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RE: IBC, Rho Factors, 10/lw, and Influece of FEMA/NEHRP Folks

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Very well put Charles. 

I agree completely with your assessment on the diaphragm issue. Another
thing that I would point out is that by making the diaphragm stronger as a
result of lack of redundancy in VLFR system, we create the mechanism to
deliver more loads to those elements and could possibly lead to their
failure. This flies in the face of the strong column week girder concept
that we have been preaching in frame design. In my opinion, it is much more
desirable to have a local failure in diaphragm than a shear wall failure.

In regard to your comment on a guideline to use in the meantime, I think the
proposal to IBC was definitely a step in the right direction. It read: "The
Rho shall be taken equal to 1.0 in the design of........Diaphragms, Except
when diaphragms transfer forces between offset lateral force resisting
systems......". Or in other words applying the Rho factor only makes sense
when the diaphragm is a transfer diaphragm (or a continuation of the VLFR
element)

Ben Yousefi, SE
San Jose, CA  

	-----Original Message-----
	From:	Charles Greenlaw [SMTP:cgreenlaw(--nospam--at)speedlink.com]
	Sent:	Thursday, April 27, 2000 12:03 AM
	To:	seaint(--nospam--at)seaint.org
	Subject:	RE: IBC, Rho Factors, 10/lw, and Influece of
FEMA/NEHRP Folks

	Thanks, Ron, for your furtherance of information on these matters.

	It sure seems to me that the Seismic Code is no less a political
football
	being struggled over, than Elian Gonzales has been. 

	Except that in the case of the Cuban boy, only himself and a few
family
	members have a material interest in the outcome. The rest of us are
	spectators. But with seismic code, all professional practitioners
subject to
	correctly heeding it have substantial, serious interests in it. But
where is
	our representation in the process? I mean the representation that
informs us
	and solicits our views, and looks out for how we will be affected in
	practice by what's adopted. We designers out here are a special
interest
	group, in many ways as the prey of predators; who's our lobbyist?

	Next: One of your Rho factor points troubles me. I do not recall
reading in
	the Blue Book Commentary about the diaphragm and other things being
put at
	risk by the effort, via the Rho factor, to compensate for "too few"
vertical
	elements.

	At 07:18 AM 04/26/2000 -0700, you wrote:
	...>The second technicl issue was with regard to the application of
rho to
	>diaphragms-
	>
	>When originally developed (as noted in earlier posts, I was a
member of the
	>Seismology sub-committee that did that) it was intended to apply to
all
	elements
	>of the structure, including diaphragms.  In fact, at one point, we
considered
	>including in the rho formulation , a consideration of diaphragm
configuration
	>relative to placement of walls, but could not figure out a way to
do this.  The
	>behavior of diaphragms in non-redundant structures is significant.
Failure
	of a
	>number of parking garages in the Northridge earthquake occurred
because the
	>diaphragms were asked to work too hard to bring shear forces to
relatively few
	>walls. This was specifically what the committee had in mind when
the rho
	>provision was first developed.
	>
	>Now - if the rho applies only to vertical elements of the
structure, and
	not the
	>diaphragms as proposed by the current SEAOC Seismology Committee,
you run the
	>risk of creating relatively strong walls (designed with a large
rho) and
	>relatively weak diaphgarms (designed with rho =1).  Since the load
applied to
	>diaphragms is a direct function of the strength of the walls, this
can lead
	to a
	>situation of diaphragm failures, just like the Northridge
structures.
	The IBC
	>action was, in my opinion appropriate.
	-----------------------------------------------

	Let's see.... You want us to put the Rho factor escalator on
everything in
	the building, when it's a "shortage" of wall panels or frames that
	supposedly harms redundancy and reliability, and makes you add
design load
	to them (rather than other improvements) in compensation for their
presumed
	shortcomings.

	Once you have compensated for this unreliability and/or
understrength, you
	should be back at parity. But you figure that you have
overcompensated and
	now caused the whole building to take a heavier hit, as a result of
it maybe
	not being fit for the "standard" hit, due to too few resisting
elements.  

	The example Northridge State U. (?) parking garages are said to have
had
	diaphragms that were too weak for the actual locations of the (too
few)
	vertical resisting elements. That sounds to me like a blunder in
diaphragm
	design (which was thin concrete topping on precast planks, yes?) or
in code
	for design of diaphragms of this sort, assuming faulty execution of
	construction has been ruled out. If none of these, then it appears a
	properly designed and constructed diaphragm will fail both when a
Rho factor
	should have been used for the vertical elements but wasn't, or when
it
	really was used. Damned either way! 

	But there's more: ANY intended or unintended overstrength in
vertical
	resisting elements would, by the reasoning you gave, warrant
stronger
	horizontal diaphragms. As such, diaphragm strength should be raised
for all
	structures across the board, not just because of excessive
	over-strengthening of walls and frames by means of a Rho factor.
Anything
	less is to turn a blind eye to the whole picture. A structure not
needing a
	Rho factor for its vertical elements would appear to need one anyway
for its
	diaphragm, as a direct consequence of the sound vertical system. 

	Lastly, whatever was under consideration in committee for "diaphragm
	configuration", that didn't mature enough for codification in the
Rho
	factor, is there any guidance or tips that could be shared for
subjective
	use in the meanwhile? If there's something of concern, it would be
nice to
	learn about, even if opinion is divided into two or more views at
present.
	We can handle varieties and differences of opinion and adapt them to
our
	situations.

	Thanks 

	Charles O. Greenlaw, SE