Column Transfer - Profile Changes - Reinforcement Detailing

[Drapes]

If a circular column is required to transfer into a blade column (or vice versa), and the overlapping area is not large enough to transfer the axial loads by directly lapping the reinforcement over the shared cross-section, is it acceptable to provide a thickening at the column interface (via a capital or drop panel) with enough depth that would allow you to simply develop the bars in compression (approx embed depth 20 x dia rebar), in lieu of providing a full compression lap length (approx 40 x dia rebar)? This would involve terminating and cogging the reo from each column within the thickening zone.

I understand that development length is generally half the lap length for compressive reo, at least according to AS3600, so I have some reservations with the above, but would like to hear what others think.

If this method is acceptable, I believe it would allow you to utilise a higher percentage of reinforcement over the column interface, as opposed to being limited to the shared cross-section.

I have assumed the circular column and blade column are concentric to one another.

 

[dik]

provide a thickening if needed... just make sure the dowels match the column above.

Dik

 

[KootK]

I advise against this for two reasons:

1) Your double compression development case is actually worse than the compression lap case because now you've got to contend with passing the forces laterally as well as vertically. This would represent a pretty serious "free lunch" if it worked.

2) Cogs do not work on bars in compression.

This can probably be made to work by some method, including a properly detailed thickening, but what you've described does not sound like the way to go to me.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

My take on this. You need the thickening, and within that thickening, you need to do a strut and tie analysis in both orthogonal directions to make sure the force can transfer. And because the column force is not directly through the slab, I think the bearing stress in the slab needs to comply without allowing for confinement.

 

[Drapes]

Thank you everyone,

hokie66, if the strut and tie model confirms that we can accommodate the transfer of forces through the thickening, would there be any issues with the detail I have proposed above re: terminating the compressive reo from each column within the thickening zone?

Also, can the strut and tie model for each orthogonal direction be carried out independently?

 

[KootK]

hokie66, if the strut and tie model confirms that we can accommodate the transfer of forces through the thickening, would there be any issues with the detail I have proposed above re: terminating the compressive reo from each column within the thickening zone?

You didn't ask me but, for what it's worth, if a valid strut and tie analysis showed the scheme to work, I'd certainly consider that adequate proof of concept. I'd need a peek at the model to say for sure whether the orthogonal treatment would suffice but my suspicion is that it probably would.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

My answer is that I agree with KootK.

 

[Drapes]

Thank you KootK and hokie66

 

[Drapes]

Hi guys, just wanted to clarify - assuming we adopt strut and tie and it checks out based on pure bearing, how relevant is the development of the compressive reinforcement from each column within the thickening zone?

Would we need to develop the compressive reinforcement (20 x dia rebar) from the top of the thickening OR beyond the face of the strut?

 

[KootK]

Hi guys, just wanted to clarify - assuming we adopt strut and tie and it checks out based on pure bearing, how relevant is the development of the compressive reinforcement from each column within the thickening zone?

If the joint works in bearing unreinforced then I'm guessing that the column also works unreinforced just above and just the below the joints? If that's the case, I would be comfortable forgoing the compression development. If the bars are actually required at a section just above the joint for compression or flexure, that's another story of course. Like most folks, my preference is to develop all bars whether I technically need to or not. Development failure is a brittle thing and we generally try to avoid it. That said, sometimes you gotta bend in the wind a bit to keep clients happy and stay competitive.

Would we need to develop the compressive reinforcement (20 x dia rebar) from the top of the thickening OR beyond the face of the strut?

Is your compressive reinforccement actually part of your strut and tie model? If so, then it probably needs to be develped starting from the face of the strut on the side at which the tie begins to cross the strut (ie you get to use the width of the strut as part of your development length). If this is the case, I'd recommend posting a sketch of your strut and tie model here for review.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Drapes]

Thanks KootK,

The compressive reinforcement is required as far as the column design is concerned (i.e. taking into account both the axial load and moment, and considering the effective length etc), but I don't need to rely on the compressive reinforcement within the slab thickening at the column interface if I adopt a strut and tie approach (i.e. the strut and nodal checks are satisfied and the joint works in pure bearing unreinforced).

Despite this, I am still intent on providing sufficient depth in my thickening to achieve full development length to the vertical compressive reo from each column. But can this development length be measured from the top/bottom of the slab thickening, without any regard to the nodes and struts from the strut and tie model?

Will send through a sketch tomorrow to clarify - I'm out of the office right now.

 

[KootK]

You're most welcome Drapes.

The compressive reinforcement is required as far as the column design is concerned (i.e. taking into account both the axial load and moment, and considering the effective length etc)

Let me rephrase this. Broadly speaking, there are three conditions leading to the requirement for column rebar:

1) You need it for buckling checks at column mid-height. This applies to pretty much every column from a code perspective.

2) You need it to resist tensile moments at the floor plate levels.

3) You need it to resist compression throughout the entire length of the column, including at the floor plate levels. Under pure axial load, concrete strength alone would not be sufficient.

If #1 is true but #2 & #3 are false, then I'd argue that bar development at the floor levels is not terribly important. This is actually a fairly common scenario.

But can this development length be measured from the top/bottom of the slab thickening, without any regard to the nodes and struts from the strut and tie model?

It can be measured from the top of the slab if the vertical bars are not part of your STM model. If the vertical bars are part of your STM model, then it depends on the model which we'll hopefully get a peak at tomorrow.





I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Drapes]

Thank you KootK,

Please see attached sketch - would be interested to hear your thoughts

 

[Drapes]

In case the link doesn't work, see below:

 

[KootK]

I like it. However, your question about where to take the development from still comes down to this from my previous post:

If #1 is true but #2 & #3 are false, then I'd argue that bar development at the floor levels is not terribly important. This is actually a fairly common scenario.

I suspect that is the case if your diagonal struts are working as plain concrete as the incline adds to the compression stress and I'd expect your column concrete to be stronger than your slab concrete. Or at least equal.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.