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RE: Rigid Diaphragm

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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 stiffness)
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 extra

PCI is currently running a series of articles by Neil Hawkins and S K Ghosh
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 to
be modeled as infinitely rigid for shear distribution purposes.  We
regularly design 5" thick slabs and treat them as rigid. Unless the existing
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 to
make it "more" rigid", to distribute seismic loads to new shear walls

Is this possible?? If so, how??

jim k