beam - slab... shear?

[mats12]

Hello guys.

If I have a column supported on a reinf. concrete slab I may have a punching shear problem. I can solve this by making a bigger column cross section dimensions or by making a thicker slab.

What I want to discuss is what happens if I add a beam that is partly integrated with a slab. Does this mean that punching shear of a slab is no longer an issue since shear forces are taken care by a beam? Is there a possibilty of shear along the beam (between the beam and a slab). For this to happen I suspect that deflection of a beam should be huge or that load on a slab is really big.

To make it short - by adding a RC beam bellow a column, we dont have to worry about a slab punching shear any longer? Everything depends on shear resistence of a beam?

 

[jayrod12]

correct. No punching shear when you've provided a beam.

 

[JAE]

Agree as well with jayrod12.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[WARose]

Agree with the others.....no (2-way) punching shear to worry about. However, some one-way stresses will occur between the beam & wall and the slab based on difference of stiffness. Should be pretty easy to check though.

 

[hoshang]

What I want to discuss is what happens if I add a beam that is partly integrated with a slab. Does this mean that punching shear of a slab is no longer an issue since shear forces are taken care by a beam?

You should use deep beam to support a column above the slab. Try using strut and tie model.
Or you can use transfer slab.
Keep in mind slabs under a concentrated load will be subject to radial positive moments and circumferential negative moments around the downward concentrated load (in contrast to slabs supported by columns).

 

[greznik91]

Why deep beam? In order for beam to be considered as deep beam it must have the height that is usually not desirable in a lot of situations...

 

[NLEng1]

I agree with jayrod12. If you design the beam to be able to resist the shear forces along its length, and ignore any additional resistance provided by the slab, you are simply providing a greater factor of safety against punching shear. Also, be sure to provide proper end bearing support for the beam to ensure it is stiff and attracting the load as you have intended. To eliminate your concerns of cracking along the beam-slab interface you can consider designing the beam as a Tee section. However, if you limit the deflections, this should not be an issue.

 

[EagleEngAus]

A quick question/comment

I would have thought there is still a chance for the column to punch through the slab and beam if the load is a large one?

Please correct me if I am wrong.

Thanks.

 

[greznik91]

Only if beam shear capacity / strenght is not sufficient, otherwise I do not see that to be possible...

 

[WARose]

[blue](EagleEngAus)[/blue]

I would have thought there is still a chance for the column to punch through the slab and beam if the load is a large one?

No....at least not from a 2-way (punching) failure......however, as I pointed out in my post above, there still is the possibility of excessive one-way shears between the slab and the beam (or wall) based on relative stiffness & loads. (Similar to the "beam shear" in the B-B sketch in the OP.)

 

[EagleEngAus]

Okay, next question.
What if the beam were a wider and shallower "band" and the critical shear perimterer was wholly within that banded area?
Then we would be back to a punching scenario right?

Thanks for helping me clear up my understanding of the topic.

 

[jayrod12]

Theoretically yes, if it was a slab band beam, there is the possiblity of punching shear, Once your account for the increased number of bars in the slab band though, it's likely that two way shear won't govern, one way shear will.

 

[EagleEngAus]

Okay, sorry for the thread hijack, to summarise:

Both punching and beam shear need to be checked when you have a concentrated load on a wide beam, where the critical perimeter is wholly within the beam area (plan).

On a side note, I don't believe extra longitudinal bars help with punching shear capacity (using AS3600). The depth to the steel is the only thing considered, in the case of PT 0.8D is used.

I'm curious how other codes handle it?

Is anyone able to post an excerpt or an explanation of how other codes treat longitudinal steel in a punching shear scenario?

Thanks

 

[jayrod12]

By the book it longitudinal bars don't increase capacity either. I'm not entirely sure what I was getting at earlier.

However, it's likely that most slab bands and beams design for appropriate deflection criteria in a scenario like this, won't have punching shear problems.