Fundamental frequency vs Shear Wave Velocity

[pelelo]

Does anyone knows an approach to compute the fundamental frequency in 60 ft deep strata, given different shear wave velocities (Vs) between ground surface and 60 feet below ground?.

 

[Titleistguy]

Don't think the fundemental period which is basically a function of a structure's mass , height, and stiffness is directly related to shear wave velocity.   Put another way, a six story moment frame of mass x and stiffness y will always have the same period, unless the analysis method to find the period changes (approx, modal, Rayleigh...etc).  Now that said the shearwave velocity and how it relates to site class and SDC will relate to the ultimate formulation of base shear along with the period.  But I believe those two are independent variables in relation to each other.

But I'm wrong all the time so that's just what I think off the top of my head.  

 

[pelelo]

Thanks,

I am just given the shear wave velocities at several depths. Based on that, how do you assume I need to compute the fundamental frequency?

 

[Titleistguy]

I'm not assuming you need to calculate the fundamental frequency I'm just going off of what your original question was.

You can average the shear wave velocities over the top 100 feet and determine a site class.  If S1 and Ss are known you can determine Fa and Fv and get a SDC.  Once you have a SDC you'll still need a building period to proceed.  The building period is essentially the x coordinate and up until now with the site class and SDC we've been developing the response spectrum but still have no x to tell us where exactly on the spectrum we are.  Which is why I say it's an independent variable.  You could have a campus of buildings all sharing a response spectrum but bc each bldg is different you would have different periods and different seismic responses in each.

 

[EBAT75]

If you need to construct a Design Response Spectrum according to ASCE 7, and site specific ground motion procedures are not used, that is where T comes into play. 

 

[EBAT75]

If you need to construct a Design Response Spectrum according to ASCE 7, and site specific ground motion procedures are not used, that is where T comes into play. 

CORRECTION AND CLARIFICATION: replace "are not used" with are required to be used.

Teitlistguy is right about T being dependent only on the structure's physical characteristics and not on what it is founded on. However, when the Site Class becomes F, and the soils are vulnerable to say liquifaction, sensitive clay responses, then, except when T is less than 0.5sec, site specific ground motion procedures are required. Ref: ASCE 7 - Ch. 11, 20, 21

Looks like pelolo was having a situation like this but the reason for calculating T is to see whether it is in fact less than 0.5 sec. Whether site specific ground motion procedures are required or not flows from that.