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RE: ASCE7-05 240 ft ordinary braced frames in seismic design category D?

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There has been no small effort to dump the OCBF.  But there is insurrection, dissention, and revolution in the ranks.  There has not been an overwhelming body of evidence to indicated the OCBF's are the bane of proper structural engineering as one would theorize. 
 
The Pre-engineered Metal Building industry will not dump the OCBF.  That is why footnote j is there.  If you can qualify it for simplified design, it is permitted.  Better yet, qualify it as a nonbuilding structure and you can dance in the streets.  Check ASCE 7-05 Table 15.4-1. 
 
The reason the OCBF was killed for most buildings is the "perception" of poor performance and the alternative was the SCBF.  Without a body of evidence to indicate poor performance for the OCBF, the nonbuilding structures people were able to retain it for power plants, towers and other structures where deflection limits were driving the design as opposed to stress.  The other reason was that a lot of towers do not have gusset plates rendering the SCBF impossible. 

Regards,
Harold Sprague



Date: Thu, 10 Apr 2008 09:51:11 -0700
From: gmse4603(--nospam--at)gmail.com
To: seaint(--nospam--at)seaint.org
Subject: Re: ASCE7-05 240 ft ordinary braced frames in seismic design category D?

Doug is 100% correct on DSA's policy on OCBF's. 35 ft limit, no exceptions and they will do everything in their power to make you abandon the system for something more ductile.

-gm

On Wed, Apr 9, 2008 at 5:40 PM, Doug Mayer <doug.mayer(--nospam--at)taylorteter.com> wrote:
I attended a DSA seminar prior to the rollout of the new code and I
remember them saying that ordinary steel braced frames were essentially
going to the dump.  The height limitation per ASCE 7-05 seems to confirm
that statement and none of the errata that I have seen changes that
value.  Sucks.

Doug Mayer, SE
Structural Engineer


-----Original Message-----
From: Haan, Scott M POA [mailto:Scott.M.Haan(--nospam--at)usace.army.mil]
Sent: Wednesday, April 09, 2008 5:28 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: ASCE7-05 240 ft ordinary braced frames in seismic design
category D?

Gerard:

They had the same note in the ASCE7-02 and IBC 2000 that allowed single
story
ordinary braced frames to be 60 feet with the 20 psf roof, but there was
an
exception for "braced frames" to be 240 when not more than 60% of the
story
shear was on a grid and not more than 20% of the force in a brace was
from
torsion.

Ordinary braced frames by ASCE7-02, IBC 2000, and IBC 2003 could be 240
feet
tall in seismic design category D if there was not a torsional
irregularity
and if your grids had an even distribution of the story shear.  It
appears
maybe they don't want to allow two story ordinary braced frames in
seismic
design category D to be taller than 35 feet in the ASCE7-05 anymore, but
you
can't tell exactly because of the poorly written wording - maybe there
is an
errata since the printing I have.

I am wondering if there is a typo in the ASCE7-05 and whether the code
committee really wants to hose people into using special concentrically
braced frames for a shoe box - two or three story building that is a bit
taller than 35 feet. I think 35 feet is too short for an ordinary braced
frame building with a bunch of braced grids for a two, three or four
story
building.

Scott.

-----Original Message-----
From: Gerard Madden, SE [mailto:gmse4603(--nospam--at)gmail.com]
Sent: Wednesday, April 09, 2008 4:02 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: ASCE7-05 240 ft ordinary braced frames in seismic design
category D?

Footnote J of Table 12.2-1 states you can increase the height to 60 feet
for
1 story buildings with roof DL 20 psf or less for OCBF's

I think this provision superseeds footnote D in the same table

hth,
-gm


On Wed, Apr 9, 2008 at 3:53 PM, Haan, Scott M POA
<Scott.M.Haan(--nospam--at)usace.army.mil> wrote:


       It used to be in ASCE7-02 that ordinary braced frames were
limited to 35 feet with an exception that they could be as high as 240 feet as long as any braced frame grid did not carry more than 60% of the seismic forces in that direction and that not more the 20% of the force in a braced frame was from torsion.

       ASCE7-05 section 12.2.5.4's wording seems to muddy the waters for me.  It says height limits in Table 12.2-1 are permitted to be increased from 160 feet to 240 ft in seismic design category D?

       Does this mean I can can have an ordinary braced frame in
seismic design category D with a 35 ft height limitation increased to between 160 feet and 240 feet per ASCE 7-05?

       Special braced frames and eccentrically braced frames have a
height limit of 160 feet in seismic design category D.  It seems like the intent of ASCE 7-05 is only to allow special braced frames and eccentrically braced frames to use the height limit exception because of the wording "increased from 160 feet to 240 feet."

       Thanks.
       Scott

       


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