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Re: Rigid Diaphragm

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The framing is reinforced concrete.

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

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

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 ??

----- 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