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WillisV (Structural) (OP)
8 Sep 10 17:00
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 (Structural)
8 Sep 10 17:28
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 (Structural) (OP)
8 Sep 10 17:43
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 (Structural)
8 Sep 10 18:30
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?
Helpful Member!  Lion06 (Structural)
8 Sep 10 18:40
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 (Structural)
8 Sep 10 21:00
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 (Structural)
8 Sep 10 21:54
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 (Structural) (OP)
9 Sep 10 7:41
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 (Structural)
9 Sep 10 8:55
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 (Structural)
9 Sep 10 9:10
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 (Structural)
9 Sep 10 9:15
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 (Structural) (OP)
9 Sep 10 9:20
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 (Structural)
9 Sep 10 9:24
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 (Structural)
9 Sep 10 12:41
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.
Helpful Member!  IDS (Civil/Environmental)
9 Sep 10 16:40

Quote:

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 (Structural)
9 Sep 10 16:50
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 (Structural)
9 Sep 10 19:00
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 (Structural)
12 Sep 10 21:14
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
Helpful Member!  BAretired (Structural)
13 Sep 10 1:26
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 (Structural)
13 Sep 10 6:30
Hurray!  At least there is one recognized code which specifically addresses what is an important issue.
cap4000 (Civil/Environmental)
15 Sep 10 11:53
I have the CRSI 2002 Design Handbook and on page 3-15 they have a 40 x 40 column with 4-#18 in the corners. Only one bar in each corner or 1% steel.
IDS (Civil/Environmental)
15 Sep 10 19:47

Quote:

I have the CRSI 2002 Design Handbook and on page 3-15 they have a 40 x 40 column with 4-#18 in the corners. Only one bar in each corner or 1% steel.

It would be interesting to know what the load capacity of such a column was, compared with one with a properly confined core.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
 

STATICPH (Structural)
16 Sep 10 7:12
the SABS 0100-1 say area 0.4 – 6% for cols in vertical,
max spacing of vert bars 300mm (out of my head), I won't bundle if I can spread them,
less grip in concrete, and I'm not genius enough to do that calc.
Anyway, we are busy adopting all the euro codes here, I don't know if you would accept the SABS
 
jheidt2543 (Civil/Environmental)
25 Sep 10 15:08
Excuse my coming late to this thread, but I have a question related to this topic, but in reverse.  The best way to illistrate my question is in the case of a metal building corner column (see attached sketch).  The columns tend to be inset pretty far sometimes and it is generally less expensive to "square off" the concrete pier with excess concrete than to form all the ins and outs for an exact pier size. So, how then do you arrange the reinforcing for the conc. col. pier?  Around the base plate where the load is or around the perimeter of the oversized pier?  
hetgen (Structural)
28 Sep 10 18:23

Eurocode-2 9.5.2-6
"Every longitudinal bar or bundled bars placed in a corner should be held by transverse reinforcement. No bar within a compression zone should be further than 150 mm from a restrained bar."
BAretired (Structural)
28 Sep 10 18:50
jheidt2543,

I think I would place the bars as shown in red in the attached.  These are not really columns, more like short pedestals.

BA

jheidt2543 (Civil/Environmental)
28 Sep 10 21:21
hetgen
The detail shows the corner bars & verticals in the 8" wall. The note calls out for this example/question that I don't show the #2 ties around the column verticals.

BAretired
I don't disagree with what you suggest, it is what I would call the traditional correct way to do a column pier.  However, much of the concrete is "excess" to what is needed to carry the loading; in fact I would guess the pier capacity is 2-3 times what is required.  The addition concrete is filled in to reduce the formwork.  So, your answer would be to reinfore the pier in the "normal" way and ignor the column base plate/load eccentricty rather then "balance" the vertical rebar around the base plate adn load.  That's the crux of my question.
BAretired (Structural)
28 Sep 10 21:48
Yes, that would be my choice.  The concrete is so lightly stressed in this situation that you need little more than plain concrete to do the job.  Balancing the reinforcement about the baseplate, in such a situation would, in my view, be unnecessary because of the extremely low stresses in the concrete.

BA

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