Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Wall loading to exist. slab

[Subject Prev][Subject Next][Thread Prev][Thread Next]
This is where prescriptive and design get a bit intermixed. I was under the impression that this was under the IRC, as it sounded residential, not IBC.  In the IRC, only exterior footings need be 12" below the surface. Also, that 12" is below the "undisturbed" surface, not below the finished grade. An exception is in D1 and D2 zones, where the bottom of the thickened slab (integral interior footing) must be 18" below the slab finished surface. The IRC does require "spread footings" to be at least 6 inches thick.

My only concern would be the likely cracking of the slab away from the wall in a code-level event - the actual slab will have a decreasing areal load placed on it depending on the stiffness of the slab and the modulus of subgrade reaction. While the shear capacity is okay for punching, and the building won't fall, there may be some damage done.

>From a non-prescriptive perspective (i.e.: engineering, the stuff we were trained in), the loads are fine.  1500psf is squat nothing for many soils without known problems, which is why its the prescriptive allowable. How many of you actually use 100psf active pressure for your basement wall designs, or 85psf for your retaining walls? That's the prescriptive value for any soil with a majority of silt or clay (which around me is anything that doesn't come out of a quarry).  You should also consider that the effective bearing allowable for many soils goes down with depth due to the need to support an overburden, and I have yet to read a geotech report on a small, shallow foundation building that tells me I can place a shallow footing with, say 2500psf, but I can use 3500 or 4000psf if I place the footing 10 or 12 feet below grade. They all give a single value which is depth-independent. What magic prevents the use of a 4" unreinforced footing?  You can set a wood foundation wall directly on crushed stone without a footing, does putting 2" of concrete under the wall invalidate the ability of the ground to bear the weight is did when there was no concrete? Of course not. And yet, we wonder why the general public sees us as over conservative wastes of money.

Joe, do you have any uplift concerns, or is your tension side always in compression?  Getting a good tension hold (and uplift resistance with such little floor mass) would be difficult, I would expect.

Jordan

S. Gordin wrote:
Joe,
 
According to IBC (apparently, this is what you are using), the "minimum depth of footings below the undisturbed ground surface" shall be 12 inches (1805.2).  The values of IBC Table 1804.2 presume that depth, and may be inapplicable to your case of the slab.  UBC is even more explicit about the correlation between the "minimum" values and the depth of footings.
 
The vertical load of 780 plf is not that low for a wall that currently appears not to carry any load (partition).  The currently unloaded slab under that wall will, most probably, settle and crack under such load. 
 
The lateral load of 216 plf is a substantial force that will require plywood sheathing, shear anchors, and, possibly, holdowns.  How are you going to embed those into a 4" slab?
 
I agree with Bill Allen regarding the possible repercussions of that seemingly small job.  Compared to those, pouring a new footing for such wall does not appear to be that big of a problem.   
 
Steve Gordin SE
Irvine CA
 
 
 
----- Original Message -----
From: Joe Grill
Sent: Friday, March 17, 2006 6:50 AM
Subject: RE: Wall loading to exist. slab

It’s vertical, I wasn’t very clear on that.  Also, It is a partition wall that wasn’t intended to be a bearing wall, but if the joists bear on it, it will assume some load, and that is the max load it would assume vertically.  The shear load to the wall is 216 plf and that is a wind load.

Joe

 

Joseph R. Grill, P.E. (Structural)

Shephard - Wesnitzer, Inc.

Civil Engineering and Surveying

P.O. Box 3924

Sedona, AZ  86340

PHONE (928) 282-1061

FAX (928) 282-2058

jgrill(--nospam--at)swiaz.com

 

-----Original Message-----
From: Mark E. Deardorff [mailto:mdeardorff(--nospam--at)burkett-wong.com]
Sent: Thursday, March 16, 2006 4:19 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Wall loading to exist. slab

 

Is that 780 plf vertical or horizontal in the plane of the wall?

 

Mark E. Deardorff, Structural Engineer
Burkett & Wong
San Diego, CA


From: Joe Grill [mailto:jgrill(--nospam--at)swiaz.com]
Sent: Thursday, March 16, 2006 10:30 AM
To: seaint
Subject: Wall loading to exist. slab

 

I am working on a small remodel.  There is a wall that has been shown as a partition wall that I would like to use as a shear wall.  The wall is supported on an existing 4” thick slab.  If the wall carries 780 plf I really don’t think that cutting the slab and adding a footing is required.  If I assume a 45 deg. distribution through the slab and add the thick of the wall (3.5”) the resulting soil pressure is only 810 psf almost half of the 1500 psf that is being used on the project.  The exist. slab is in very good condition.  Is there a reason why this just can’t be done?

Thanks,

Joe Grill

 

Joseph R. Grill, P.E. (Structural)

Shephard - Wesnitzer, Inc.

Civil Engineering and Surveying

P.O. Box 3924

Sedona, AZ  86340

PHONE (928) 282-1061

FAX (928) 282-2058

jgrill(--nospam--at)swiaz.com

 

 


-- 
Jordan
******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********