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RE: seismic cracking of SOG

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You don't say how thick the slab is or whether it was refinforced with bar or mesh.  Is it possible that it is a very thin slab, with maybe too light a mesh?
 
Unless the slab was poured over an old foundation or some uneven ground profile that was levelled up with dry sand - or some such explanation, your scenario sounds most plausible.
 
If the uneven slab is not an issue, then sealing it would probably work but if you need a level floor, you will have to do something with it.  How about levelling it up with a good concrete and mesh.
 
Sharlie Huffman, P.Eng.

Ministry of Transportation
Bridge Seismic Engineer
4B - 940 Blanshard
Victoria, BC V8W 3E6
(B) 250-370-2566
(F) 250-387-7735

-----Original Message-----
From: Steve Gordin [mailto:scgordin(--nospam--at)hotmail.com]
Sent: Monday, December 29, 2003 3:26 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: seismic cracking of SOG

Gail,
 
I could not see rebars - it would require too much demolition.  I would assume they were there, first, because it was a high-end house (for what it's worth), but mostly, because I've seen too many cracked SOG's with and without rebars. 
 
These cracks are different - approximately straight and parallel, at, say, 48" or more, but without regular spacing and correlation to the footprint layout (at an angle). They exist throughout the house and patio, but not in the exposed garage slab (in that, there are several definitely old cracks of different /"regular" appearance, similar to cold joint distortion). 
 
The single-level slab is divided into several "sub-slabs" with elevations varying 12 inches max. and steps between them.  All steps are cracked horizontally.  
 
All cracks of interest are definitely fresh; any relevancy to columns, drains, conduits, etc. is unlikely.  
 
The best scenario I could come up with is the slab was poured against the surfacing inclined shale that was disturbed by the intensive seismic movements.  That would be consistent with the appearance of the cracks. 
 
But if so - what (if anything) should be done?
 
SG 
 
----- Original Message -----
Sent: Monday, December 29, 2003 2:51 PM
Subject: Re: seismic cracking of SOG

My first guess would be that the cracks were linear because they were occuring over reinforcing steel.  But if you are getting 1/2 in. offsets at the cracks,  there is likely no reinforcing perpendicular to the cracks,  and probably none in the other direction either.  And if that isn't a plug for slab on ground reinforcement, I don't know what is.  Fibers aren't going to do anything to prevent faulting.

What is the spacing of the cracks?

Is it possible the cracks were already there (covered with tile) but weren't visible until they opened wide enough to cause faulting?

Is there conduit or something in the slab that could cause cracking?  Is the cracking actually following some weakened plane caused by columns or drains or some other embed?

Gail Kelley