getting reported as an actual 'mode'

How would you prevent that? I generally have shear walls set to a low stiffness out of plane, like 10%.