Horizontally offset concrete columns 7

[milkshakelake]

I'm not talking about walking columns. I mean columns placed on top of each other with an offset. Are these okay? What things do I need to be concerned about?

My punching shear senses are tingling. I'm immediately thinking of punching shear issues for overhanging portions of columns sitting on slabs. But then again, can I visualize a diagonal strut connecting the lower and upper column going through the area where they align? This would actually make the punching shear perimeter larger, for which I'm having trouble wrapping my head around how to calculate it.

I'm realizing that my knowledge of punching shear is extremely lacking. I'm using the textbook from Wight and MacGregor, which doesn't cover weird cases at all. Does anyone have a reference for getting into it in more depth for things like the situations below?

I'm looking at these older threads, still not sure what to do about punching shear or if it's even a concern:

https://www.eng-tips.com/viewthread.cfm?qid=422266

https://www.eng-tips.com/viewthread.cfm?qid=376477

 

[hetgen]

Any p x e moment here shouldn't be involving the slab...

I don't believe that is a correct statement. The moment induced at the joint, arising from the eccentricity of the columns and the unbalanced loading on the slab panel, should be distributed between the columns and the slab strip based on their relative stiffness. The unbalanced moment resisted by the slab needs to be considered in both punching shear and flexural design.


@milkshakelake
There are some good discussion on this thread that may help you visualize the load path.

 

[ALK2415]

@milkshakelake
if you consider the following for punching shear reinforcement?
it will work as grillage system and prevent dense reinforcement (Idea came from actual paper)
give it a thought

 

[cliff234]

ALK2415: We frequently have to deal with walking columns such as you illustrated in your sketch. In your sketch there is very little overlap between the upper and lower columns. When we have this situation (very little overlap between the columns) we always insist on having a larger transition column enveloping the footprint of the two of them for at least one floor. This allows axial load to flow through the columns without inducing excessively high punching shear stresses. Alternatively we could walk the column over several floors to provide a larger overlap between columns and reduce the horizontal kick force.

 

[ALK2415]

thanks cliff234 for your illustration

 

[dik]

I'd be reluctant to introduce a 'shear' plane in the slab, even reinforcing with headed studs.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Janni]

Items you should verify
1) Column rotation, ensure there is additional tension reinforcements in top slab check this with the inclined Strut&Tie method. The load to be safely transferred within the slab and down to foundation

2) Verify column capacity for the reduced section

Ideally the reinforcements should follows this profile, otherwise you will have a section at the bottom with less reinforcements.
3) Verify slab concrete strength at the transition. When higher strength columns are used with lower strength slabs, there are concerns about the ability of the concrete in the slab
to transfer the stresses from the upper to the lower column.
Refer to CONCRETE ADVICE No. 34 (by the Concrete Society) or ACI 318 (suggests that:
fce = 0.75 fcc + 0.35 fcs
where
fce = effective strength of the joint concrete
fcc = characteristic strength of the column concrete
fcs = characteristic strength of the slab concrete.
4) Rather than a traditional punching check, given the proximity of the support, you should examine this section of the slab using the localized Strut & Tie method for verification, for the residual force to be transferred.

 

[dik]

These lateral forces have a big impact on the moment...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[ALK2415]

@milkshakelake
"LIFE IS FUNNY", friend asked if there is away to solve same of your problem but after casting the column
so i came up with the following solution;
my assumption is for strengthen the slab side form left and right to prevent punching shear as much as possible
wish to hear your opinion and others also are welcome to participate

 

[milkshakelake]

Sorry for the late response, I've been on vacation. Anyway, I'm getting a lot of pushback from the architect about using corbels, so I'm going to investigate this a bit further with strut and tie model.

@rapt That makes sense. I'll have to run some calculations and see where this gets me. I suspect I'll have a punching shear issue.

@hetgen Thanks for the link to the thread. It does help a bit, and it's a similar situation.

@ALK2415 I've seen a similar setup (W beams instead of WT beams) used for punching shear. I'm not sure how to calculate it, because the guides I have are using rebar or stud rails. Maybe I can treat the WT members as area of steel similar to rebar. In your second sketch with the brackets, that's a good solution for retrofitting. I had to do something similar with retrofitting but with concrete (for offset beam instead of column).

@dik Totally agreed, I don't like doing it. But others in my area are doing it, so I need to think of something good. And yeah, I will design the columns for these added moments. I'm making a spreadsheet to handle the additional moments. This will be a pain to design, with a lot of P-M diagrams...should have charged more.

@janni Thanks so much! It's very helpful. Can you share where you got the last diagram? Maybe the reference will be useful.

 

[bookowski]

@MSL In your market doing a column rotation or partial offset as you originally proposed is quite common. So from a business standpoint I'd recommend staying away from embedding steel shapes, doing corbels, or anything else that's going to set you apart unless you really feel uncomfortable with the proposed offset/rotation. Per my earlier statement, the slab forces can't be the same as if you didn't have an offset column but I think that for most reasonable conditions the difference will be small and you do in fact have a larger punching perimeter as you originally noted. Someone posted an image of a condition with a tiny overlap, you'll need to use judgement but of course having a 3" x 3" overlap is not the same as rotating a column and having a 16x16 overlap slightly offset etc.

Look at any of the design guidance for the typical column walk, the condition where the walking column extends to the extent of the full column above and below. The walking column centroid is often completely outboard of the column below, and at a minimum it's always offset from the column below. I've never seen anyone recommend going granular to find the impact to punching. I've got the internal design standards for column walk design for two of the big firms in your area and I just skimmed them to make sure, no mention of punching checks. Again, I agree that the slab moments can't be the same as the condition with nothing going on but that doesn't mean that you necessarily need to study it for most conditions (similar to many things we ignore in our day to day designs) and if you've got a particularly ugly condition then you would need to use judgement but the general concept is done quite often successfully in your area so don't screw yourself too much with going down the etips rabbit hole. I've always treated these like a column walk - get your thrusts and resolve those, check the strut, enlarge the column above and/or below as needed to ensure the overlapping area works, etc. and move on.

 

[Tstruct]

If possible, provide a beam beneath the offset column which will act as a base for it, the column should rest entirely on that beam. We call them picked up columns here. We usually do it in every other project, and it bears load of several floors.

 

[milkshakelake]

@bookowski Thanks for the reality check! I'll probably end up doing some of the rabbit hole stuff anyway so I feel comfortable, but it's good to know that I shouldn't worry too much about it. I wish nobody started this trend in my area in the first place, but the cat is already out of the bag.

@Tstruct That was my original idea, but the architect is not buying it. I usually use that, or a walking column.

 

[BFstr]

Milkshakelake,
Did you execute any correction or remedy to the column?
I don't know my idea will be too late?