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Re: Rigid vs Flexible residential diaphragm discussion

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Bill Allen wrote in part:
".... IMO, it would be prudent to include an
exception for residential, wood framed structures
three stories or less...."


Hear! Hear! I applaud Bill's comments and I
generally agree with the concept.  Where are the

I would, however, offer one change to his
proposal.  I think that there should still be the
OPTION of considering the stiffness of the
diaphragm, as would still be required for other
buildings.  It is sometimes useful to use this
approach if one wants to reduce the tributary load
on a short shear wall by allowing the diaphragm to
redistribute some of the tributary force to
adjacent walls.

Regarding horizontal distribution of shear in
general, the amazing thing is how long this
requiremet has been in the code (and applicable to
wood all along), and yet has been so routinely
ignored.  It is not just the 1997 code that has
this requirement, and it didn't start in 1988
either.  The 1988 UBC simply TRIED to define what
a flexible diaphragm is, which had not been done
in the UBC before then.  If the diaphragm doesn't
calculate to be flexible, then one is REQUIRED to
"consider" the rigidity of the diaphragm.  If the
diaphragm does calculate to be flexible, then the
consideration is finished, and the implication is
that tributary loads are to be applied, since 97
UBC Section 1630.5 says in part that "At each
level....the force....shall be applied over the
area of the building in accordance withe the mass
disribution at that level."

With no offense to the original author, the
definition of "flexible" diaphragm (that first
appeared in the 1988 UBC) is not well-defined (Ed
Zacher already knows this), and therefore does
VERY LITTLE to alleviate possible liability for
those who distribute the forces based on tributary
area.  There are many, many residential buildings
where if one were to calculate the relative
stiffnesses based on the definition, the
diaphragms would NOT be deemed to be flexible.

The problem is that almost no one had performed
this "flexible diaphrgm" calculation until
recently (even though it has been in the code for
11 years), because nearly everyone relied on the
assumption that "everyone else does it this way",
and also relied on the numerous references (deemed
reliable by the reputation of the authors) that
say "wood diaphragms are flexible".  Relying on
either of these, and ignoring the code, is to
subject oneself to possible future litigation.

It is time that the code is brought back to
reality.  The code should be one step BEHIND the
state of the art, but in this instance, the code
has been AHEAD of the state of the art for some
time, and I don't see the art progressing in any
significant way with regard to this requirement,
especially for wood structures.

The calculations required to determine the
stiffness of plywood shear walls are poorly
understood and poorly defined.  In addition, the
methodology is cumbersome to apply since it
requires numerous assumptions to be made that are
not defined in the code, which is compounded by
the fact that iteration must be done to arrive at
the final "solution."

In my opinion, designing residential buildings
with wood diaps and shear walls (of 3 stories or
less) for seismic forces by the tributary force
method ("flexible" diaphragms) will result in
structures that are no less safe than if they were
designed by the more rigorous procedure.  That's
the bottom line, isn't it?

Well I've blabbed enough already....

Mark Swingle, SE
Oakland, CA

These are my own views and not necessarily those
of my employer.


    Dennis Wish wrote:

    <<This would be section 2303 of the 1991 
    UBC I could not locate my 88 code) and 
    is similar to section 1605.2.1 in the 
    1997 UBC.

    Dennis S. Wish PE>>


    and Bill Allen wrote:

    <<See also section 1630.6 (1997 UBC). 
    This is where, IMO, it would be prudent 
    to include an exception for residential, 
    wood framed structures three stories or 
    less, so that diaphragms are considered 
    flexible. Rationale? I hope that, at the 
    August 13th Seismology Committee meeting, 
    someone acts as Frank Lew's proxy by 
    standing up and shouting: "Where are the 

    I would be very interested if someone 
    could cite specific (real world) cases 
    where there was a structural failure 
    attributed solely to the fact that the 
    original analysis was performed based on 
    a horizontal distribution of forces 
    assuming the diaphragms were flexible AND, 
    based on an analysis that accurately 
    modeled the stiffness of the diaphragms 
    and shear walls, the shear walls failed 
    based on this analysis. I am not 
    interested in cases where the original 
    designer used (or misused) the "rotation" 
    provisions of past codes since this 
    provision can be addressed specifically 
    without affecting the entire methodology 
    of lateral analysis of wood framed 

    Also, shouldn't the seismic provisions 
    for wood framed structures be compared 
    to the Conventional Framing Provisions? 
    For example, if a structure was *slightly* 
    deviant from the Conventional Framing 
    Provisions and had to be designed in 
    accordance with section 1630, shouldn't 
    the structural design produce only 
    *slightly* more hardware, nailing, etc? 
    The only "anti-argument" I could see 
    here would be that a "prescriptive 
    method would be *most* severe (not less) 
    as they are in other model codes such as 
    the Title 24 Energy Compliance regulations. 
    In these regulations, there are (or at 
    least used to be) three methods of 
    compliance: Prescriptive, Points Method 
    and Computer Analysis; each progressively 
    more rigorous and each progressively less 
    restrictive. This is opposite in concept 
    from the seismic design of wood framed 

    Those who have an influence in the code 
    editing procedure and who know the truth 
    about the flaws and ridiculous 
    impracticalities of the 1997 UBC with 
    regards to wood framed construction 
    should stand up now and fix this mess 
    with an addendum.


    Bill Allen, S.E.
    Laguna Niguel, CA>>