Concrete rebar ?

[EightFingers]

I'm getting ready to pour four short square concrete columns 18 x 18 x 24" deep. All but six inches will be in solid rock that I've already jackhammered out.
What I need to know is how much and where to put the rebar. At first I figured on dropping in 4 18" sticks of #3 going no closer than 3" from the outside.
When I googled it, I got a lot of stuff either for huge projects or theoretical essays from universities....
Any suggestions from someone with experience?

[Wild Turkey]

What are the columns going to hold?

Rebar give a concrete structure tensile strength so what forces might try to pull the concrete one way or the other?

If you're just setting something on top of them you might just need some rebar sticking up to attach things to.

[EightFingers]

They will hold posts to a pergola. It will measure 12 x 18 feet. Posts will be about 12 feet high.
So far, the maximum wind speed here was about 100 mph.
Maximum earthquake was 5.2.

But that could change

[blackhat]

How will the posts connect to the concrete? I would look at post bases with j bolts into the column. If the bolts are 2/3 or so of the concrete depth I likely wouldn't add anything else. Why square?

[EightFingers]

I'll be using 8" j-bolts. I think they way overdo stuff in Hawaii. I agree in rock the concrete should well supported. I'll be calling for an inspection so I'll ask one of those guys.

[Splinter Puller]

From how you describe the setup the only real loads your "footings" will need to handle is potential wind uplift. I'd got with your plan of 4 vertical boards a couple inches in from the corners. A couple bars would work or even longer anchor bolts.

[theeviltwinn]

It's a pergola. No rebar needed. Even 18 x 18 is overkill.

Twinn

[Herb G]

I never built anything with anything bigger than 1/2" rebar.
This included driveways & sidewalks.

[mike4244]

Herb G said:

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I never built anything with anything bigger than 1/2" rebar.
This included driveways & sidewalks.

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The OP is using #3 rebar,also known as temperature bar for some reason. #3 is 3/8" in diameter. 1/2" is #4 rebar etc.
mike

[Robert Adams]

I'd just do a couple sticks of 1/2" or... If you are bolting it down just get a couple long j bolts like the ones for street lights and make a plywood plate to hold them in place and stick them in after pouring the concrete.

Since basically everything is in rock I wouldn't worry too much about rebar. Course I have been known to over do it and do something like use 4 long sticks of 1/2" and use 3/8 rings and wire it all together and drop it in the hole first. Problem is without a vibrator it's easy to get air pockets.

I used to live on Oahu and I remember how great the soil was there. Haven't been back since 89 so it's past time to go. Course all the places I liked going to are now gone.