Reinforcement congestion - How much is too much?

[ronjon322]

Help me wrap my head around this?

I am familiar with ACI 318 and ACI 350 minimum and maximum reinforcement spacing, flexural and temp & shrink limits, shear, etc. However, I have concerns about just how much rebar congestion results in using a typical 6" o/c spacing with #6 and #7 rebar or even #8 and #9 when needed. I prefer to use 8" or 9" o/c with a larger bar size. What is the preferred and more common design practices? Nominal aggregate sizes are 3/4" - 1" dia. I get nervous about designing an entire structure using 6" o/c spacing, each way.

This is typically for in-ground rectangular wastewater treatment tanks with 18" - 30" walls and foundations and other heavy flood control hydraulic structures. Typical dimensions of structures may be 50' x 50'x 25' H or larger.

Thanks for your input.

P.S. Sorry if this is a repeat. I tried to post this earlier this morning, but it didn't seem to work.

 

[canwesteng]

I've never had issues with 6" c/c spacing, though don't go much bigger than #8. I prefer 6" spacing for large mats actually, it's easier to walk on, so a bit safer. 4" would be probably be my limit, as it gets harder to get the vibrator in.

 

[TRAK.Structural]

I've also done a lot of 6" spacing each way when needed, without much issue. Contractors who build these water bearing structures are used to heavy bars and tight spacing. The harder part is getting staggered laps and additional bars around openings.

 

[ronjon322]

Great to hear back from people with hands on physical experience. I've never gone tighter than 6" except maybe to add supplemental bars.
Question No. 2: Is 7" c/c rebar spacing ever used. I've never seen it use on any old as-built drawings. Any reason or just designer preference.
It's usually 6", 8", 9", 10" or 12"

 

[TRAK.Structural]

Never 7" or 9". I've always either used 6/12, or 8/16 on any particular job. Be consistent with modular units on the same project.

 

[Tomfh]

6” is nothing.

It’s too much when the concrete can’t get through.

 

[canwesteng]

The thing with 7" is it is just a weird spacing to lay out, since it only rolls into a foot nicely at 7' intervals. More regular spacings, you can get a tape on them say for 9" spacing, I make sure there are four spaces in a 3' length that I tape out when checking bar. I imagine that also makes it simpler to install correctly.

 

[JStephen]

I've used odd spacings before, nobody seemed to have a problem with it.
On the #8 @ 6"/4", if it gets too close, you can use a double layer each way or bundled bars.

 

[JoelTXCive]

I use 5" and 6" vertical rebar spacing on ACI 350 walls often.

For vertical bars, this is a nice setup because I can do 5" spacing on the lower half, and then transition to 10" spacing easily by letting half the bars die out. (Or I can transition from 6" to 12" spacing.)

 

[ronjon322]

I use 5" and 6" vertical rebar spacing on ACI 350 walls often.

For vertical bars, this is a nice setup because I can do 5" spacing on the lower half, and then transition to 10" spacing easily by letting half the bars die out. (Or I can transition from 6" to 12" spacing.)

Yes, does work well with transitioning.

 

[ronjon322]

Thanks everyone for the helpful hands-on experience. Much appreciated.