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RE: IBC vs UBC vertical acceleration
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- Subject: RE: IBC vs UBC vertical acceleration
- From: Glen Underwood <gunderwood(--nospam--at)clarkpac.com>
- Date: Thu, 28 Apr 2005 08:47:50 -0700
Thank you for the responses.
Yes - a further decrease in DL below the 0.9 factor seems conservative (at least by CBC/UBC standards) - but not necessarily the correct thing to do. (Although per IBC looks like that's what is required).
In my line of work (precast cladding) a 20% swing in DL is very large - especially considering we know the dead weights of the panels very accurately - so it is something I will need to pin down.
Glen Underwood
Glen-
0.9D + E is for overturning/uplift anyways. Decreasing D is conservative in
this case.
Eli Grassley
-----Original Message-----
From: Glen Underwood [mailto:gunderwood(--nospam--at)clarkpac.com]
Sent: Wednesday, April 27, 2005 10:16 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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