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RE: Seismic Response Spectrum

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Thanks all for the very informative posts.

 

Rich

 

 


From: bcainse(--nospam--at)aol.com [mailto:bcainse(--nospam--at)aol.com]
Sent: Tuesday, May 23, 2006 12:48 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Seismic Response Spectrum

 

Rich-

I strongly agree with Chris on this one (on the probabilities) however, good seismic performance is as much dependent on having a good and ductile load path as it is on force level. The force level is fiction since it includes an R factor based on structure type to attempt to reduce elastic spectral levels to a design value pre-supposing that inelastic deformations will occur.

 

As far as adjusting the PGA to spectral level, it usually uses a 2.5 to 2.75 multiplier on the PGA in the acceleration controlled region of the spectrum (the code assumes damping at 5%) based on some work done by John Blume and Nathan Newmark (two separate studies for the old AEC) many moons ago (although Newmark is usually credited for the factors eventhough they both came out with about the same numbers at about the same time).  If you are interested in the factors used, there is an Earthquake Engineering Research Institute Monograph on the subject obtainable from EERI. Contact juliane(--nospam--at)eeri.org for details. Unfortunately, their "Publications Store" on their website show it is down for maintenance until October, but my experience is that they respond promptly to emails.

 

Bill Cain, S.E.
Berkeley CA

 

 
-----Original Message-----
From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
To: seaint(--nospam--at)seaint.org
Sent: Tue, 23 May 2006 11:25:07 -0500
Subject: Re: Seismic Response Spectrum

On May 23, 2006, at 9:50 AM, Rich Lewis wrote: 
 
> I was wondering how this information can be utilized in an equivalent > static force analysis. 
These levels do look like ground accelerations--too low to be response spectrum ordinates. If it were me I'd want to verify what the accelerations really represent. If they are indeed ground accelerations, you can probably get hold of some standard texts on how the UBC morphs ground accelerations into seismic coefficients. There are 3 or 4 bugger factors to account for the type of structure, the site-structure resonance, the importance factor and such. You can just mimic that process with your ground accelerations. 
 
You shouldn't use just the ground accelerations. That would leave out the effect of the building dynamics and would be very unconservative. 
 
As far as the probabilities are concerned, my own personal opinion is that it's generally nonsense. You never know where that 50 or 100 year repetition cycle actually started or how carefully mother nature times these things. I've always been something of a skeptic when it comes to statistical methods and I figure the big one will happen the day after closing. 
 
Christopher Wright P.E. |"They couldn't hit an elephant at 
chrisw(--nospam--at)skypoint.com | this distance" (last words of Gen. 
.......................................| John Sedgwick, Spotsylvania 1864) 
http://www.skypoint.com/~chrisw/ 
 
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