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

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AISC 341-05 requires the connections to fail by a ductile limit state and
requires the braces to be seismically compact. This drives you to connections
stronger than the member and for your brace to not fracture.  Chevron braces
have the same requirements for ordinary or special frames.

When I took earthquake engineering I remember the professor saying the
special load combinations were put into the codes to approximate the maximum
force that could be delivered by the system.  

The one benefit with ordinary braced frames now is that you could design the
connections with a envelope of member internal forces with the special load
combinations - they still have to fail by a ductile limit state.  I detail my
gussets with the 2 thickness offset so why can't the building be taller than
35 feet and to meet the 70%-30% compression-tension rule.

They need to add an exception that allows OCBF's to be several stories tall
if the gussets are detailed to buckle out of plane and if 70%-30% rule is
met.


-----Original Message-----
From: Harold Sprague [mailto:spraguehope(--nospam--at)hotmail.com] 
Sent: Thursday, April 10, 2008 12:45 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: ASCE7-05 240 ft ordinary braced frames in seismic design
category D?

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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