Footing Rebar - 2 Matts?
Footing Rebar - 2 Matts?
(OP)
The only time I typically specify (2) rebar matts for footings is when I'm dealing with pre-fab metal buildings with high uplift.
In this conventional building of joist and girders, I'm not anticipating uplift to be as much a requirement, if at all.
My 2-way punching shear forces me to go with 18" thick footing.
At what footing thickness might it be necessary to add a 2nd matt? And why?
In this conventional building of joist and girders, I'm not anticipating uplift to be as much a requirement, if at all.
My 2-way punching shear forces me to go with 18" thick footing.
At what footing thickness might it be necessary to add a 2nd matt? And why?






RE: Footing Rebar - 2 Matts?
RE: Footing Rebar - 2 Matts?
Wouldn't you put a top mat in for temperature steel?
RE: Footing Rebar - 2 Matts?
RE: Footing Rebar - 2 Matts?
RE: Footing Rebar - 2 Matts?
RE: Footing Rebar - 2 Matts?
1) with only bottom steel, you may get bottom side restrained, differential shrinkage curvature in the footing leading to cracking at the top. Flexure, when present, would tend to close those cracks. And I'm generally not too concerned about the cracks anyhow unless the footings are in an especially corrosive environment.
2) at some depth, perhaps the integrity of the sides of the footing becomes an issue and it would be best to have a continuous "basket" of reinforcing around the whole thing. This I don't buy at all.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Footing Rebar - 2 Matts?
In a large raft foundation it seems like positive moments would incur if the reactions on the foundation is large. A typical footing is one thing but a massive thick raft foundation I would replicate the bottom flexural steel.
RE: Footing Rebar - 2 Matts?
Dik
RE: Footing Rebar - 2 Matts?
http://www.ccaa.com.au/imis_prod/documents/Library...
RE: Footing Rebar - 2 Matts?
RE: Footing Rebar - 2 Matts?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Footing Rebar - 2 Matts?
A pile cap as you describe is similar to a continuous concrete beam / floor system with tension forces alternating between bottom of cap/slab at "mid-span" and top of cap over the pedestal supports.
This is a different situation that i will defer to the apparently more experienced elaboration of hokie... please...
RE: Footing Rebar - 2 Matts?
Did you carefully read that Leftwow is inquiring of a 100' x 100' cap?
I must be misunderstanding something...
Will there not be tension in the top half of the 2 foot thick cap requiring rebar in the system of which Leftwow inquires?
RE: Footing Rebar - 2 Matts?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Footing Rebar - 2 Matts?
RE: Footing Rebar - 2 Matts?
BSVBD has answered for me. Surely a pile supported mat slab, or an earth supported mat slab for that matter, of those dimensions, would have both top and bottom steel. My answer was about leaving out top steel where it has no benefit, and where I know plastic settlement cracking is probable.
rapt's suggestion of a mat for workers to stand on has merit, but there are other ways to do that. That is in the nature of constructability, what some here call "means and methods".
RE: Footing Rebar - 2 Matts?
RE: Footing Rebar - 2 Matts?
I don't see where any offense was implied.
It appears as though you were getting credible and considerate answers to a serious and legitimate question.