Subject: RE: IBC, Rho Factors, 10/lw, and Influece of FEMA/NEHRP Folks
From: Charles Greenlaw <cgreenlaw(--nospam--at)speedlink.com>
Date: Thu, 27 Apr 2000 00:02:57 -0700
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
At 07:18 AM 04/26/2000 -0700, you wrote:
...>The second technicl issue was with regard to the application of rho to
>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
>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
>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
>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
>situation of diaphragm failures, just like the Northridge structures.
>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
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
Charles O. Greenlaw, SE