Reinforcement Spacing 7

[Shark4]

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

 

[dik]

North American codes stipulate 3x slab thickness <= 18" or 2x slab thickness for critical areas... The closer the reinforcing spacing and the greater the %, the smaller and closer shrinkage cracking will be.

Dik

 

[rapt]

dik,

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"!

 

[JedClampett]

One thing I like to check for is space for hooks, if your bars are hooked at the ends. For instance, a no. 8 has a 16 inch hook. It's going to be hard to fit in a 12 inch slab. No. 5's have 10 inch hooks. You can twist them a bit and fit them in a 12 inch slab.
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.

 

[dik]

Jed... I wouldn't normally use hooked bars and to put a 16" hook into a 12" slab, tilt it a bit, until it fits...

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