Max bar spacing in concrete columns 4

[WillisV]

Is there a limit in ACI pr elsewhere for the maximum spacing between longitudinal reinforcing bars in a column resisting only compression loads? For instance if I have a 48"x48" column, is it allowed to only place say bundles of 4 no. 11 bars in each corner without any reinforcement in between?

 

[DaveAtkins]

You should provide the required minimum area of steel for a compression member--0.5% or 1% (engineers disagree on this, but I use 0.5%).

Either way, 4-#11 is not adequate.

DaveAtkins

 

[WillisV]

Dave - of course - that's why I said bundles of 4 no 11 in each corner (16 total) - but the specific example is not really relevant to the intent of the question.

 

[frv]

DaveAtkins-

Why do you say engineers disagree on the min. reinforcement, I think ACI 10.9.1 is pretty clear that it's 1%. Is this one of those relatively recent changes to the provisions that has older engineers crying foul?

 

[Lion06]

I don't know of a provision that limits it, but I would look pretty hard before doing something like that. The only limitation I know off the top of my head is if the clear spacing is greater than 6" then every longitudinal bar needs to be enclosed by a 135 degree hook (or better).

 

[hokie66]

I thought there was a limiting provision on spacing, but I can't find it. The main reason for distributing the reinforcement along the faces is to have bars for terminating lateral reinforcement. We often think of ties only in terms of restraining the vertical bars, but they also confine the core concrete, making the column behave in a more ductile manner. This is especially important in high strength concrete columns.

 

[JAE]

ACI has a maximum limit on slab rebar spacing (18").

I usually limit the space between bars at about 12" on a column...no specific reasn - just a feel good.

 

[WillisV]

So the consensus appears to be basically what I thought - there is no limit - it is up to judgment as long as you can meet minimum reinforcing.

So my initial example of a 48x48 with bundles of 11s in each corner is code compliant - but maybe not a good idea!

 

[DaveAtkins]

WillisV--

Sorry! I didn't read the word "bundles."

frv--

The Code says that if a column which is half the area of the column you are designing would theoretically be adequate, you only need to use half of the normally required area of steel.

DaveAtkins

 

[ToadJones]

Interesting discussion here. I have wondered the same thing many many times.
I have even thought the same about T&S steel. Intuitively it seems more logical to me to have the steel even dispersed.

 

[paddingtongreen]

Why bundles? development lengths suck and the splices are murder.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[WillisV]

ARRRGHH!! it was just an extreme example that met minimum reinforcing requirements. Pretend I put a No. 18 in each corner - doesn't matter - the point is there are no code-specified maximum spacing for bars in columns beyond the practical limitations of meeting minimum reinforcing ratios.

 

[steellion]

For compression loads, location of the bars wouldn't matter. For flexure loads, location of bars does matter but only in relation to how close the bars are to the edge, and you've got that covered.

So really it comes down to judgment, but I'm not sure why you would bundle the bars when you have plenty of room to spread them out along the column perimeter. It's just not standard practice.

 

[JAE]

WillisV - I think ACI 10.9.2 is the most applicable section dealing with your question. I don't believe there is a stated maximum spacing on bars in compression members.

If your column receives large bending moments and is a beam-column element, then perhaps sections 10.6.4 and 10.6.6 might apply.

 

[IDS]

For compression loads, location of the bars wouldn't matter.

I can't agree with that. Confinement of the core is an important requirement, and a the core of a large column with bars only at the corners will not be adequately confined.

The requirement to provide confinement to every bar when the spacing exceeds 150 mm but every other bar when the spacing is below 150 mm suggests to me that the intended maximum spacing is 300 mm, even if the code does not explicitly state that.


Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[msquared48]

I full agree with IDS on the confinenment issue here. There will be little to none between the column corner bundles without additional verticals with the associated confining crossties.

Spread out the vertical steel you need - 12" max as JAE suggests is a good start. Blown out columns are not good for your reputation.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

The importance of confinement of the core is clear when you consider how a circular column with spiral reinforcement performs. The core can fail only when the confining spiral yields in hoop tension. Studies have shown that, for a given mass per length of column, spiral steel is at least twice as effective as longitudinal bars in increasing column capacity.

 

[dik]

I've done 24x24 with corner bars only numerous times. For 48x48, I likley have a face bar with a second set of ties. Even at 48x48 with corner bars only, the ties will be in tension, but I'd spend the extra few dollars for the added ties...

Dik

 

[BAretired]

CSA A23.3-94 Clause 7.4.1.3 states "The clear distance between adjacent longitudinal reinforcing bars in compression members shall not be greater than 500 mm (say 20").

BA

 

[hokie66]

Hurray! At least there is one recognized code which specifically addresses what is an important issue.