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Re: Seismic Design Manual, Volume 3

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Interesting question, as I just had some discussions regarding these
sections with another structural engineer whom I am collaborating with on a
small, 2 story braced steel building.

If you are designing connections for braced frames (the requirements vary
depending on bracing style), the connections are required to remain elastic
during the seismic event.  Therefor, you need to start with E (which is the
seismic action divided by R for inelastic behavior) and multiply by Omega(0)
the overstrength factor.

E x Omega(0) = E(m) which is the "true" ultimate force the connection must
resist.  It is analogous to the old 3Rw/8 from the 1994 UBC.

Now that you have a force, the question arises as to how you are designing
the connection.  If using LRFD, then design for E(m) directly.  If using
ASD, then the E(m) must be reduced, and I believe the factor to reduce by is
1.7.  The 1.7 is found in several places, but most quickly look near the
front of Section 2213 (I think, I'm at home and don't have the Code in front
of me) where there is a list of stress increases to apply to E(m).  The
factor 1.7F(s) shows up for bolted connections which you would be applying
in reverse for this instance.

In short, for a brace connection I think you need to design for E x Omega(0)
/ 1.7 if using ASD.  The reduction would be different for concrete, &etc.

Open to debate, however, like many of the overlapping bandaids put in the
Code every time the ground moves.

Larry Oeth
----- Original Message -----
From: Casano, Karen <Karen.Casano(--nospam--at)dgs.ca.gov>
To: <seaint(--nospam--at)seaint.org>
Sent: Wednesday, January 17, 2001 2:41 PM
Subject: Seismic Design Manual, Volume 3


> I recently was asked a question in regard to braced frame connection
design
> per CBC 2213.9.3.1.  The question was in regard to whether the "design
> seismic force" in item 2 is E/1.4 per the load combinations in section
> 1612.3 for allowable stress design.  I had always thought this to be the
> correct procedure if using section 2213; however, it was pointed out to me
> that the SEAOC Seismic Design Manual, Volume 3 contains an example of a
> braced frame design in which it appears that the 1.4 factor is omitted.
The
> example is Design Example 1A, Special Concentric Braced Frame, step 6a.
Is
> this an error, or is there some reason that the seismic design force for
> brace connections should not be reduced by the 1.4 factor?
>
> -Karen Casano
>
>