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RE: Lighter Loads in IBC vs. UBC

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You are not incorrect.

The UBC wind was originally derived from ASCE 7 with conservative
assumptions, and therefore when deriving actual ASCE 7 values they will
typically be slightly lower (regardless of the three extra days to
derive the value).

Seismic is also going to be lower in many instances, but not all.  Under
UBC all structures in a "zone" were equally penalized.  Under the IBc
provisions the seismic requirements are much more site specific, and in
most cases will be lower until you reach the type of site that set the
former threshold for the zone.

Paul Feather PE, SE
pfeather(--nospam--at)SE-Solutions.net
www.SE-Solutions.net
 
-----Original Message-----
From: chris.slater(--nospam--at)gmail.com [mailto:chris.slater(--nospam--at)gmail.com] On Behalf
Of Chris Slater
Sent: Wednesday, January 09, 2008 12:29 PM
To: seaint(--nospam--at)seaint.org
Subject: Lighter Loads in IBC vs. UBC

So I think I need a sanity check.  Last time I asked a question here I
got blasted at first and then wound up getting some very useful
information.  So I suppose I'll give it another shot!

We're working on making the transition from 1997 UBC to 2006 IBC.
While the wind loads are more complicated, I'm finding that as long as
topography isn't a factor, even the highest load (that of the end
conditions) is the same or lower than what we were using in the UBC.
So we can simplify and use the "A" load across the whole projected
area, and still generate Wind loads that are lower than what we were
designing for before.

Similarly, when I go through the USGS software and get values for S1
and Ss, then put that through the equations in ASCE7-05, I come up
with a Base Shear equation that is a fair bit lower.   Where a one
story home in the Bay Area would have previously had a V of say 0.14W,
 the new code puts it at 0.108W.

I can send more details of how I'm coming up with these figures, but I
just wanted to start by finding out if other folks are seeing the same
thing.

Thanks in advance.

Chris Slater, PE

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