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RE: IBC and Rho factor

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Hi Ben,

Thanks for sharing this. I hope we can have open dialogue like this and
not have it come down to being considered attacks on the messenger. I
respect your thoughts and effort in the code process, but I have to
disagree with your statements on this issue.

Now for my disagreements with you and Dr Gosh (with all due respect).

A 4 foot concrete shearwall is commonly referred to as a wall pier in
the code, 1921.6.13 in UBC. As are 3', 3.5', 5',6' walls segments or
even longer depending on the thickness of the wall. On a tilt-up
concrete building these wall piers may be all that we have to resist
shear on the street side of the building. If walls of these lengths
aren't good for anything, why even have a classification for wall piers?
Of course they are good for something and in many cases they are all we
have to resist shear. Depending on their fixity (located at a window or
full height opening), these segments can take significant amounts of
shear. The rigidities of fixed-fixed wall piers can suck almost all the
load out of similarly long wall piers that are cantilevered. Then we
have a problem getting rho to work simply because of the length of the
segment being less than 10 feet. Another effect of RHO on tilt ups is
that interior braced frames (preferred by architects to keep traffic
flexibility in warehouses) are automatic RHO=1.5 and are not used unless
absolutely demanded by the Arch.

I take exception to the Tri-Chapter and your plan checking staff on
their assertion that it is not a major point of contention with
designers. To be honest, I doubt they even look at the calcs that
closely to know. I have done several homes in the bay area and have
never been asked to comply with 10/lw > 1. And also to be honest, I
doubt most designers understand the implications either, they are told
to ignore it by there boss which was the case at my previous employment.
Designers probably don't want to bring it up to plan checkers in fear of
being asked to comply with the provisions.

Almost every house with a strong wall or hardy panel (loaded close to
allowable capacity) would have a rho = 1.5 and thus force all walls in
the structure to be designed for this increase. Plan checkers tend to be
overly conservative and without any responsibility of the design, why
would they care to not to make each house a bunker since they don't have
to answer to a client or contractor. Lightly loaded is a relative term,
but if you are going on a shear per foot basis, the wall length issue
becomes even more critical. An eight foot shearwall with 450 plf of
shear is penalized while a 10'-1" shearwall on the same line with the
same shear per foot is not penalized. Why if they are loaded the same
and "lightly loaded"? We already have the 2:1 aspect ratio for walls -
shouldn't that take care of these concerns?

Try making a discontinuous upper floor shearwall calc out with a rho=
1.5 and the omega of 2.8 and try to prove the hold-down wont just blow
out the side of the foundation. I don't call this lightly loaded. Half
the engineers out there say HD14 bolted into a double 2x4 and call it a
day - This is the type of stuff plan checkers should be looking out for,
not speaking on the behalf of designers they have an adversarial
relationship with.

Why is there a need for a penalty at all when the wall is less than 10
feet long? It was never the intent. The base shear for most structures
has gone up 20% minimum from 94 UBC to 97 UBC within 10km of a fault
(almost all of the bay area). How many houses designed under 94 UBC (or
91 UBC for that matter) performed poorly? Now we are asked to design for
50% more force on top of that because SK Gosh or a committee of building
officials thinks it's "not such a bad idea"? Why do we need to invent
reasons to justify a mistake (RHO in the 97UBC was the mistake)? This is
the problem with the codification process. In this case, SK Gosh spoke
for everyone and vetoed the intent of the original provision that no
penalty should occur for a shearwall of any material shorter than 10
feet long (See Ron Hamburger's comments on this list in 2000).

Hopefully you understand my point of view as I tend to ramble. Again, No
personal attack intended, just a professional disagreement :>).

Santa Clara, CA

-----Original Message-----
From: Yousefi, Ben [mailto:Ben.Yousefi(--nospam--at)] 
Sent: Tuesday, October 01, 2002 1:58 PM
To: 'seaint(--nospam--at)'
Subject: RE: IBC and Rho factor


A point of clarification here. The limitation of Rho to 1.0 for light
shear walls was actually approved to the be incorporated to the 2003 IBC
April of 2000. Long before ASCE 7 incorporated it per your email. The
proposal was submitted by the SEAOC Seismology. It intended to include
concrete and masonry shear walls also. However, Dr. Ghosh, rightfully in
opinion, spoke against extending it to those types of walls. (what good
is a
4 ft concrete shear wall anyway?!)

On a side note:

The change has also been approved by the LA Basin Regional code Group
will be adopted by ordinance throughout southern California
We took up the same issue in our Tri-chapter (Bay area) code committee.
didn't think this is a real issue in residential (or light frame)
construction anyway, since you normally have many shear walls and most
lightly loaded. We even asked our Plan check staff to see if this is a
point of contention with designers, and all indications were that it is
And, some of the committee members were of opinion that it is not such
bad idea to penalize short walls even in light frame constrion.

Ben Yousefi, SE
San Jose, CA

-----Original Message-----
From: Allen Adams [mailto:aadams(--nospam--at)]
Sent: Tuesday, October 01, 2002 11:38 AM
To: seaint(--nospam--at)
Subject: Re: IBC "Oops" (Was Residential Design Discussion)

One of Dennis Wish's core complaints in regard to this issue is the
that the rho factor inflicts on short walls. This problem HAS been
addressed and is in the process of resolution. The Draft copy of ASCE
defines rho for walls essentially the same as in UBC 97, but then adds:
"... where the ratio 10/lw need not be taken greater than 1.0 for
of light frame construction." Once ASCE 7-02 is formally approved, it
be incorporated into the IBC.

If California would adopt the IBC, the problem that Dennis has addressed
would soon go away. But it is now a political matter, not a technical
matter: rather than directing your ire at volunteers who work on the
committees, contact your California State Assemblyman and State Senator.
They are the only ones now who can solve the problem (by getting the
to adopt the IBC). There is not a mechanism in place to make changes to
UBC (ICBO will soon be absorbed into the ICC), and I doubt the State has
the will to finance a replacement (it seems that the State was getting a
free ride on Code development). We can yell 'til we are blue in the
but it seems obvious to me that the UBC isn't ever going to be changed
again, no matter how outrageous it might be. As long as California
to stay with the UBC, SEAOC is powerless - like eveyone else - to
the problems. So direct you energy to your Assemblyman and State Senator
tell them to support adoption of the IBC (you are wasting your time
them to support changes to the UBC - it isn't going to happen). The UBC
a dead horse. I'm not saying anything about how I thing things SHOULD
be, I
am only telling it like I see it.

But somebody did address the issue of the rho factor, and they did push
hard enough to get it approved in ASCE 7. Somebody worked within the
to correct a problem. I don't know who on the ASCE committee was behind
change; maybe it was even SEAOC members (my guess is that SEAOC had a
in it). Whoever it was, they should be thanked with as much energy as
expended previously criticizing them.

Allen Adams, S.E.
RAM International

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