Need a book? Engineering books recommendations...

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

RE: depth of footing above bottom reinforcement

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Title: Message
I won't bother with whether or not one should consider a slab as a "structural foundation", but instead avoid that mess and just stick to ACI 318.  ACI 318 tends to make the issue of slabs on grade easy...it is does NOT cover slabs on grade...per section 1.1.6.  Now, it does get a little bit "murky" when you consider that it specifically states: "This code does not govern design and construction of soil-supported slabs, unless the slab transmits vertical loads or lateral forces from other portions of the structure to the soil."  That last part kind of mucks things up a bit.  Would this mean that a load bearing light gage wall on a slab "force" that slab to be under ACI 318 since it would be transfering load from "other portions of the structure" (i.e. transfering load from a roof or floor that the load bearing wall supports)?  But, then a slab that only supports a storage rack NOT be per ACI 318 (after all, it would be taking the "load" directly, unless you consider the storage rack to be "part of the structure")?
 
Now, in relation to the original provision, it would NOT apply to "slabs" anyway as section 15.1.1 specifically limits chapter 15 of ACI 318 to isolated footings, combined foots and mats.
 
As to the original question, I would say it is pretty clear that the dimension in question is the minimum depth from the bottom most layer of reinforcement, which will be "d", unless there are multiple layers of bars.  After all the provisions specifically states: "Depth of footing ABOVE BOTTOM REINFORCMENT shall not be less than 6 in for footings on soil, nor less than 12 in for footings on piles." (emphasis is mine).  The point is the provision specifically goes out of its way to define where the depth of footing is relative to...the bottom reinforcement.  Now, it is not clear if that means location of centroid of bottom reinforcement or absolute bottom of reinforcement.  For one layer of bottom steel, the difference will be neglibile...for multiple layers of bottom steel, it would still be small, but not as small as one layer.
 
If you really want to know the intent behind the provision, I could see what I can find out.  I likely would not be able to get an answer for a little bit...the Fall ACI Convention is taking place right now in St. Loius, so I don't know how quick of a response I will get from my friends/contacts.
 
Regards,
 
Scott
Adrian, MI
-----Original Message-----
From: Tom.Hunt(--nospam--at)fluor.com [mailto:Tom.Hunt(--nospam--at)fluor.com]
Sent: Friday, October 31, 2008 6:29 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: depth of footing above bottom reinforcement


Chris,

I am not sure what the original intent of this requirement is.  May be Scott can find out from one of his committee friends.  Most building slabs are not considered structural foundations so it may or not apply to some of the conditions you site.  If I was to guess, I would say this requirement just comes from experience that there should be some type of minimum thickness requirement for footings and pile caps just like a lot of other minimum concrete requirements such as minimum temperature steel.  May be it was just a bunch of old farts from the 1940's smoking cigars around the table and thought it was a good idea?!?

Thomas Hunt, S.E.
Fluor



"Christopher Banbury" <christopher.banbury(--nospam--at)gmail.com>
10/31/2008 08:10 AM
Please respond to seaint
To
seaint(--nospam--at)seaint.org
cc
Subject
RE: depth of footing above bottom reinforcement





So is this specification intended to be a cover requirement for the steel? What is its purpose? Does it matter if there is no earth above the footing as in monolithic construction? Could this be interpreted to prevent the use of structural slabs on grade used to support industrial racks; or monolithic footing & slab mat construction where the slab is reinforced? What of interior monolithic wall footings for light gauge steel partions as in slab construction for self storage facilities?
 
Thanks again. These are all hypothetical questions as this point. I'm just researching the history and application of this requirement.
 
 
Chris/David,

I would disagree and say it is a just a dimension/distance value of 6 inches or 12 inches and has nothing to due with "d" (i.e. distance from extreme fiber.....).  So if you had a pile cap with #8 bar each way for bottom reinforcement and 4 inches of cover then your overall depth (h) of the pile cap would need to be a minimum of 18 inches (4" clr + 2 -#8 bar + 12").


Thomas Hunt, S.E.



--
Christopher Banbury

Ark Engineering, Inc.
PO Box 10129, Brooksville, FL,34603
(352) 754-2424

------------------------------------------------------------
The information transmitted is intended only for the person 
or entity to which it is addressed and may contain 
proprietary, business-confidential and/or privileged material.  
If you are not the intended recipient of this message you are 
hereby notified that any use, review, retransmission, dissemination, 
distribution, reproduction or any action taken in reliance upon 
this message is prohibited. If you received this in error, please 
contact the sender and delete the material from any computer.  

Any views expressed in this message are those of the individual 
sender and may not necessarily reflect the views of the company.  
------------------------------------------------------------