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RE: Rigid Diaphragm

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-----Original Message-----
From: frp2000 [mailto:frp2000(--nospam--at)]
Sent: Saturday, March 10, 2001 10:40 AM
To: seaint(--nospam--at)
Subject: Re: Rigid Diaphragm


The framing is reinforced concrete.

******** My question was really aimed at finding out how the 4" slab is
supported.  4" is thin for an R/C slab, and unless the frames are spaced at
about 10 feet on center, I'm assuming that you have secondary framing (e.g.
joists).  Is this the case, or is the 4" slab framing between closely spaced
beam-column frame lines.  This may be important, if R/C joists are present,
since they will add to the slab section and should be accounted for in the
analysis assumptions.

They don't want to increase the dead load by adding a concrete topping.

******** Why?  It may very well be that this is a parameter that they THINK
they want to impose (btw, who is "they"?), but at the beginning of a
problem, I find it is best to leave ALL options open in order to increase
the chances of finding the most efficient (structurally and usually more
importantly ECONOMICALLY) solution.

In your 2nd scenario, do you mean to install diagonal bracing, as is
sometimes done with shear walls ??

******** If you have soffit space under the slab, then adding a steel
bracing system (such as an X brace with the braces terminating at the
columns) may be a viable solution.

As to your last question, how does one decide when a slab is not rigid ??
Are there degrees of rigidity ?? Is there a deflection limit that separates
rigid from flexible ??

******** 1999 (7th) Edition of the Bluebook, section 105.6 on page 13
defines a flexible diaphragm as one where the max lateral deformation  of
the diaphragm is more than two times the average story drift of the
associated story (the diaphragm "bows" out to a point 2x the average drift
of the story itself).  Note that this is a definition of convenience, and
you will need to look at the system you have to determine if this is
applicable.  Furthermore, if you have a structural plan configuration
wherein the aspect ratio is slender, then you may get a lot of cracking
under the maximum deformation under EQ loading, and the section modulus (in
plane) of the diaphragm should be reduced to account for this (resulting in
increased deflections).

Perhaps you could give us a bit more detailed description of the building -
bay spacing, height, approx member sizes, framing layout etc.

Hope this helps anyway.

T. Eric Gillham PE

----- Original Message -----
From: T. Eric Gillham PE <teric(--nospam--at)>
To: <seaint(--nospam--at)>
Sent: Friday, March 09, 2001 3:46 PM
Subject: RE: Rigid Diaphragm

> 4 inches thick sounds pretty darned thin.  Is there a joist system or
> something along those lines?  What type of framing supports the 4 inch
> slab, and if it is concrete (joists for example) how about accounting for
> those in your calcs?
> It also depends on the configuration of the slab.  What is the aspect
> Depending on the configuration, you may want to :
> 1) Thicken the slab
> 2) Create a steel bracing system under the slab using rolled sections to
> take the in plane shears to the shear walls
> Or a combination of the two.
> First place to start I guess would be with the question "on what
> basis is the conclusion that the slab is too flexible being based?"
> Hope this helps.
> T. Eric Gillham PE
> -----Original Message-----
> From: frp2000 [mailto:frp2000(--nospam--at)]
> Sent: Saturday, March 10, 2001 8:31 AM
> To: seaint(--nospam--at)
> Subject: Rigid Diaphragm
> I have a situation where an existing slab is 4" thick. Owners would like
> make it "more" rigid", to distribute seismic loads to new shear walls
> equally.
> Is this possible?? If so, how??
> jim k