Slender Column Design - Reinforcement Joggle

[Drapes]

As far as slender column design is concerned (assuming braced column), the moment amplification can often be extremely sensitive to the concrete cover. Even increasing the cover from 30mm to 60mm to account for the joggle in the reinforcement at the slab level can have a significant impact.

Can this joggle in the reinforcement (and increased cover) be disregarded for slender column design, given moment amplification will probably be more critical at mid-height? I am doubtful this is ever accounted for.

 

[Settingsun]

Disregard it. The moment amplification procedure has many assumptions built in and this effect wouldn't be anywhere near the likely error magnitude. For k=1.0 assumption (pin-pin), it's truly negligible. For k<1.0, you're taking advantage of the stiffness at the ends so it's only pretty much negligible.

 

[slickdeals]

There's details for moment frame columns where the offset bend is provided in the column above. The column below bars continue straight up so that you have the higher effective depth, in case you so choose to use.

 

[KootK]

X2 for steveh49's answer in general.

There's details for moment frame columns where the offset bend is provided in the column above.

Do you move it to just above the floor level or to the column mid-height? If it's just above the floor height then, in a way, you're just moving the soft zone to a different part of the column. Or is the goal to simply have thing simple and at at maximum effectiveness over the height of the beam-column joint where most of the action is?

I am doubtful this is ever accounted for.

Agreed. I've pondered this myself in the context of moment frame / sway columns. For those, I'd expect P-Big-Delta column curvature to be dominated by the stiffness at the column ends. And, in the field, the joggles sometimes compromise the effective depth more than they do in our drawings. Of course, conventional drift calculation are often done on a %Ig and so do not drill down to a granular level of detail that would account for this.

 

[Drapes]

Thanks steveh49 and slickdeals.

In our case, the slender column design has been based on single curvature and minimum moments, which are the governing moments at the ends of the column. An effective length factor ke<1 has been adopted.

When combined axial plus moment are assessed at mid-height (with standard 30mm cover as per original design) there are no issues, however when assessed at the ends of the column (with approx 60mm cover to account for the joggle), the column fails due to the moment amplification effect.

Now in reality this column will be in double curvature as we have confirmed from our analysis under vertical loads, however AS3600 enforces you to take single curvature if you are using minimum moments. But even conservatively taking single curvature, the column design will still be adequate at midheight.

Would it be reasonable then to say that the end moments are less susceptible to any moment amplification effects and the onset of buckling, if the column design is confirmed to be okay at midheight? In other words, if the column is designed as 'slender' at midheight and treated as 'stocky' at the ends, the design can be justified.

 

[Drapes]

Also thanks KootK, missed your response!

 

[KootK]

Would it be reasonable then to say that the end moments are less susceptible to any moment amplification effects and the onset of buckling, if the column design is confirmed to be okay at midheight? In other words, if the column is designed as 'slender' at midheight and treated as 'stocky' at the ends, the design can be justified.

I feel that is reasonable. Restated, it's comparison between:

1) A fictitious column with no end restraints but non-joggle cover at mid-height which one would expect to dominate the buckling response of such a column.

2) A more realistic column with end restraint at the ends but stiffness at those location based on the joggle cover.

One could run some numbers to verify this easily enough but I feel confident that #1 would always govern for a non-sway, K<1 column design.

 

[Settingsun]

I suspect that the end moments would reduce due to second order effects if you have uniform moment with some end restraint.

 

[Settingsun]

Here is linear analysis on the left with P-Delta on the right. The axial force is 800 kN.