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Site Period

Site Period

Site Period

(OP)
Over the years I have frequently seen this formula used to figure the period of a site (in seconds): T=4H/Vs

Where:

T= period
H= layer thickness
Vs= shear wave velocity

Does anyone know if this is accurate if you were considering just the upper layers of the soil? In other words, if I wanted to determine the period for (say) the top 3 meters......is that what I would plug in for "H"?

Thanks in advance.

RE: Site Period

What are you trying to figure out exactly? The natural period of the soil mass itself? The frequency response of the soil is built into the site class modifiers.

RE: Site Period

(OP)
Basically I am trying to figure how much site amplification will happen from a artificial (i.e. a machine, not a earthquake) source of Rayleigh waves. Figuring the site period would (I thought) be a good place to start.....BUT you run up against the problem I stated in my OP. The Rayleigh wave is a surface wave (i.e. it only travels in a depth of about 1.5* the wavelength). Ergo, going too deep in the formula I cited may not be appropriate. Then again, most of the time I've seen people figure this, the used depth has been around 30 meters. So I am not sure....ergo, my question.

RE: Site Period

Everytime I've needed the site period (to squeeze a bit more out of the seismic coefficient, not used it for your purpose), I've got the geotech to do some specific test (can't remember the name of it though, but involves sending/receiving vibration waves across the site). I'd imagine this is more accurate than any empirical method. Usually it's quite cheap, but they are usually on site doing other stuff anyway, cost is usually justified in the structural savings achieved from use of a lower seismic coefficient, however for a single foundation may not be so, but worth asking in case you need to sharpen your pencil to get things over the line.

Otherwise if a firm answer from a geotech isn't forthcoming I'd bound the problem by assessing the impact on foundation size for different site periods (perhaps plus/minus some percentage either side of your interpretation of the site period as will vary in reality). May not even be an issue depending on the foundation mass for other design checks if there is a large difference between the forcing frequency and your ground harmonic frequencies.

Resonance could occur over a very short frequency range, so you need to assess a wide range of likely frequencies at relatively small increments to assess if their are likely to be any vibration/resonance l issues. Simply choosing one frequency to check is potentially ignoring resonance occurring just to one side of the chosen site frequency/period.

RE: Site Period

(OP)

Quote:

Everytime I've needed the site period (to squeeze a bit more out of the seismic coefficient, not used it for your purpose), I've got the geotech to do some specific test (can't remember the name of it though, but involves sending/receiving vibration waves across the site).

You are likely thinking of a surface plate test or cross-hole shooting. I would do it here.....but I don't have the time.

In any case, digging through more of my resources, I have a bit more confidence (at this point) in letting "H" be the depth that the Rayleigh wave is active in.

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