Minimum eccentricity as per CSA A.23

[GalileoG]

The Canadian concrete code requires that a minimum eccentricity of P(15+0.03h) shall be required for all columns.

My question is: Does this not mean that all columns shall have biaxial moments? Or is moment from the minimum eccentricity requirement to be considered and taken on the same direction of a uniaxial column with moment due to applied loads.

I hope the above made some sense.

Clansman

 

[JAE]

Not sure about the exact wording of the Canadian code - in the ACI I think the minimum eccentricity only applies if you have moment in that particular direction.

 

[GalileoG]

I can't write down the exact wording for you folks right now as my handbook is at the office. But I do remember it being very vague. I will post it tomorrow so I can hear your opinion on its interpretation.

Clansman

 

[ChadV]

From the design examples, it looks like that minimum moment due to eccentricity is only checked (against a magnified moment for slender columns) for a direction in which there is already a moment.

 

[asixth]

You should always account for a minimum moment due to construction tolerances and yes, you will need to apply the load at a minimum eccentricity about each principle axis. For minimum moment condition that you are describing, the line of action will be near the centroid of the column so it would be a valid assumption to account for bending about each principle axis seperataley.

For reinforced concrete columns that are cast in-situ, it is unlikely that your design should be goverened by minimum moment due to a notional eccentricity, there is always going to be some moment transferred into your column from the slab spanning onto it an I would guess this is larger than your minimum design moment (at least that is what I have found from my experience).

Likewise, there will also be biaxial bending in columns so it is best to rotate the section when designing for strength opposed to the simplifications that are set out in the code.

 

[WillisV]

ACI requires minimimum eccentricities to be checked even for axial-only columns (if there is such a thing) about each axis independently (not biaxially).

 

[Lion06]

ACI already recognizes a minimum eccentricity in the column strength equation. That is why if you draw out the interaction diagram it starts off with a horizontal line from the vertical axis (P). The point where the horizontal line is no longer horizontal is the minimum eccentricity (emin=M/P). This is obviously slightly different for each section given the variables (f'c, section dimensions, rebar pattern, etc), but is considered nonetheless. That is the only "minimum eccentricity" that I'm aware of for short columns. Slender columns do require a minimum eccentricity of 0.6" + 0.03h from ACI 318-05 10.12.3.2, but that is only for determining if a column is slender or not and to use with a slender column. It's not to use with a short column.

 

[kslee1000]

I remember it was once popular to include e(min) = b/6 or h/6 to account for construction tollerance/mistakes. The minimum eccentricity was applied one axis at a time, it applied to concentrically loaded columns, as well as piles and piers.

StructuralEIT: I recall the horizontal cut-off line on the interaction diagram is corresponding to the strength reduction factor (phi). Check me on this.

 

[Lion06]

kslee-

phi contributes to the vertical location of that horizontal line, but doesn't contribute to the fact that it is horizontal. If phiPn,max were phiPn (and didn't have the additional factor of 0.8 (for tied columns) or 0.85 (for spirally reinforced columns)) then the interaction diagram would not have the horizontal line. The reason it does is because to the left of the horizontal line phiPn is greater than phiPn, max due to that additional factor. Once you get past that point, phiPn is no longer greater than phiPn,max.

 

[Lion06]

Just reading the commentary to 10.3.6 and 10.3.7, it explicitly says that the factors of 0.85 and 0.8 (for spirally reinforced and tied columns, respectively) are intended to account for minimum eccentricities and that the minimums were deleted from the code.

 

[kslee1000]

StructuralEIT: Thanks.

 

[WillisV]

Yes - I should have inserted the word "slender" in front of the word "columns" from my previous post.