Thursday, August 25, 2011

RE: High frequncy or low frequency response

You are absolutely correct.  We do not properly address the seismic vertical component unless it is a nuclear facility or a critical military facility.  Having designed both, I am very uncomfortable not properly designing other structures for the vertical component and the in-structure response.  The vertical ground shaking generally occurs in about the same time domain as the P waves. 

Regards, Harold Sprague

Subject: High frequncy or low frequency response
Date: Thu, 25 Aug 2011 18:20:14 +1000

One opinion:
On 8/24/11, Harold Sprague <> wrote:
> This will be interesting on an academic level because of the transmission of
> high frequency ground motions versus low frequency ground motions and the
> in-structure response.  The 20th floor of a building should not transmit a
> significant high frequency ground motion, but would transmit low frequency
> ground motions.
> The underlying rock of the East Coast is not highly fractured as the classic
> slip faults in California.  The ground motions (especially low frequency) do
> not attenuate quickly in unfractured rock.  This is the 20th earthquake of
> Modified Mercalli V or higher in Virginia since 1774.  There was a Modified
> Mercalli VIII in 1897 in Virginia.
> My son serves on an aircraft carrier, but is living on shore now in Norfolk,
> VA.  He felt it, but was not too alarmed.
> ...The aircraft carrier sustained no damage;>)
> Regards, Harold Sprague
Another one:
Harold, you are all right when focused on lateral motion, as most of seimic engineering seems to be.
But if your quake happens to have a strong vertical component, the building will be transmitting
high fequency as well, vertically.
Gregory from Oz