Subject: Re: Ron Gallagher's Comments in SEAOC Plan Review
From: Lynn <lhoward(--nospam--at)silcom.com>
Date: Sat, 08 May 1999 10:35:01 -0700
Okay, refer to section 2513 of the 1988 UBC, second to last paragraph.
"In masonry or concrete buildings, lumber, plywood and particleboard
diaphragms should not be considered as transmitting lateral forces by
rotation". In other words, it does not behave as a rigid diaphragm.
The paragraph directly above this one does allow for plywood diaphragms
to transmit lateral forces by rotation for wood framed buildings, but
with severe limitations on the aspect ratio of the diaphragm and the
number of story's of the building.
If your diaphragm calculates out as "rigid" per section 2312 of the 1988
Code, then why did the same Code not allow you to use the plywood
diaphragm as rigid in your design unless it also met the very
restrictive requirements of 2513? Bottom line is it just did not make
sense. One section of the Code says it is rigid, the other says it is
not. Given the apparent discrepancy, it was always thought that is was
"conservative" to just consider the plywood diaphragm as flexible.
And the truth of the matter is that almost ALL design engineers using
the 1988 UBC assumed their plywood diaphragms were flexible. It was
"assumed" that 2312 applied to metal decks or other diaphragms, not
To now make the claim that since 1988 we should have all been
considering that our plywood diaphragms may have to be considered as
rigid in our design is ridiculous. We ALL know (or at least I hope we
all know) that this is not the case. It just simply wasn't done, and
still to this day is not done by the overwhelming majority of design
Can you imagine trying to get a DSA (at that time OSA, school) or OSPHD
(hospital) project approved if you had tried to use a plywood diaphragm
as rigid? We have a project right now in for DSA review and I spoke
with the chief plan checker about this issue, and they still will be
very hesitant to accept a plywood diaphragm as rigid. He basically told
me that I had better be able to show that the lateral system can work
considering the diaphragm as flexible. If I wanted to strengthen
elements to consider the possibility that it acts rigid that he would
Lets not muddy the waters here. Nearly all practicing design engineers
have been designing plywood diaphragms as flexible since the invention
of plywood. There have been provisions in the code for many years that
say using plywood diaphragms to transmit lateral loads by rotation is a
no-no unless some very restrictive requirements are met. I believe the
City of LA and County of Ventura even made those restriction more
restrictive after the Northridge earthquake.
"Ron O. Hamburger" wrote:
> Here's the facts with regard to the rigid diaphragm requirement,
> Refer to your 1988 UBC (yes that's right 1988!!)
> Section 2312 sub para 5.
> The desgin story shear Vx, in any story is the sumof the forces Ft and Fx
> above that story. Vx shall be distributed ot he various elements of the
> vertical lateral force resisting system in proportion to their rigidities,
> considering the rigidity of the diaphragm....
> Section 2312 sub para 6.
> Provision shall be made for the increased shears resulting from horizontal
> torsion where diaphragms are not flexible. Diagphrams shall be
> considered flexible for the purpose of this paragraph when the maximum
> lateral deformation of the diaphragm is more than two times the average
> story drift of the associated story....
> Seem familiary????
> Also Ron Gallagher is not the Seismology chair. So - I don't think he was
> agrandizing himself.