Permissible margin in selection of required column reinforcement 1

[PSEPK]

Hi All,

After designing a concrete column, we get required area of reinforcement. While selecting suitable bar size and number, sometimes the actual cross-sectional area of selected bar size & number combination is slightly less than the required reinforcement area (calculated either manually or obtained from a structural software like ETABS, SAP2000 etc.). For example, required vertical reinforcement area for a certain column in a multi-story (6 or more stories) is 3.37 percent, whereas actual area of my selected bar combination is 3.27 percent of cross-sectional area.

Design is based on ACI 318, with grade 60 reinforcement, and 3 ksi concrete.

In this connection, I am curious to know:
a. Should I use the selected bars with slightly less rebar area, or go for larger bars , or larger column size?
b. What others do in such a situation, especially when next higher combination may require use of a larger bar size, not otherwise used in the project, or use of a larger column.
c. What is the maximum margin, that can be utilized in such a condition, without compromising overall strength and stability of a structure?
d. Is there some definite figure for this kind of margin (e.g., 2%, 3% etc), written or referenced in some code or guideline?

Thanks.

A good structural engineer is often a blessing for others.

 

[hokie66]

The difference in .0327 Ag and .0337 Ag is not worth worrying about.

But if you are interested in economical column design, this amount of reinforcement is too much. You can achieve better economy by using higher strength concrete, and limiting the amount of reinforcement. I would never use less than 4000 psi concrete in columns.

 

[PSEPK]

hokie66,

The reason for using 3,000 psi concrete is that both the clients & the contractors in our area, are more comfortable with it, as it does not require a special mix design, etc.

Thanks for your response & advice.


A good structural engineer is often a blessing for others.

 

[JAE]

a. Should I use the selected bars with slightly less rebar area, or go for larger bars , or larger column size?
[red]I would never use less rebar than required. The code provides minimum design values. [/red]
b. What others do in such a situation, especially when next higher combination may require use of a larger bar size, not otherwise used in the project, or use of a larger column.
[red]The preference, for economy, is what hokie stated - use as few bars as possible to reduce the required amount of cross ties, etc.[/red]
c. What is the maximum margin, that can be utilized in such a condition, without compromising overall strength and stability of a structure?
[red] Not sure, that would depend on the true variability of your loads and design accuracy/assumptions.[/red]
d. Is there some definite figure for this kind of margin (e.g., 2%, 3% etc), written or referenced in some code or guideline?
[red]No

...and 4,000 psi doesn't require any special mix design. For many building structures I've done I've used 5,500 to 6,000 psi concrete for the columns.[/red]



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[KootK]

I know of no code reference but 5% +/- is fine by me. I don't feel that we "know" anything to an extent that would justify greater precision than that. This, especially with concrete columns where your moments and rebar stresses are going to be significantly impacted by creep etc.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JStephen]

That general question (Item X is overstressed 2%, do I use it or not) has been kicked around here some before, with varying opinions.

Generally, on knowing the actual forces on a structure versus the calculated- you're looking at +/- 50% or more, particularly if wind or seismic are involved.

On the other hand, if you're writing a code, you can determine compliance with it to 15 decimal points if so desired.

And different people think differently as to the best compromise between these. Sadly, everyone does not all agree with me.

If you specify a tolerance- 5% say- then somebody else is going to come along and say, "My column area is 5.49% too low. Is that close enough to 5%?"

And then, too, whatever arguments you make for accepting 5% deviation are pretty much equally valid for 6% deviation or 8% deviation...

 

[KootK]

Part of setting own your personal tolerance, even if it's none at all, is sticking to your guns with regard to what it is and what it is not.

Does it embody some rationality? Yes.

Is it overtly scientific or probabilistic? No.

Does it obligate me to extend the margin when pushed? Certainly not.

All roads lead back to engineering judgement and integrity.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[IDS]

Essentially I agree with JStephen. There has to be a definite cut off somewhere, and there is no logical reason to say that it should be 95% of the code design strength, rather than 100%.

However, if you have something in your "back pocket", so that you can show the design does fully comply with code requirements if challenged (for instance you may have been generous with load calculations, or with buckling factors etc), then you may choose to use some of your back pocket reserve to keep the bar size down. On the other hand, if you have nothing in your back pocket, increase the bar size (or number of bars), then you do.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[KootK]

My argument isn't for a design tatget other than unity. My recommendation stems from my belief that there isn't enough of a meaningful difference between 0.95, 1.00, and 1.05 to justify rework or adding complexity to the drawings/project. And, unless I miss my mark, I believe that to be the crux of the original question.

There was a time when, if I was a little over stressed, I'd go back to the well get that extra 2% or whatever. After a while, I noticed that my success rate in those endeavours was 100%. It seems that I've generally got enough tricks up my sleeve that I can always pull 10-15% out of a routine design if I'm feeling motivated to do so. So now, in the 5% cases, I don't bother going back to the well. It saves on fee and allows me to be very responsive to my builders.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

It seems as though this has already been hashed out from every conceivable angle, and by most of the usual suspects, here: Link. Lawyers, code intent, significant figures, new v retro...

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rapt]

I would use 100% as the cut-off. That is the minimum I have calculated that I need, so that is the minimum that should be supplied.

And I do not think 3000psi concrete should be used in a 6 storey building frame!

 

[PSEPK]

Thank you all for your valuable comments.

I really appreciate the effort you people have made to throw light on various aspects of the issue.

Thanks again.

Does any one know the exact reference to the "5% permissible overstress" allowance, mentioned in the 2008 thread referred to by KootK above?

A good structural engineer is often a blessing for others.

 

[KootK]

I don't remember quite where that resides but I'm fairly certain that it is in reference to renovation work.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[straub46]

Chapter 34 of the 2012 IBC (and 2009) has this provision:

3404.3 Existing structural elements carrying gravity load.
Any existing gravity load-carrying structural element for which an alteration causes an increase in design gravity load of more than 5 percent shall be strengthened, supplemented, replaced or otherwise altered as needed to carry the increased gravity load required by this code for new structures.

This is in regard to alterations (with similar wording for additions) after initial construction, and where the "5% permissible overstress" is mentioned in the building code. I wouldn't use the provision in new design, as the wording in 1604.2 is pretty clear:

1604.2 Strength.
Buildings and other structures, and parts thereof, shall be designed and constructed to support safely the factored loads in load combinations defined in this code without exceeding the appropriate strength limit states for the materials of construction.

Go Bucks!

 

[KootK]

In new construction, I think that it would be quite irrational to have a "5%" provision actually built into the code. If the code writers wanted you to have more wiggle room, I imagine that they'd just build that into the basic load and resistance factors. It would be weird to have the code read "Do all this factoring based on structural reliability theory. And now cheat it by 5%".

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Teguci]

Engineers should not be playing Jenga with their designs. If the numbers don't work at first, sharpen your pencil and get it to work. If you can't get the numbers to work, then repair the design so that you can. Yes, the reality is that a 5, 10 or 15% "under design" will probably still work, but you never want to be in possession of information that informs you that your structure, already built, doesn't work (enter loss of sleep and more damage to the liver).

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