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RE: IBC vs UBC vertical acceleration

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Yes this is correct. That is what I always do. Your load combination for
12-6 will be something like 0.68DL+ - p Eh (depending on your Sds)


 

 



-----Original Message-----
From: Glen Underwood [mailto:gunderwood(--nospam--at)clarkpac.com] 
Sent: Wednesday, April 27, 2005 9:16 AM
To: seaint(--nospam--at)seaint.org
Subject: IBC vs UBC vertical acceleration


Preface:  In the 2001 CBC, they amended load combinations in 1612.2 (eqs

12-5 and 12-6) in such a way that you could not be forced to subtract 
0.5CaID from the already reduced 0.9D value for dead load 
effects.  (Equation 12-6 was re-written to be 0.9D +/- 1.0*rho*Eh).

This makes sense, since the 0.5CaID value was created solely to make up
for 
the "missing" 0.2 factor on Dead Load that appeared when the positive
dead 
load factor was reduced to 1.2 (between the '94 and '97 codes).  It
makes 
no sense to use this factor to further reduce the 0.9 coefficient in 
equation 12-6.

Question:  Now - I am trying to work with the IBC/ASCE 7 - and I do not
see 
any provision for this.  It appears that if you plug IBC equations 16-50

and 16-51 (or 16-28/16-29 for 2000 IBC) into the load combination
equations 
16-5 and 16-6, that you will end up reducing the Dead load effects by 
taking 0.9D minus another 0.2Sds*D.

So, for a case where Sds = 1.0, you would end up with a DL effect of 0.9

minus 0.2 = 0.70*D.  Is this correct?

Thanks,

Glen 


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