ACI "Column Shear Cap" vs "Column Capital"

[engjg]

In looking at ACI 318-14 I see where Shear Caps are addressed in section 8.2.5 where it is required they extend a distance from the column face equal or greater than their depth below. I don't see where column capitals are addressed. I take the distinction between a shear cap vs. capital is one is cast with the slab and the other with column. I see most capitals being 45 degrees or less from the column vertical which is opposite from the shear cap dimension requirement. Am I a missing where ACI addresses capitals and their requirements. What about reinforcement detailing for shear caps and/or capitals, does aci provide any guidance on this?

 

[KootK]

I don't see where column capitals are addressed.

I believe that, in the context of ACI 318, shear caps and column capitals are the same thing.

What about reinforcement detailing for shear caps and/or capitals, does aci provide any guidance on this?

I don't believe that it is common to reinforce shear caps / column capitals other than running the prismatic column reinforcing cage through the capital. In a way, capitals are sort of akin to corbels with the tension reinforcing being the slab top reinforcing at the column.

 

[hokie66]

Don't you just hate it when the boffins change the nomenclature? These were traditionally called column capitals. I suppose the capital was just shortened to "cap", as is the modern trend. Then, because the capital is part of the column, but really not there to assist the column, but rather to address punching shear in the slab, they are now called "shear caps". My guess, anyway.

 

[rapt]

No, in ACI a shear cap is between a capital and a drop panel.

It has the shape of a drop panel but normally is only about 4' square, independent of the span lengths. Normally 2" to 4" deeper than the slab.

It increases the punching effective depth. And therefore it also increases the punching perimeter because d is larger. Punching perimeter is not controlled by the lateral dimensions of the shear cap.

A capital is normally sloped at 45 and increases the support perimeter and thus increases the punching perimeter. But it gives no increase in the effective depth.

A shear cap should only contribute to punching shear capacity, it should not contribute to flexural capacity as it is not wide enough relative to the span lengths.

They are very different in the way they work.

 

[hokie66]

I stand corrected. Don't recall ever using a shear cap, then. Don't really see the point. If you are going to form a little drop, why not make it a proper drop panel?

 

[rapt]

Hokie66,

That has always been my opinion.

Unfortunately you now get some US "experts" suggesting in some of their text books that you can ignore the width limitations and consider a drop cap to be a drop panel in flexure, as engineering continues its downhill slide. I think they proved it using FEM and using the full panel width moment, so that "proves" it works that way. Apparently!

 

[KootK]

Yes, thanks for the correction rapt. As I read ACI 318, it basically amounts to either:

1) A shear cap is a capital but with the potential for a critical shear section within the cap OR;

2) A shear cap is a drop panel minus the benefit of increased flexural depth.