2-way slab punching shear at edge columns

[cliff234]

I have a 2-way slab and am checking punching shear at the edge columns. The critical section at an edge column typically has 3 sides. How far past the edge face of the column must the slab cantilever before I can consider the punching shear critical section to wrap around all 4 sides of the column? (I can't find anything definitive in ACI 318 nor in the PCA notes that clarifies this.)

 

[Lomarandil]

Can you check it both ways to see what governs? 3 side critical section, with two of those sides extended to the free edge, vs a 4 sided section?

I'd imagine it may be a case-by-case decision (informed by experience and maybe a helpful rule of thumb from one of our wiser members).

 

[rapt]

Yes, you check it both ways and the worst case (shortest perimeter) governs.

 

[KootK]

How far past the edge face of the column must the slab cantilever before I can consider the punching shear critical section to wrap around all 4 sides of the column?

Some countries have code recommendations for this and some don't. I believe that Canada and Europe both have (or had) recommendations of five times the slab thickness beyond the column face. That said, rapt's method strikes me as eminently rational.

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.

 

[structSU10]

Its definitely an 'engineering judgement' kind of thing. I actually had a brief session about how ADAPT handles it, and if I recall correctly the program checks for 8*slab thickness from the edge to determine if it is a corner, edge, or interior column.

 

[KootK]

There's this from ACI 351.2-89. I'll try to dig up the 2011 version.

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.

 

[calvinandhobbes10]

ACI 318 11.11.1.2 and the commentary make it reasonably clear that the designer needs to find the controlling punching section.

RAM Concept which I've used and ADAPT etc will do this as long as the programs are given proper inputs.

If you're hand-calcing, you need to run it a few ways. Consider not merely the perimeter lengths but also the slab-column moment (ACI 318 Fig R11.11.7.2), which will be somewhat increased by increasing cantilever distance. If you have a lot of conditions, a spreadsheet will be useful. Plus architects are known to shift slab edge dimensions.

 

[KootK]

It did change with the 2011 version of ACI 352.1 - Guide for Design of Slab-Column Connections in Monolithic Concrete Structures.

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.

 

[cliff234]

Wow. Thanks for all of the responses. You answered my question.