Ballast Question on Earthquake Acceleration

Structural Layout Vignette and Multiple Choice

Ballast Question on Earthquake Acceleration

Postby Jessican » Fri Sep 02, 2016 10:09 am

I have a question on Ballast Question #93. In this "choose all that are true" type of question, the answer says that the statement, "A building's acceleration is generally greater on dense soil than it is on soft soil," is false. However, I really thought that this statement is true. From studying, I've learned that soft soil causes a longer building period, which relates to a lower acceleration and therefore, a smaller seismic force. Wouldn't this mean that if a building is on dense soil that it has a high acceleration?

If not, what part(s) of the assumptions I'm making above are incorrect?

Any guidance is appreciated!
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Re: Ballast Question on Earthquake Acceleration

Postby tmston2 » Tue Sep 06, 2016 4:53 pm

•question is basically pointing out the difference between soil stiffness (acceleration) and building stiffness (acceleration)
• you are correct about acceleration decreasing as period increase; but it is my understanding that only applies to ductility of building (soft soil acceleration is btwn .4 & 1.5
(typ. 4 story bldg is .5) im not sure how you could economically make soil more stiffer.
•Soft soils generally have a tendency to increase or amplify ground shaking as much as 2 to 6 times as compared to stiffer soil.
•in a nut shell the more stiff something is the less it moves which is basically the kind of couple (stiff soil and stiff bldg.)l you want together.
• but on soft soil: bldg w/ longer period( which such bldg has shorter acceleration or f/a=m or will experience less lat force in earthquake)

silly but gud example: palm tree anchored to stiff ground in wind storm, ductile tree moves w/ lateral wind force which increase period (acceleration) but tree survives storm.
BUT if tree was in a large pot sitting on ground (soft soil) it would collaspes from lateral wind force.

hopee this helps
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Re: Ballast Question on Earthquake Acceleration

Postby Jessican » Wed Sep 07, 2016 6:46 pm

Ok, thanks for the response!
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Re: Ballast Question on Earthquake Acceleration

Postby 22design » Sun Feb 25, 2018 11:31 am

Jessican, you are correct that Ballast question is true. It must be a typo on their part. I also came across this and thought it was true, and was thrown into confusion seeing the false. After much study, I finally resorted to calling an engineer friend who agreed with me that it was true. They pointed me to the following example of the Bangkok towers: www.iitk.ac.in/nicee/wcee/article/1466.pdf
There was a study done because the site had soft clay and this paper presents the "amplification of earthquake ground motions in Bangkok." Based on the study's results the solis underlying Bangkok(soft clay) has the ability to considerable amplify earthquake ground motion, both in peak ground acceleration and spectral acceleration.
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Re: Ballast Question on Earthquake Acceleration

Postby lali » Sun Feb 25, 2018 8:37 pm

I found this from FEMA:
https://training.fema.gov/emiweb/earthq ... 102330.htm

Basically earthquakes get worse in soft soils.
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Re: Ballast Question on Earthquake Acceleration

Postby lali » Sun Feb 25, 2018 8:52 pm

Jessican wrote: From studying, I've learned that soft soil causes a longer building period, which relates to a lower acceleration and therefore, a smaller seismic force. Wouldn't this mean that if a building is on dense soil that it has a high acceleration?


When the period is longer the acceleration is higher. Read FEMA 454 page 4 of chapter 4
I hope this helps

Lali
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Re: Ballast Question on Earthquake Acceleration

Postby jdunks » Mon Feb 26, 2018 10:49 pm

Jessican, I don't pretend to know what the final answer is to this. But I did check on what Mike SE said about acceleration, and 'Greater acceleration = more shaking' is what I found. So maybe that fits with the interpretation above, "the more stiff it is, the less it moves", and that's how the question meant itself to be interpreted. Just one other thing, from FEMA: Chapter 4, Figure 4-7 plotting acceleration vs. period surely supports your take - long period, low acceleration, low velocity, high displacement. So idk. I'm shrugging my shoulders and moving on.
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Re: Ballast Question on Earthquake Acceleration

Postby lali » Tue Feb 27, 2018 1:40 pm

lali wrote:
Jessican wrote: From studying, I've learned that soft soil causes a longer building period, which relates to a lower acceleration and therefore, a smaller seismic force. Wouldn't this mean that if a building is on dense soil that it has a high acceleration?


When the period is longer the acceleration is higher. Read FEMA 454 page 4 of chapter 4
I hope this helps

Lali



I am completely confused now.
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