Eccentric Column Loading

[BT2910]

Usually when designing an eccentric or 'walking' column, I will tie resultant forces back into the slab or beam that are caused by this eccentric loading.
However I have a case where a 400x400 column is landing only 65mm eccentric to a 250x1200 column in it's major direction. Hence, it is quite close to being centric.

Is there ever a situation in which people think that there is no need to tie these forces back? Can it be argued that the load spread over a much larger column will not have an adverse affect on overturning?

Thanks,

 

[IDS]

I suggest you include the eccentricity in your calculations, then if it is negligible it won't make any difference, and if it isn't you will have it covered.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Guest090822]

I’ve used a higher effective length factor to account for eccentricity in the past. When compared to a lower effecive length factor an accounting for the eccentricity it usually ends up being fine. Either way you should check it and prove to yourself that it’s OK

 

[KootK]

I wouldn't go to any special, real world trouble to tie that back. 65 mm is only about 5% of the low column dimension. At such a small eccentricity, it's difficult to even say by what mechanism it will get resolved. It may just present as a slightly eccentric soil pressure at that foundation of the low column.

To satisfy my curiosity;

1) how many stories are above the transition?

2) For how many stories is the 1200 column that size?

3) Does the 1200 column carry though to the foundation level?

4) Does the transition happen at or below grade as you transition to a parking level? Or is it a demising wall thing higher up?

5) what percentage of the building weight is coming through this column? 5? 25?

 

[KootK]

Another perspective on this is that, at such a small eccentricity, you're approaching that which might be induced by normal construction tolerances on things like column plumbness and footing locations. And we don't normally sweat those things. We're making buildings after all, not pianos. If a five percent centricity would bring down the house, we probably shouldn't be creating something so fragile to begin with.

 

[BT2910]

Thanks for the responses guys.

I tend to agree with what KootK says. For such an eccentricity that is only small in relevance to the below column, even if the force was tied back I can't see it ever truly being resolved in this way.

1) For more information, this is a 28 story tower and this is at level 23. The column load above is around 3000kN.
2&3) The 200x1200 blade grows every 6 or so levels until it becomes a 400x1400 column at the footing location.
4) The column is higher up at apartment levels.
5) I'd be inclined to say the total load is closer to 5% of the total building load.

What makes me wary though is that a 65mm eccentricity requires an almost 1000kN tie force by traditional methods (P x e/D) but I just can't fathom that this would be required. My assumption is that this load will adequately strut through the column below and I'd go as far to say that if the dowels can take the resolved push / pull force from this moment, then the column will be fine.

 

[r13]

...IS code has provided minimum eccentricity if load acts within that we can consider the column as axially loaded column.
e min= (unsupported length of column/500)+(least lateral dimension/30). I.e..e min = (L/500)+(D/30) or 20 mm whichever is maximum

I think you have to make sure the slab or beam can resist this moment without adverse effect, although it is well within, close to, the minimum eccentricity.

 

[Settingsun]

Is the short side 250mm or 200mm? It seems to have changed since the first post.

Are you using 200mm to get the 1000kN force? Is that the right dimension to be using?

What's the basis of the P.e/D tie force? I'd have thought P.e/L to produce an opposing force couple but nor sure that's what you're trying to achieve with the slab tie.

 

[KootK]

What makes me wary though is that a 65mm eccentricity requires an almost 1000kN tie force by traditional methods (P x e/D) but I just can't fathom that this would be required.

.

Me neither, I'm getting 11kN considering the transfer to occur over just one six story segment. You could probably make a similar argument to take it right down to the foundation. See below for how I'm envisioning your situation and let me know if I've gotten it wrong. I agree, 1000 kN needs explicit consideration. 11KN is an order of magnitude less than what you'd want just to brace the column at each floor level with no eccentricity.

 

[rscassar]

Transitions are always difficult, and when dealing with strut-tie in column transitions applications even the slightest of angle change can result in very large tie forces. For your application, I would be looking to put the 400SQ column as an eccentric loading onto the 1200x250 column requiring reinforcement from the 1200x250 column lapped into the 400SQ.

On the provisions that the 400SQ can transmit thru the slab complying with axial load transmissions thru floors & strut-tie, comply with bearing stress and comply with column-moment design, then the column can transmit load without significant diaphragm forces redistribute themselves throughout the floor plates.

But yes, it should always be considered.

 

[BT2910]

Sorry for the extremely late reply, very rude of me.

Thanks for the responses. I'll admit that I made a few oversights in the engineering justification and what KootK posted seemed to jog my memory on previous examples of where I have dealt with similar situations.

The force at each level is small enough to be taken out by the slab diaphragm as the reinforcement over each column is in excess of what I would be adding as tie reinforcement.

Thanks for your help guys.

 

[hoshang]

Hi
Please refer to 6.6.4.5.4 of ACI-318 for minimum eccentricity in columns. If your 65mm is less than ACI minimum eccentricity provision, you should use ACI minimum eccentricity provision.