Reinforcement Spacing
Reinforcement Spacing
(OP)
Hello All,
When deciding the reinforcement type and spacing per foot width in slabs or footings, should you go with larger rebars with larger spacing or smaller rebars with smaller spacing? and please explain why? for example, I need 0.77 in^2 of rebar per foot width in a footing, I have a couple of options, 1) I can use 1 # 8 bar at 12" or 2) 4 # 4 bars at spacing of 2" or similar combination of smaller bars with variable spacing to get it. What's the thought process when making these type of decisions? Thanks
When deciding the reinforcement type and spacing per foot width in slabs or footings, should you go with larger rebars with larger spacing or smaller rebars with smaller spacing? and please explain why? for example, I need 0.77 in^2 of rebar per foot width in a footing, I have a couple of options, 1) I can use 1 # 8 bar at 12" or 2) 4 # 4 bars at spacing of 2" or similar combination of smaller bars with variable spacing to get it. What's the thought process when making these type of decisions? Thanks






RE: Reinforcement Spacing
Small bars give better crack control, but that is not normally important in an isolated footing.
Large bars are less labor intensive, but you need to be careful about development of the bars.
RE: Reinforcement Spacing
BA
RE: Reinforcement Spacing
Dik
RE: Reinforcement Spacing
BA
RE: Reinforcement Spacing
Dik
RE: Reinforcement Spacing
Another thing: ACI 318 minimums for T&S are not adequate to control T&S cracks in slabs and walls. You need about twice the minimum (as required by ACI 350). It can be anywhere in the slab but the closer the reinforcement is to the surface, the tighter the cracks in that surface will be.
RE: Reinforcement Spacing
Dik
RE: Reinforcement Spacing
In spread footings the structural depth to the reinforcing will vary 1" with #8 bars, but only 1/2" with #4 bars. Could make a difference in the As required in the opposite direction.
Not of consequence for retaining wall footings or very thick footings.
Mike McCann, PE, SE (WA)
RE: Reinforcement Spacing
- As few bars as possible for economy.
- spacing < 3h or 18" for the technical reasons (distribution/crack control).
- bars a lone human can carry for light residential and "detailing bars" in commercial.
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: Reinforcement Spacing
Dik
RE: Reinforcement Spacing
RE: Reinforcement Spacing
RE: Reinforcement Spacing
Dik
RE: Reinforcement Spacing
BA
RE: Reinforcement Spacing
RE: Reinforcement Spacing
There's a different standard of care between review and inspect.
Dik
RE: Reinforcement Spacing
INSPECTION = Contractor Quality Control
REVIEW = Consultant Quality Assurance taken on behalf of the owner
Silly but apparently there were problems at some point. The part that gets me is that, in Alberta, we have to sign off on schedules for the building code and the language of those very much makes it sound as though I was out there tying the rebar myself, without friends even.
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: Reinforcement Spacing
RE: Reinforcement Spacing
They also suggest the smaller bar size the better for crack control. So I would be looking at maximum spacing of 12" and probably maximum bar size of #5 for slabs
RE: Reinforcement Spacing
Dik
RE: Reinforcement Spacing
Other codes also used to use 18 - 20", but as reinforcement strength increases, this needs to reduce. ACI has not reduced this since the 70's but reinforcement strength has increased significantly.
These days in Europe/Asia/Australia, reinforcement is mostly 500Mpa, so about 72.5ksi (if I got the conversion right) and when we went to that strength the maximum spacing reduced from 18-20" to 12"!
RE: Reinforcement Spacing
So my vote is more smaller bars (but not a 2 inch spacing).
Another thing is I've trained everyone in my office to use either 6 inch spacing or 12 inch spacing, except in footings supporting CMU. It's easier to put in add bars at corners and the like.
RE: Reinforcement Spacing
I try to space rebar so it can be readily walked on, but, normally don't use 6" or thereabouts spacing. Way too close. There is no question that closer spacing is much better for performance and I typically try to space between 12 to 16 inches.
Dik