Transfer Slab Shear Checks 1

[Slurgi]

Please review the attached sketch.

I am curious to see what others would do to check shear in a situation like this:

I would think that one-way shear would also be quite critical but cannot find any guidance on what effective width of slab to consider.

Thank you in advance.

 

[KootK]

The whole width of the slab has to shear for oneway shear failure (highly unlikely for most floors).

I disagree although, as you'll see, my own understanding is far from complete.

I feel that local, one way shear stresses should be considered rather than assuming the shear to be spread across the design strip. Here's what I've got for backup on that:

1) As I understand it, one way shear is not as ductile failure mode and, as such, significant lateral redistribution is not possible.

2) My intuition tells me that the one way shear stresses will be highest right in front of the column.

3) Every elastic FEM model that I've run indicates that shear stresses will be highest right in front of the column.

4) Both of the two documents shown below suggest that localized one-way shear is of some importance, albeit in somewhat different situations. The second one is NEHRP's guide on the seismic design of raft foundations.

This all leads me to another, related question: why is it that traditional two way slab design allows us to assume the entire design strip width is effective for one way shear? Frankly, I don't know. Presently, all that I'm able to think of is:

5) Code slab design provisions seem to have been largely developed considering uniform loads and situations where there's ample "time & space" for loads to make their way over to the column in a well distributed manner.

6) In my opinion, the code one way shear provisions are not suitable for heavy concentrated loads near the supports.

 

[Tomfh]

I think you need to check punching shear at the wall-ends with critical perimeter as shown below..

The clip below is for slabs supported on wall, but I think it is applicable for slab supporting a wall. You case is just upside-down.

The need to check punching shear around the columns is obvious.

When to apply the cut offs to columns? If the punching load should only be considered 1.5D from end of wall, why assume a punching periemter all the way around the column?

 

[Retrograde]

I think you need to check punching shear at the wall-ends with critical perimeter as shown below..

Which fib bulletin is that from?

 

[hetgen]

@ Tomfh,

The cut-off according to the model code is (C[sub]max[/sub] / d > 3)... where [C[sub]max[/sub]] is the support cross-section maximum dimension and [d] effective depth of the slab. So it is not specific for walls, elongated column support could also exhibit a concentration of shear stress.

@ Retrograde

The clip is from this paper.

The research paper shows that, because of the one-way shear contribution, the MC 2010 punching shear perimeter becomes increasingly conservative for C[sub]max[/sub] / d > 6. But I think the model code won't be conservative considering the geometry the OP is looking at (i.e, column support right next to the wall end).

If possible the OP should follow @rapt advice and convince the architect to change the geometry.

 

[Enhineyero]

I disagree although, as you'll see, my own understanding is far from complete.

I feel that local, one way shear stresses should be considered rather than assuming the shear to be spread across the design strip. Here's what I've got for backup on that:

1) As I understand it, one way shear is not as ductile failure mode and, as such, significant lateral redistribution is not possible.

2) My intuition tells me that the one way shear stresses will be highest right in front of the column.

3) Every elastic FEM model that I've run indicates that shear stresses will be highest right in front of the column.

4) Both of the two documents shown below suggest that localized one-way shear is of some importance, albeit in somewhat different situations. The second one is NEHRP's guide on the seismic design of raft foundations.

This all leads me to another, related question: why is it that traditional two way slab design allows us to assume the entire design strip width is effective for one way shear? Frankly, I don't know. Presently, all that I'm able to think of is:

5) Code slab design provisions seem to have been largely developed considering uniform loads and situations where there's ample "time & space" for loads to make their way over to the column in a well distributed manner.

6) In my opinion, the code one way shear provisions are not suitable for heavy concentrated loads near the supports.

I'll bite. My engineering way is to simplfy complex behavior, not saying its the 'right' way, but its my way and its limited by my imagination (hence not error free). One way shear failure to me is shearing in one direction, in this case the full width of the slab. If the whole width of the slab doesn't shear off then its not oneway failure. Yes, there will be stress concentration in front of the column, however that is usually checked using punching shear failure as described above (in eurocode).

The paper from ACI looks interesting, I was able to find it online but can only view the introduction (it is pay walled). They have conducted some test (big fan of this) and indicated in the introduction that there was a reduction in the mobilised effective width. I would be interested in what sort of failure they noted (if the whole slab hasn't shear, then this is likely a punching failure) and though there is a reduction in effective width there may be other factors that contributed to strength similar to punching shear behaviour.

 

[KootK]

@Enhineyero:

Do you not find the NEHRP document blurb persuasive? They pretty much come right out and say that the full strip width should not be assumed to be mobilized for raft slabs. That one's a freebie: Link

...I would be interested in what sort of failure they noted...

The dissertation that was the basis of the ACI paper can be had for free here: Link. And here: Link.

The types of damage observed are shown below and are characterized as one-way punching. One way and two way shear failure at concentrated loads are both "punching" after a fashion I believe.

If the whole width of the slab doesn't shear off then its not oneway failure.

1) I think that's debatable and a matter of definition.

2) Either way, I believe that the rational fear is a local, non-ductile fracture followed by an unzipping that may well compromise the entire slab width. I've shown this conceptually in the first sketch below.

 

[Retrograde]

@ Retrograde

The clip is from this paper.

Thanks!

 

[Tomfh]

1) I think that's debatable and a matter of definition.

I tend to agree. Punching shear is really just one way shear that’s taking a short cut around a column. And one way shear is just punching shear that’s largely confined along a strip.