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Transfer Slab Shear Checks

Transfer Slab Shear Checks

Transfer Slab Shear Checks

(OP)
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.


RE: Transfer Slab Shear Checks

Where is the transfer column?

If you hit the search button on this site, I believe this topic has been discussed a few times previously.

RE: Transfer Slab Shear Checks

(OP)
No transfer column.

It's a wall transferring along the green line.

RE: Transfer Slab Shear Checks

Unless your slab is very stiff, or endowed with intermittent soft joints, the wall will probably wind up being the spanning element over most of the slab span.

The one way shear question is an interesting one. This NEHRP doc offers some guidance on that in the context of raft foundations.

RE: Transfer Slab Shear Checks

What kind of wall is it? RC in-situ wall? Segmented precast? Reinforced CMU blockwork?

What's the distance between each end of the wall and the adjacent column?

As a side note... the more detail you offer in the original post, the better answers will be coming back. The original post is incredibly vague.

RE: Transfer Slab Shear Checks

(OP)
Thanks KootK; interesting read, will review over the weekend.

First post, will try to give more detail in the future. All RC concrete.

This is happening at a few places; distance to wall is between 250mm and 2m.

RE: Transfer Slab Shear Checks

No worries, Slurgi - most engineers on these forums want to give useful answers and it's hard to sometimes with limited info.

I agree with KootK - as you have a 700mm/800mm thick transfer slab, walls that terminate within around 500mm distance to the column face will mean the majority of the wall load will be dumped within 'd' from the face of the column. Most national codes will give you a one-way shear enhancement benefit in this case. Essentially the load tends to strut itself into the support. See Eurocode shear enhancement provisions below.

When the wall stops greater than say 500mm from the column face, then you could think of a "hidden" beam within the two way slab equal to the lesser of the width of the column slab strip or say column width + 600mm (effective depth 'd') either side. Then you could apply the one-way shear provisions and design beam links as required.

RE: Transfer Slab Shear Checks

(OP)
Trenno, it is the width of this hidden beam that I am most interested in.
I am unsure about whether taking the column strip would be unconservative. For bending I am taking a much smaller thickness, based on this (found in BS8110):

And taking x at the middle of the wall.

RE: Transfer Slab Shear Checks

What is the thickness of the wall? What is the wall height? How many floors is this wall supporting?

I would say the wall will span between each end if it's tall enough. Assume the wall reaction onto the transfer slab acts over a length of 4 * wall thickness. This "hidden" column base within the end of the wall will be your new concentrated load location. You could then use your british code width equation with "x" being the distance from the centre of the "hidden" wall end column to the centre of the support column below.

RE: Transfer Slab Shear Checks

(OP)
200mm thk wall; 3m; 10 levels
The "hidden" column approach is interesting.
What I am conscious of taking an overly narrow depth leading to an overkill of shear reinforcement.
Though probably the same can be said for going too wide and providing too little.

RE: Transfer Slab Shear Checks

Engineering judgement comes into play here. It seems you're not too far off an answer.

Say your shear load is around 7m x 850 kN/m / 2 = 3000 kN. Let's say 3MPa one-way shear is the upper limit, then you're looking at width of around for 1500mm for the 700mm deep slab. This isn't accounting for shear enhancement. The shear links required for this indicative situation aren't unexpected and would appear reasonable.



RE: Transfer Slab Shear Checks

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.




RE: Transfer Slab Shear Checks

Quote (KootK)

Unless your slab is very stiff, or endowed with intermittent soft joints, the wall will probably wind up being the spanning element over most of the slab span.

Yes. This is what happened at Opal tower in Sydney. There were bearing failures at the wall ends (which sat on columns) and then some punching failures at the blue dots as the wall settled. Good times :)

RE: Transfer Slab Shear Checks

I dont believe oneway shear would be critical. The whole width of the slab has to shear for oneway shear failure (highly unlikely for most floors).

I would check it for punching shear considering the wall as a whole and local failures. Remember that loads travel on the shortest path, so if you wall is highly loaded at the edges, then loads would concentrate there before it goes to the column.

For the transfer slab, assuming its quite thick, I would consider designing it using strut and tie.

RE: Transfer Slab Shear Checks

Tomfh, shouldn't you expect bearing or punching problems, not both? That is, bearing problems if you assumed distributed support, or punching if you assumed concentrated support at the columns.

RE: Transfer Slab Shear Checks

Quote (steveh49)

Tomfh, shouldn't you expect bearing or punching problems, not both?

If the wall fails in bearing at the columns the load can start going through the slab instead, which can punch/shear.

RE: Transfer Slab Shear Checks

That's interesting Tomfh. This is kind of what I'm imagining. Although it's probably less a case of the strut migrating but, rather, expanding to achieve sufficient bearing width until a bunch of the load has to transfer over via punching shear rather than direct strut delivery.

RE: Transfer Slab Shear Checks

Quote (Tomfh)

If the wall fails in bearing at the columns the load can start going through the slab instead, which can punch/shear.

I'm picturing that they assumed the load/reaction was along the wall length so would have designed for punching shear. The bearing failure was a stiffer but unconsidered path, otherwise they would have provided adequate bearing. So, one or the other unless something else is at play.

RE: Transfer Slab Shear Checks

steveh49,

I would assume they treated the wall as a beam carrying the full load to the support and would not even have considered punching shear. And as a deep beam they had to consider the compression strut coming through the wall onto the column below.

Slurgi,

Send it back to the architect and tell him/her it does not work and the walls need to extend at least to the far face of the columns below! Then make sure you check bearing.

RE: Transfer Slab Shear Checks

(OP)
Thank you all, great responses.

Re punching shear, here is a guidance based on EC2, which is very similar to what hetgen showed:


RE: Transfer Slab Shear Checks

Quote (Enhineyero )

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.



RE: Transfer Slab Shear Checks

Quote (hetgen)

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?

RE: Transfer Slab Shear Checks

Quote (hetgen)

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

Which fib bulletin is that from?

RE: Transfer Slab Shear Checks


@ Tomfh,

The cut-off according to the model code is (Cmax / d > 3)... where [Cmax] 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 Cmax / 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.

RE: Transfer Slab Shear Checks

Quote (KootK)

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.

RE: Transfer Slab Shear Checks

@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

Quote (Enhineyero)

...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.

Quote (Enhineyero)

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.



RE: Transfer Slab Shear Checks

Quote (hetgen)

@ Retrograde

The clip is from this paper.

Thanks!

RE: Transfer Slab Shear Checks

Quote (Kootk)

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.

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