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Re: Rigid Diaphragm

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Thanks for your prompt reply.

I am attempting to gather the relevant information.

The project is in Central America, which is a high seismic area, equivalent
to Zone 3 or 4.

Are the PCI articles by Hawkins and Gough available on the internet?

----- Original Message -----
From: Nicholas Blackburn <nblackburn(--nospam--at)>
To: <seaint(--nospam--at)>
Sent: Friday, March 09, 2001 3:36 PM
Subject: RE: Rigid Diaphragm

> A few questions:
> What is the plan geometry of the slab?
> Any openings or large reentrant corners?
> What/where are the existing lateral supports?
> What is the strength of the existing concrete?
> Is the existing slab cracked (enough to substantially reduce the
> or uncracked?
> What Zone is the building and what is the soil type, distance to nearest
> fault etc?
> Given reasonable answers to the above, a topping slab could be used.  You
> would have to check the capacity of the existing system to support the
> load.
> PCI is currently running a series of articles by Neil Hawkins and S K
> discussing the use of precast lateral systems in seismic zones which would
> be a good starting point for topping slabs.
> If the existing slab is in good shape and you are adding walls it is quite
> possible that the slab will be ok in its current form.  The slab is going
> be modeled as infinitely rigid for shear distribution purposes.  We
> regularly design 5" thick slabs and treat them as rigid. Unless the
> cracking is substantial, the distribution should still be performed as a
> rigid system.
> Good luck
> Nicholas Blackburn, PE
> -----Original Message-----
> From: frp2000 [mailto:frp2000(--nospam--at)]
> Sent: Friday, March 09, 2001 2:31 PM
> 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