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# RE: IBC and Rho factor

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: IBC and Rho factor
• From: "Structuralist" <dennis.wish(--nospam--at)verizon.net>
• Date: Wed, 2 Oct 2002 14:29:07 -0700

```Ben,
I'm no longer an SEA member so I won't be receiving the notes from this
presentation. What I want to ask is this:

1. The 20% increase that you are refering to is the difference between
2.5Wd/(1.4R) and 3.0Wd/(1.4R) I take it???

2. Don't you require V=3.0Wd/(1.4R) to be multiplied by a Rho of 1.5?

3. Do you consider plywood covered in lightweight concrete to be
flexible or rigid?

4. Doesn't the code still require the engineer to calculate the
deflection of the diaphragm before assuming it is flexible and using the
Simplfied Static Method? This was my interpretation of the code as I
could not find any specific wording that stated the engineer may use
flexible diaprhagm analysis for a wood structure three stories or less
based on Simplified Static proceedures. My interpretation has always
been that the engineer must first calculate the diaprhagm deflection. Is
the assumption of flexible diaphram only the Tri-County's comprimise on
the issues?

Thanks
Dennis

-----Original Message-----
From: Yousefi, Ben [mailto:Ben.Yousefi(--nospam--at)ci.sj.ca.us]
Sent: Wednesday, October 02, 2002 11:56 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: IBC and Rho factor

Dennis

We are still using our Tri-Chapter policy which requires increasing the
base shear by 20% if you do not wish to verify diaphragm flexibility and
would assume flexible diaphragm. We will soon be using the IBC criteria
which pretty much says the same thing, which is if you design per
simplified method 1630.2.3 (20% increase in base shear) you can use the
flexible approach for untopped steel decks and plywood diaphragms. I am
not reviewing many calcs directly, but based on my assessment from our
plan check engineers, most are using that approach.

There was an interesting presentation on this issue in the SEAOC
convention. You should be getting a copy of proceedings if you're a
member. The SEAONC had done a survey of building departments in the Bay
area and the majority are still allowing the flexible approach with or
without a written policy.

Kelly Cobeen, the program manager for the CUREE wood frame project also
opined on this issue, that based on the research so far, for majority of
the wood framed buildings, the flexible approach would yield acceptable
results and in reality could result in a better design, since you would
tend to distribute the vertical LFRS in a more reasonable manner.

The last bullet of the SEAOC presentation on this issue was:

"many engineers continue to be RIGID! in their opinions on this issue"

Ben Yousefi, SE
San Jose, CA

-----Original Message-----
From: Structuralist [mailto:dennis.wish(--nospam--at)verizon.net]
Sent: Tuesday, October 01, 2002 11:40 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: IBC and Rho factor

Ben,
>From your experience in plan check, what percentage of light-frame
designs (residential, multi-residential, commercial and industrial) less
than three stories in height and designed with wood exterior walls and
wood diaphragms (with or without interior partitions) are designed using
consideration for rigid diaphragms verses those that are designed
specifically using the previous UBC methods considering the diaphragm as
flexible? Also, how many (before you provide the corrections) are
designing using the Simplified Static design assuming a flexible
diaphragm but increasing the base shear by a Rho of 1.5 and increasing
the Base shear calculation using a variable of 3.0 rather than 2.5?

Dennis

-----Original Message-----
From: Yousefi, Ben [mailto:Ben.Yousefi(--nospam--at)ci.sj.ca.us]
Sent: Tuesday, October 01, 2002 1:42 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: IBC and Rho factor

Allen

A point of clarification here. The limitation of Rho to 1.0 for light
frame shear walls was actually approved to the be incorporated to the
2003 IBC in April of 2000. Long before ASCE 7 incorporated it per your
email. The proposal was submitted by the SEAOC Seismology. It intended
to include the concrete and masonry shear walls also. However, Dr.
Ghosh, rightfully in my 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
and will be adopted by ordinance throughout southern California
jurisdictions. We took up the same issue in our Tri-chapter (Bay area)
code committee. Most didn't think this is a real issue in residential
(or light frame) construction anyway, since you normally have many shear
walls and most are lightly loaded. We even asked our Plan check staff to
see if this is a major point of contention with designers, and all
indications were that it is not. And, some of the committee members were
of opinion that it is not such as bad idea to penalize short walls even
in light frame constrion.

Ben Yousefi, SE
San Jose, CA

-----Original Message-----
Sent: Tuesday, October 01, 2002 11:38 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: IBC "Oops" (Was Residential Design Discussion)

One of Dennis Wish's core complaints in regard to this issue is the
penalty 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 7-02 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 buildings of light frame construction." Once ASCE 7-02 is formally
approved, it will 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
State to adopt the IBC). There is not a mechanism in place to make
changes to the 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 face, 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 chooses to stay with the UBC, SEAOC is powerless - like
eveyone else - to correct 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 telling them to support changes to the UBC -
it isn't going to happen). The UBC is 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
it hard enough to get it approved in ASCE 7. Somebody worked within the
system to correct a problem. I don't know who on the ASCE committee was
behind the change; maybe it was even SEAOC members (my guess is that
SEAOC had a hand in it). Whoever it was, they should be thanked with as
much energy as was expended previously criticizing them.

RAM International

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