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earthquake design

earthquake design

earthquake design

when designing a structure for earthquake, would the earthquake forces be higher at the top of the structure or at the base? or would the earthquake loads be dependent of the dead loads on the specific floor? thanks

RE: earthquake design

Doesn't that depend on the response of the building?  If the building decouples, then the top might have significantly higher accelerations than at the base.


RE: earthquake design

Usually, the lateral forces - using static procedures, are larger as you proceed up the building.  

RE: earthquake design

If you consider the building as a vertical cantilever obviously the internal forces will be maximum at the base but the amplitude will be maximum at the top.

RE: earthquake design

The seismic forces normally occur at the center of mass of the structure.
The seismic design using international codes and standards can be done using an equivalent static method or dynamic analysis.

The equivalent static method calculates seismic forces, which is normally higher on the top and reduces as you go down the structure.


RE: earthquake design

Xpert, the load path dictates that the seismic forces must be transferred to the base or foundation.  As such, by using the static method you will see that the force that must be resisted actually increase toward the bottom.  The tally of forces for the bottom or foundation will be the base shear.

It should also be noted that the above assumes a linear elastic relationship.  Large displacements don't always imply large forces!


RE: earthquake design

thank you all for your suggestions. they have been very helpful. i was just wondering though, how would a foundation take the horizontal forces? aren't foundations designed in such a manner in where it only takes vertical forces?

RE: earthquake design

Doesn't that depend on the expected seismic motion as well as the soil and the type of building?  One could imagine that a tall building will require a foundation that resists lateral force more than a two story house's foundation.  


RE: earthquake design

If I remember right there are two separate kinds of seismic waves that are generated, "S" and "P". The "S" wave is horizontal or a shear wave that moves the base of the building back and forth.  The "P" wave has a vertical component like a water wave, and in thick gravels produces ground waves which are apparently very disconcerting to see. I did a search on "Seismic Forces" and found this site, based in California, that provides free information.

RE: earthquake design

Yes, there are those 2 waves. Yes, the foundation must be designed to withstand the seismic load expected in that location. Otherwise the seismic effect will not get into the building  but the foundation will just stand there  and let the earth wiggle under it!!

FL Wright claims to have designed his Tokyo Hotel that way. It may have been an accident but that is the way the building  sitting on long piles, reacted.

And why not create a new thread here, called Seismic, or Seismicity??

RE: earthquake design

It is my understanding that the structure of the Tokyo Hotel was very rigid - I believe it was a masonry shear wall building.  Therefore the period of vibration of the building is very short, probably less than 0.5 seconds.  I believe the site has a very deep layer of soft clay or mud.  This type of soil can not transmit the high frequency, high acceleration ground motions that a rigid shear wall building would be especially vulnerable to.  It was a matter a picking the best type of structure for the particular site.

RE: earthquake design

Yes you are right. Frank Wright was on goo so put his stiff buildings on piles and let the goo vibrate around them under their stiff cap. I wonder if he really expected that.

But to the origianl question--surely the foundation has to be designed to accept and handle the seismic ground motion. Since the earth will move it must lean on the footings as it moves so they should not skid around nor break up unless you are designing a floating building. Which can be done.

RE: earthquake design

To absorb energy the structure have to resist the movement of the crust (earth) to do that the period  between both (structure and crust) have to be near, in few words the capacity of absorb energy is a funtion of its period, weight (m/g) and rigid (k).
 If you have more mass and rigidity in one point that in another point. The point with more mass and rigidity absorb more energy and have to have the capacity to send these energu to others poinst of the structure.
  A hand rules: soil rigid lihe rock or similar flexible ( beams and columns)  structure, soil flexible like clay rigid strcuture like tube, shear wall and so on.

RE: earthquake design

The source ( point where slip the crust)produce wave of several period but the principals run one streak and another elliptical. The streak wave arrive first that the elliptical wave but the elliptical wave produce more dammage. Besides when the wave arrive to the surface the contact produce another type of waves (Ralight) that produce dammage too. Also, the energy that transmit the wade depent of the different layer of soil that the wave found.

RE: earthquake design

Yes the foundation has to be designed to accept the lateral load.  Usually this is done through bending in deep foundations, or friction in spread footings.

RE: earthquake design

Greetings. please give help me out on how to do analysis for new building with reinforced concrete roof supported by masonry wall only, with no columns and beams. this building is in Texas with z=0. I'm used to doing frame analysis for buildings in Seismic zones 3 & 4 only. thanks.

RE: earthquake design

Please start a new thread.

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