Development length for main bar in tension in pad footing with ribs 1

[Aakalim103]

I have an foundation that has a rather thin bottom slab i.e. 120mm (with a single rebar mesh) but has 8 inclined concrete ribs that connect the column to the bottom slab, one at each column corcer and one at mid point of each column face. The hight of the ribs is almost 500mm. The size of bottom slab is 1.8m x 1.8m. The column height is 1.2m and the size of column is 600mmx600mm. I have attached a picture which shows the foundation.
The depth of the bottom slab is so small that it is almost impossible to provide the minimum development length for the the vertical column rebars. However, I was thinking that for the tradition pad foundation, the critical section for moment is at the base of the column/top of the slab. But in my case, there are ribs that will essentially stiffen the slab and the column and the critical section for moment will be at the start of the ribs i.e the maximum moment in the column will be at the start of the ribs. Do you think in this case it will be justified to provide the anchorage length from the start of the ribs?

 

[hardbutmild]

Yes.

This foundation looks really unusual, I've never seen anything like that.

 

[Aakalim103]

Thanks for the reply @hardbutmild. Yes this is indeed a very unusual geometry for a foundation. I am working with a client who are 3d printing these foundations and varius other concrete elements and this is a optimised geometry they have come up with.

 

[DoubleStud]

They save concrete but the forming man hours is going to skyrocket. Either that or they have to invest in a nice forming system. It only makes sense if this was a precast foundation IMO.

 

[KootK]

Do you think in this case it will be justified to provide the anchorage length from the start of the ribs?

I vote no. Or, more precisely, I feel that it doesn't matter and paying attention to it suggests that the real load path here may not being properly considered.

The whole point of those stiffening ribs is to shift the incoming axial stress in the column out to the perimeter of the pad. Trying to turn rebar compression forces is usually a big ask. Consequently, when your column compression forces hit the stiffeners, the compression probably needs to be able to be handled in the diagonal struts within those stiffeners without the benefit of compression reinforcement. See the sketch below.

In my sketch I've consider a case of pure axial load. That's because I don't consider this setup to be viable for resisting any moment that would require a tension connection to pad. More on that next.

 

[KootK]

This is why I deem this setup to be questionable for resisting moments.

 

[Aakalim103]

Thank you KootK for the reply and apologies for such a late response. The ribs do have rebars in them to resist the tension. The ribs seem puny but, there will be a few cms of 3d printed concrete layer around it which will act as a kind of permanent formwork.

I get your point about the inability of the foundation to resist moments, however, as per in-house testing, the foundation seems to be capable of resisting moments.

 

[KootK]

Thank you KootK for the reply and apologies for such a late response.

No worries. Your thread, your pace.

I get your point about the inability of the foundation to resist moments, however, as per in-house testing, the foundation seems to be capable of resisting moments.

Fair enough. I consider appropriate testing to represent higher quality knowledge than does theoretical analysis in most cases. I also feel that the predictive value of our theories tends to diminish when the scale of things gets small. I can design a transfer beam under a fifty story building no problem. Give me a ship's ladder rung and I'll choke up hard.

 

[Aakalim103]

Your insights are really valuable Kootk. I have a question related to this topic; lets say we anchor the bar in the rib to the rebar mesh in the slab. By anchor, I mean the bar hooks around the main bars of the slab mesh (a 180degree hook), would that mean that the tensile force will be transferred to the slab reabrs? I suppose such a connection would risk a blowout or a concrete cone faliure however, the presence of the rib above the connection may somehow mitigate for this type of faliure. What do you think? Maybe I am getting it all wrong.