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RE: minimum fc for Residential slab on grade

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Dennis:
 
Want to see some magic?
 
The code makes a distinction between design strength and specified strength.  Design based on no greater than 2500 inspection is not required.  UBC 1701.5.  Then specify that 3000 or 4000 psi be used to pour the concrete if you want.  UBC 1921.2.4 .  The idea is not to penalize someone who just wants to pour higher strength concrete if there is no life safety issue.
 
Our practice is to design and specify no less than 2500 psi.
 
Regards, George Richards, P. E.
-----Original Message-----
From: Dennis Wish [mailto:Dennis.wish(--nospam--at)verizon.net]
Sent: Wednesday, July 09, 2003 2:28 PM
To: light_framing(--nospam--at)structuralist.net
Cc: seaint(--nospam--at)seaint.org
Subject: minimum fc for Residential slab on grade

I designed a single story residence (spec home but large) using a slab on grade which is typical in this area. Historically, we placed f'c=2,000 psi concrete for everything but where proprietary frames such as Hardy Frames are used. The typical soil in the area is silty sand with a bearing pressure of 1,500 psf and with increases allowed for depth and width.
I received a plan check correction based on the 1998 CBC and 1997  UBC. He quoted section 1921.2.4 which requires high strength concrete but has an exception that allows for concrete not less than 2,500 psi. This is generally required when inspection of the concrete is called for and not for simple slab on grades. The city has, during the 1997 UBC cycle allowed for f'c=2,000 psi concrete, but the contract plan check company requires 2,000 psi concrete so as not to force deputy inspection.
 
Are there any comments as how I should address the plan check correction since deputy inspection can be expensive and there are no problems with subsidence or expansive soils in the area.
 
Am I wrong on this one????? If so, it comes as a surprise.
 
Thanks
Dennis