Punching Shear - Self Climbing Columns 1

[Drapes]

Punching shear is a clear and apparent mode of failure when the column is poured to the underside of the slab (that is, when the slab bears over the column, as is conventionally seen in most situations).

Does this failure mode also apply if the slab does not inherently bear over the column, but is rather fixed to the face of the column on all sides (via couplers, pull-out bars, post-installed reo etc), as you would typically see for self-climbing columns that overshoot the slab level? In other words, you get a vertical cold joint all around the column face, as opposed to a horizontal cold joint at the underside of the slab.

I can see the logic for doing a shear friction type check here, but a punching shear mode of failure is not as apparent, although I get the sense it cannot be simply disregarded.

 

[HTURKAK]

I understand the question is for precast flat- slab system. You should provide embedded steel collar to shift the punching surface apart from the column. I
I searched the web for ( lift slab technology , which is an old technology ) and found the picture below, which depicts the collar and hidden beam reinforcement..;

Of course you are expected to check two -way shear.

If you provide a sketch with your specific question, you may get better specific responds..

 

[r13]

I wouldn't treat this case lightly, a plate analysis can provide better picture on stresses distribution around the column. The resulting stress would be in between one way and two way shears, and the critical location should be right on the slab-column interface, unless proven by code permitted research/methods.

 

[HTURKAK]

The lift slab is supported on shear collar fixed to the column. The picture shows the concept for Steel column and RC column alternatives...and one can imagine the critical section and the punching failure surface ..

 

[KootK]

I can see the logic for doing a shear friction type check here, but a punching shear mode of failure is not as apparent, although I get the sense it cannot be simply disregarded.

1) Firstly, cudos to you for recognizing that a shear friction check is not sufficient in this instance. There's just no substitute for good structural engineering intuition. The shear friction check addresses the vertical shear plane against the column face but not the diagonal shear plane near that same location. So it would have to be [SHEAR FRICTION and PUNCHING SHEAR] not [SHEAR FRICTION or PUNCHING SHEAR].

2) Like HTURKAK, I would be very reluctant to ever rely on shear friction for the shear check on the vertical face of the column for this unless the average shear stress around the interface was pretty darn low. Rather, I'd find some way to move the support reaction delivery point to the underside of the slab such that a full depth, slab shear mechanism could be mobilized. So that's either:

a) Some kind of collar element or;

b) In a very thick slab, or a slab with a drop panel, maybe a shear key placed low.

3) Once you have an appropriate collar element in place, I don't think that the shear friction mechanism is required. Rather, it's down to:

a) Design of the collar/corbel element and it's fastenings.

b) Punching shear about a perimeter defined by the collar thing. For this, I would deem it appropriate to use one way shear capacity rather than two way shear capacity. The two way capacity, which is higher, depends on developing significant flexural compression stress in the slab at the joint. Obviously, with the slab discontinuous at the column, you won't have that convincingly.

c) Slab bearing stresses on the collar thing, perhaps acknowledging that these will be non-uniform and disproportionately concentrated near the corners.

d) Whatever you gotta do for slab flexure and integrity reinforcing at the joint. It sounds as though you've already got this covered however.

 

[Tomfh]

Punching shear is a clear and apparent mode of failure when the column is poured to the underside of the slab (that is, when the slab bears over the column, as is conventionally seen in most situations).

Does this failure mode also apply if the slab does not inherently bear over the column, but is rather fixed to the face of the column on all sides (via couplers, pull-out bars, post-installed reo etc)

The punching shear failure surface is a cone outside of both the horizontal interface between column and slab (Eg traditionally formed slab on columns), and the vertical interface between columns and slab (Eg pull out bars), and thus remains a potential failure regardless.

 

[MIStructE_IRE]

Would shrinkage of the slab around the column perimeter not greatly reduce your connection capacities here? I don’t like the idea of a cold joint at this interface and I don’t think I’m brave enough to design it!

Personally, I’d steer clear of this construction technique as there have been too many lift slab failures. If I HAAD to do it, I’d introduce some sort of collar around the column.

 

[hokie66]

I agree with MIStructE about lift slab construction. That idea has come and gone. Good riddance. But it primarily used steel column, I believe.

With precast concrete columns, it is common practice to leave a void (other than reinforcement) to allow the slab to go through. That also allows the required slab reinforcement to pass through the column void.

Shear friction, uggh! The designers of the Miami pedestrian bridge which collapsed used that theory.

 

[Tomfh]

Shear friction is ok for checking shearing failure across an orthogonal plane. The problem is it’s upper bound. It only checking shearing at that plane, and people often assume it covers surrounding failure modes, Eg diagonal shearing failures.

 

[hokie66]

The whole theory is ill defined and depends on guesses. It depends on developed reinforcement in a flexural member for clamping action, but how can reinforcement on the compression side of a member ever be credited with clamping, as it is not in tension? And if I remember correctly, the inventors of this theory attributed some of the capacity to dowel action, but that portion of the whole was undefined. Perhaps later research has cast some light on these issues, but I still reject it. And the last time I looked, it is not even addressed as an acceptable design procedure in the Australian code.

 

[Drapes]

I agree with MIStructE about lift slab construction. That idea has come and gone. Good riddance

Agree, the detail is uncommon nowadays as far as lift-slab construction is concerned, but contractors still pursue it (and engineers will generally oblige) particularly on a high rise where jumpforming larger sized columns on typical slabs improves cycle times, or where top-down construction is being proposed and you end up with plunge columns or stanchions installed prior to pouring the intermediate basement slabs.

Thank you everyone, very helpful feedback. It appears the general consensus across the board would be to account for punching shear.

Some follow up questions:

1. Assuming the punching shear capacity is shown to be adequate around the column, could you simply detail the slab to column interface via either couplers, threaded inserts, or pull-out bars, instead of resorting to a collar or angle bracket arrangement, and then check to ensure shear friction is satisfied?

2. Would it be necessary to also include a small rebate or key (say 20 to 25mm deep) around the column?

3. How would you pro-rata the shear resistance b/w the reo and the rebate/key?

4. If no rebate is provided, can the vertical joint still be justified via shear friction provisions using couplers, threaded inserts or pull-out bars in isolation? And would this satisfy the requirement to fully develop fy in the bars on either side of the joint?

For arguments sake, lets say we have a column that is at least 300 x 1500mm.

 

[KootK]

1) Answered previously.

2) Suggested previously.

3) I'd use the distance from the top of the slab to the bottom of the key as the shear depth and use one-way shear capacity as I mentioned earlier.

4) Sure, at least numerically. As I'm sure you've surmised, this is going to a judgment thing for most.

For arguments sake, lets say we have a column that is at least 300 x 1500mm.

That's basically a short wall and actually makes me feel a bit better about the whole thing. And it definitely warrants my suggested treatment of this as one way shear. I'd get all of the shear friction done:

5) On the long sides;

6) Utilizing only the top bars as shear friction bars and;

7) Still using some kind of shear key for good measure. This should be pretty easy in that 1500 mm column face.

8) Coupling the top bars across the narrow dimension of the column so that they are effectively continuous side to side, thus eliminating the development concern.

 

[HTURKAK]

I believe a picture is worth 1000 words.. I found a picture with 50 mm key provided with styropor during casting of wall .