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RE: Ron Gallagher's Comments in SEAOC Plan Review

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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)]
> Sent:	Saturday, May 08, 1999 10:35
> To:	seaint(--nospam--at)
> 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
> plywood.
> 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
> engineers.  
> 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.
> Lynn
> "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
> 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.
> >