Building and Soil Resonance and Period 1

[mes7a]

When you design structure or do renovations.. do you always check if the soil seismic properties can be in resonance with the structure? I heard tuff rock or stiff soil has resonance approaching that of 2 to 3 storey. And I read that some has to change the properties of the building so it won't be in period resonance with the soil in case there is seismic activity. Do you do it too? What is your computations of the resonance of very stiff soil or tuff rock?

 

[WARose]

When you design structure or do renovations.. do you always check if the soil seismic properties can be in resonance with the structure?

I have to admit I haven't. I do run the lateral forces (of course) to see what I have.....but I've never compared periods.

What is your computations of the resonance of very stiff soil or tuff rock?

I've calculated the period of a site before, but it wasn't for a rock base. The formula I have used is T=4H/Vs (Where H=thickness of the layer; Vs=shear wave velocity of that layer.)

 

[JoshPlumSE]

The only times I've heard of folks doing that was for Mexico City where the soil basin has a very well defined period that will resonate with buildings of a certain height / period.

Probably for nuclear sites where they have all kinds of extra info on soil properties and expected seismic events and such.

 

[sponton]

You do check them but you do it indirectly through the design response spectrum, the higher A/g value would be located in those periods that are within the range of the natural period of the soil, that's why response spectra are location dependent. Mexico's city response spectrum is a bit different, due to the soil-structure interaction there's a more considerable participation of the higher modes, so it's not exactly only the natural period of the structure which you should be concerned with. Chopra's Structural dynamics book actually talks about these things and explains them very clearly, you should check it out, it's a good place to start.

 

[mes7a]

Are there specialized equipment like ground penetrating radar (and services) to check the soil or rock properties and periods and possible resonance with the building?

 

[sponton]

You would use a seismic accelerometer and do a reading at the place where the new construction is set out to be. Then using the results you could make your own Response spectra which may or may not have actual reduction in forces (normally in mexico city you would do this sort of study to reduce applied seismic forces and save on construction costs), it might work the other way around where it turns out you actually end up with a higher seismic acceleration.

 

[mike20793]

sponton,

Is that something that is done in coordination with the geotechnical engineer, or is it done by specialty firms?

 

[sponton]

Well, my company was solely structural engineers but we used to rent the equipment from one of the seismic accelerator equipment vendors and we would do all the reading ourselves and generate their respective response spectra [of course we charged for it as an extra service]. I don't see why a geotech engineer shouldn't be able to do it given the tools, in fact you can do it yourself, I think chapter 21 ASCE-05 [I don't have ASCE-10 at the moment so i can't tell you which chapter it is], has the provisions you need to comply with, as for the procedure you'd need to do some research online. I had some matlab routines to perform all the work given the seismic data (they'd clean up the signals), if I find them I'll post them here.