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Re: footings a beam or a slab?[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: Re: footings a beam or a slab?
- From: "Mike Brown" <mike.brown(--nospam--at)cshqa.com>
- Date: Wed, 30 Sep 1998 16:56:30 -0600
Marlou: Several things can be argued about this situation. To begin with the footing will act like a beam across any weak soils in the longitudinal direction (I think this issued is resolved for you). As for the reinforcing in the transverse direction, an argument can be made stating that there is no flexure in this direction (assuming the wall is centered on the footing). To back this argument up, the maximum flexural stress occurs at a distance 'd' from the face of the wall ('d' is the depth of the footing to the tensile reinforcing). With a 16 inch deep footing, d=16-3-0.25(#4bar)=12.75 inches. Assuming an 8 inch wall thickness and the wall is centered on the footing, you have 8 inches from the face of the wall to the edge of footing which is less than the 12.75 inches, therefore no flexure. The vertical forces from the wall are transferred directly to the soil without putting the footing in flexure in the transverse direction. In other words, there is no tension zone in the transverse direction. However, we do use transverse reinforcing in our footings. If anyone does not agree with the above, please respond. I would like to hear other opinions, especially if I'm not seeing the whole picture. Mike Brown, P.E. -----Original Message----- From: MRodrig273(--nospam--at)aol.com <MRodrig273(--nospam--at)aol.com> To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org> Date: Wednesday, September 30, 1998 4:14 PM Subject: footings a beam or a slab? >I have a case where I have a footing that is 24 inches wide and 16 inches >deep. I found that the footing only requred minimum steel reinforcement. In >the longitudinal direction I used 3 #5 bars and in the transverse direction I >used #4 @ 6". I was told that my #4 bars were not needed and that I can just >space them at 18 inches. This was argued that the footing acts like a beam, >which I understood, and in a beam you don't need to satisfy the 0.0018bt >critiria. But as I was reading the ACi. In the flexural reinforcement >section 10.5.4 it states that "For Structural slabs and footings of uniform >thickness the minimum area of tensile reinforcement in the direction of the >span shall be the same as that requred by 7.12. Maximum spacing of this >reinforcment shall not exceed the lesser of three times the thickness and 18". >If my footing acts like a beam, does it span the length of the footing or does >it only span the 24 inches in the transverse direction. > >What does everyone else use as the minimum reinforcement for the transverse >direction for a 24 inche wide beam. > >Thanks in Advance > >Marlou B. Rodriguez, EIT >Robert Englekirk Inc. >Honolulu, HI > > >
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