0.5% Reinforcement in Existing Concrete Columns

[mcc202]

Hello,

I am analyzing an early 1900s existing concrete structure for a potential overbuild. Based on a field investigation, the columns appear to have been reinforced with about 0.5% vertical steel. Per ACI 318-14 section 10.3.1.2, you would reduce your concrete strength proportionally to account for the less than 1% minimum reinforcement. Obviously, the intent of this provision was for new structures. Am I obligated to also reduce this strength when checking the existing columns for the new loads? I understand that we are obligated to meet current code requirements, but the intent of this provision does not seem intended to apply to new structures. The original design was likely based on a set working strength of 540 psi as specified in the early versions of ACI, which gets me much more capacity than the current code does, specifically due to this 10.3.1.2 provision. I am currently proceeding on applying this provision, but would just like some opinions on the matter.

Thanks

 

[JLNJ]

Columns are pretty important. I don't think you want to over-estimate the capacity using an extremely out-of-date code provision. Additionally, I think the IBC prohibits this, if that's your enforced code.

 

[ProgrammingPE]

You need to design the column using the reduced effective area. This is necessary since columns will creep and transfer additional stress to the reinforcement. The effect of this is more significant in columns with low reinforcement ratios.

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[dik]

There used to be provision in the codes for reducing reinforcing for oversized columns... If I recall, you could consider the column size being 1/3 of the actual size (or maybe by 1/3) to accommodate reduced size reinforcing... if memory serves; it's been decades since I've done anything serious with concrete.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[mcc202]

Yes, I am currently analyzing using these provisions for the reduction of strength. It just seems a little odd that if you keep the same size rebar and increase the column size, there's the potential to get less strength because the percentage of steel decreases, even though the column size increases.

 

[JoshPlumSE]

There used to be provision in the codes for reducing reinforcing for oversized columns... If I recall, you could consider the column size being 1/3 of the actual size (or maybe by 1/3) to accommodate reduced size reinforcing... if memory serves; it's been decades since I've done anything serious with concrete.

Yes, it's still in the code, Section 10.3.1.2 (of ACI 318-14): Design Limits: Dimensional limitations
For columns with a cross section larger than required by considerations of loading, it shall be permitted to base gross area considered, required reinforcement, and design strength on a reduced effective area (Ag) not less than 1/2 the total area. This provision shall not apply to columns in special moment frames or columns not part of the seismic-force resisting sytsm required to be designed in accordance with Ch18.

This basically means concrete columns in SDC D, E or F.

For columns that old, I would be much more concerned about "shear confinement" of the longitudinal bars rather than the strength of the bars themselves.


Note: Even for SMF columns or non SMF columns in seismic design categories D, E, or F you could get away with still using these columns. But, I think you'd have to do it more like a "performance based design" where you demonstrate that these columns have adequate ductility. You'd have to know a lot about the materials though....

 

[dik]

It was likely proportioned by working stress design, usually with ample reserve for flexure and compression... and less limited reserve for shear... but ample.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[ProgrammingPE]

It just seems a little odd that if you keep the same size rebar and increase the column size, there's the potential to get less strength because the percentage of steel decreases, even though the column size increases.

How are you accounting for the reduced effective area Ag? The code doesn't tell you exactly how to do this, but I've seen examples where the capacity of a 20"x20" column (Ag = 400 in^2) with 0.5% reinforcing is determined by checking it as a 14.1"x14.1" column (Ag = 200 in^2) with 1% reinforcing. If you use this method, then the capacity does not change as the column size increases.

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[mcc202]

I proportioned f'c down based on the actual reinforcement ratio from 1%...so for 0.5%, you get half the f'c. I believe that has the same effect as reducing the effective area proportionally. This is also the method in which SP Column calculates "Architectural" column strength based on their manual.

 

[BridgeSmith]

I proportioned f'c down based on the actual reinforcement ratio from 1%...so for 0.5%, you get half the f'c. I believe that has the same effect as reducing the effective area proportionally.

I'd have to run the numbers on that, but I don't think reducing the concrete strength has the same effect as reducing the area of the column. Especially at low reinforcement ratios, concrete strength is almost irrelevant. When you're only engaging a fraction of an inch of concrete in compression, the effective depth doesn't change much by cutting the concrete strength in half and doubling that fraction of an inch to a larger fraction of an inch. reducing the area of the column by half OTOH, reduces the strength considerably, especially if it becomes a slender column due to the reduction.

The AASHTO spec is also specific about reducing the area of the column.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[mcc202]

From what I can tell, it does appear to have the same effect for purely axial strength calculations and it is actually what SP Column says they do in their program:

"By default Architectural option is selected for which the capacity of the section is reduced. For the ACI codes, the reduction results from multiplying the maximum concrete stress, f'c, by the ratio of reinforcement area to 0.01Ag. This produces the same effect as reducing the effective concrete area to achieve ratio of reinforcement area to gross concrete area equal to 0.01."

I'll have to dig into the slenderness provisions a bit. I am not sure it is the intent to now base slenderness calculations on the reduced area.

 

[Settingsun]

Reduced area seems like a nice, simple idea until you try to produce the interaction diagram. Reduced f'c will work better there.

Slenderness should be based on the actual column size, with the stiffness calculated by a method that takes account of the low reinforcement quantity, and probably ignoring the 'stocky' limits, ie calculate the design moment. If the column is stocky, the moment will remain small. (I'm assuming the columns are braced.)

Do check that the transverse reinforcement is somewhat close to current requirements. If not, look into the implications in detail.

As another data point, the Australian code requires the steel to have capacity for at least 15% of the ultimate axial force when <1% is provided.

 

[BridgeSmith]

From what I can tell, it does appear to have the same effect for purely axial strength calculations

For that, I suppose it would. I was considering bending capacity, since a column that is only axially loaded would theoretically not need any reinforcement.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[dik]

short ones anyway...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BridgeSmith]

short ones anyway...

That's why I said "theoretically", since it would be rare to find a column that's not subjected to at least some lateral load. In my work, it's rare to find a concrete column where the axial load is even significant in terms of design capacity.

Rod Smith, P.E., The artist formerly known as HotRod10