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

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I should mention that AISC 341-05 7.1 forces your connection to be controlled
by a ductile limit state which means for an ordinary braced frame normally
the connection is probably going to be as strong as the brace in tension
anyway.  So why can't a two story ordinary braced frame be 36 feet tall?

-----Original Message-----
From: Haan, Scott M POA 
Sent: Wednesday, April 09, 2008 5:12 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: ASCE7-05 240 ft ordinary braced frames in seismic design
category D?

Follow this code path:
Table 12.2-1  --> Building Frame System B. 1.  ordinary steel braced frame
seismic design category D height <35 feet. - note d  for seismic design
category D -->"12.2.5.4 Increased height limits for steel braced frames...
The height limits in Table 12.2-1 are permitted to be increased from 160 to
240 feet for structures assigned to seismic design category D or E..."  They
don't mention it has to be a special frame - leaves a lot open to
interpretation and I must be a structural engineer because what I think it
means affects my state of harmony.

With ordinary braced frames you are allowed to design the connections for the
special load combinations and there aren't the stupid protected zones.  That
is why people would want to use them on a three story building.  

They destroyed the benefit of chevron bracing in ordinary braced frames in
AISC341-05.  They already reduced the R to 3.25 for ordinary braced frames
and the OMEGAo=2 so you are practically designing the stupid connections to
be elastic.  You can pick the envelope internal force out of your model and
plug it into a connection spreadsheet.  I already detail them so the gussets
can buckle, why can't it be three stories? -- it is !#$%@%^$^&*% stupid. Also
I don't want to argue it with someone if I write an RFP.


-----Original Message-----
From: Doug Mayer [mailto:doug.mayer(--nospam--at)taylorteter.com]
Sent: Wednesday, April 09, 2008 4:40 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: ASCE7-05 240 ft ordinary braced frames in seismic design
category D?

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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--
-gm 

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