Equivalent lateral force procedure 8

[Ahmed A. Alamin]

This may sound silly, but how could i check that resonance is not a problem using Equivalent lateral force procedure, or the lateral force method in EC8

 

[r13]

I think the chance that a building will get into resonance with natural dynamic force is near zero, if not zero. The reason, for correct or wrong, is that it must have the same form of wave, and the wave length (period) is in the range that produced by the surrounding soil (earth). I expect you need to get into seismology and soil/rock mechanics to get correct answer.

 

[Lomarandil]

Pure resonance the way we discussed in a dynamics class is rarely of concern with seismic loads, because of the natural variation (as retired13 mentioned).

However, the Spectral Response Acceleration portion of ELF deals with this approximately, requiring design for higher level forces for buildings which have low period of vibration (more typical of the range of forcing frequencies found in earthquakes).

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just call me Lo.

 

[r13]

Lo is more correct. See the attached article for concerns of building period for seismic and drift control. Link

 

[KootK]

My take on this is a little different.

Seismic motions are generally very complex and irregular, containing a number of frequencies of periodic motion simultaneously. In the literature, this is often referred to as the "frequency content". It's kind of like the white noise that you might play to fall asleep except much less regular than even that. The white noise signal contains sound at many frequencies in the hope that one or more of them interferes destructively with whatever ambient noise might be messing with your sleep (kind of the reverse of resonance).

I don't believe that an ELF procedure can help you to preclude resonance because, when using an ELF procedure, resonance is actually what you're designing for. You're assuming that, somewhere within the noisy frequency content of the design seismic event, there is a periodic component of that motion that may excite the fundamental vibration mode of your structure. This is why you enter your response spectrum with the fundamental building period as your entry parameter. If some degree of resonance didn't matter much, neither would your building period.

To the extent that you can use ELF to produce a resonance free design, the only thing that I can think of is to make your structure so very flexible that it'\s fundamental period is out past the long period transition point such that your design mass acceleration values get very low. Doing this would effectively be to decrease your structure's natural frequency to the point that your design seismic event would be expected to have very little frequency content within that range. And, for most structures, this simply won't be a realistic option. I would love it if I could take any old shack and persuasively tell the owner that seismic performance would be greatly improved if it were turned into a 75 story skyscraper but, obviously, it just 'aint so.

It's also interesting to note that there have been some seismic events that really do hammer a particular natural frequency. I believe that some of the Mexico city earthquakes hammered a particular height of building terribly and, for the most part, to the exclusion of all other building heights. As I understand it, that was primarily because the seismic input motion was filtered and attenuated in a very specific way by the seismic waves having to travel through a large patch of muck soil en route to the structures that got beat on. Stuff like this is one of several reasons why codes might require site specific geotechnical evaluation and/or analysis methods more advanced than ELF in some situations.

 

[WARose]

Have to agree with Kootk. (And the others.) The forcing frequency of the event is random and typically changing. Low frequency seismic waves tend to propagate at great distances without much frequency change. (But it does come at some point.) The opposite is true for higher frequency waves.

The odds of hitting it are not good. (Sort of the anti-lottery.)

 

[KootK]

Pages 4-7 through 4-11 of this NERHP document speak pretty directly to exactly the issues discussed here, including the Mexico stuff: Link

 

[r13]

Good read. Thanks.

 

[HTURKAK]

This may sound silly, but how could i check that resonance is not a problem using Equivalent lateral force procedure, or the lateral force method in EC8

I do not think your question is the resonance effects of soil which occurs at certain frequencies,where the layers act as wave filter and amplifies the waves at some frequencies when the elastic modulus of the layers too different from one layer to another.

I looked to Eurocode 8 , ( Design of structures for earthquake resistance ,Part 1: General rules, seismic actions and rules for buildings ) 2004 version and do not know the latest version or not.

EC 8 limits the use of Lateral force method /Equivalent lateral force procedure.

-The structure shall be reasonably assumed that the fundamental period will be dominated . That is, a single mode should represent the dynamic behavior.
- the structure shall satisfy the EC 8 regularity criteria .
- the fundamental period T1 shall be less than 4*Tc and 2.0 sec.

The fundamental period T1 can be estimated with simple approximate methods. Compare the fundamental period with 4*Tc and 2.0 sec.

Tc is the the period at the end of of the constant spectral acceleration part.

Regarding the Tb and Tc values, you may look to national annex applicable at your region. For example , for soil type A, spectrum type 1, Tc=0.4 sec.

For this case , the fundamental period T1 shall be less than 1.6 sec.

I have atteched the typical Shape of elastic response spectrum for explanation of Tb,Tc, Td…

 

[MSUK90]

Pages 4-7 through 4-11 of this NEHRP document speak pretty directly to exactly the issues discussed here, including the Mexico stuff: Link

Very nice article. Author has very simply explained the seismic philosophy..