To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
Subject: RE: Ron Gallagher's Comments in SEAOC Plan Review
From: "Baltar, Joseph P SPK" <JBaltar(--nospam--at)spk.usace.army.mil>
Date: Mon, 10 May 1999 08:46:59 -0700
I'd like to ask our Canadian readers if they have any consideration for
flexible vs. rigid diaphragm action in their building
codes..................J. Baltar P. E.
> -----Original Message-----
> From: Lynn [SMTP:lhoward(--nospam--at)silcom.com]
> Sent: Saturday, May 08, 1999 10:35
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Ron Gallagher's Comments in SEAOC Plan Review
> 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
> not object.
> 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:
> > Gentlemen-
> > 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
> > above that story. Vx shall be distributed ot he various elements of the
> > vertical lateral force resisting system in proportion to their
> > considering the rigidity of the diaphragm....
> > Section 2312 sub para 6.
> > Provision shall be made for the increased shears resulting from
> > 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
> > agrandizing himself.