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RE: Special Truss Moment Frames - AISC Seismic Prov

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Thanks for the reply, but it's #3 that I'm getting a little confused on.  I'm not understanding how to use Vne to "calculate the required strength in the elements outside the special segments".  

John C. Jones, PE
Barnett Associates
Pell City, AL
205-884-5334
205-884-0099 (fax)


-----Original Message-----
From: Simon [mailto:shchao(--nospam--at)umich.edu]
Sent: Friday, September 12, 2003 11:12 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Special Truss Moment Frames - AISC Seismic Prov



Actually, it is like this:

For elastic analysis and design, first, you put all your forces including
gravity loading and earthquake loading on the frame (you can roughly assign
initial sizes to all the elements), then you get the required moment
strength at the ends of  the special segments from computer analysis. Pick
up the sections (T, double angles, or others) for special segments according
to the required moments. Second, calculate the expected vertical nominal
shear strength Vne based on Eq. 12-1. Third, use Vne to calculate the
required strength in the elements outside the special segments (you can use
computer do this). Fourth, design all the elements outside the special
segments. Note that for the vertical member in the vicinity of special
segment you can use the same size as the special segments. No iteration is
needed for designing the special segments. You might need a little bit
iteration in the design of elements outside special segments. But usually it
is not a big problem.

You can also use plastic design procedure, this will further reduce the size
of special segments and optimize the whole frame.
----- Original Message -----
From: "John C. Jones" <john(--nospam--at)struct-engr.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Friday, September 12, 2003 11:22 AM
Subject: Special Truss Moment Frames - AISC Seismic Prov


I have a question about 12.4 on page 24 of the 2002 AIC Seismic Provisions.
It has to do with the Non-special segment members.  Here goes...what the
heck is 12.4 telling me to do?

"Members and connections of STMF, except those in the special segment
defined in Section 12.2, shall have a Design Strength to resist the effects
of load combinations stipulated by the Applicable Building Code, replacing
the earthquake load term E with the lateral loads necessary to develop the
expected vertical nominal shear strength in the special segment Vne given
as: ..." and then it gives a formula where all the variables are moment,
tension, and compression strengths of the truss members.

Here is what I think it means.
Design the truss as you usually would and get the Vne based on the members
you are using in the special section of the truss.  Do another frame
analysis and iterate until the shear in the special section equals Vne.
This is the lateral load you use in your E for the load combinations.
Recheck truss.  When you do this you will likely get different member sizes
for the special section and then you have to start the whole process over
again.

I really hope this isn't correct, but that's what it sounds like.  Have any
of you run into this.

PS.  This project is in the Memphis area and is IBC 2000

John C. Jones, PE
Barnett Associates
Pell City, AL
205-884-5334
205-884-0099 (fax)


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