Resisting Shear Stress at exterier Columns

[dtmilad]

Hello,

I have a flat plate (180 mm) thick supported on columns, I have an issue with the punching shear check at exterior columns, the thickness is not enough to satisfy punching stress due to shear and shear due to unbalanced moment. Also, Punching reinforcement can not be used because the shear stress exceeded the allowable code value.

I decided to use an exterior spandrel drop beam (200 mm / 500 mm) to resist the punching shear. However, our seiner designer asked me to check whether we can use an embedded beam instead with a geometry of (600 mm / 180 mm) and ignore the punching shear while checking the one way shear in the beam only and design it accordingly.

My question is :
Is it right it ignore the punching check by adding this slab thickness embedded beam ?
Is there any limitations on the ratio of beam stiffness to slab thickness that shows I can't use this beam?

You will find attached a picture showing the exterior edge I am designing.

Note : I am using ACI 318-11

Thank you

 

[KootK]

1) technically, if this approach works by rhe numbers, it would be okay for strength. You might wind up with some unpleasant serviceability cracking, however, due to the beam not being very stiff relative to the slab (your concern). The elastic behavior will still fundamentally be two way.

2) since your beam will be much wider than the supporting columns, there's still a version of punching shear to be checked at the beam supports. It's not just one way shear. Since your beam is no thicker than your slab, it's unlikely to work by the numbers any better than two way shear.

3) It sounds as though what you need is thicker concrete. Either your drop beams or some drop panels perhaps.

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.

 

[rapt]

I disagree. You cannot tell the concrete that it is experiencing one way shear and design for that.

It is a punching shear problem and has to be designed as one. If you have real spandrel beams, they can be included in the punching shear calculations.

Also, you should be wary of reinforcing for punching shear in such a thin slab. BS and Eurocode both require at least 200mm thickness and I would be wary even at that depth.

 

[hokie66]

rapt is correct. You can't call a section of slab a beam. The slab knows better than you. You need a spandrel beam, so provide one.

 

[rapt]

I will qualify my reply slightly.

if you select a slab/beam that is the width of the support + d / 2 in each direction and design both of them for all of the actions on them, flexure + shear + TORSION to account for the moment transfer into the side of the support, you will actually be doing what the punching shear approaches in ACI and AS3600 actually do, albeit as simplified approach.

But you must allow for torsion and that will be the problem.

The code formula for punching shear tries to simulate this and it is saying it fails, so doing the same from 1st principles should also show that it fails!

In other words, I do not think it is worth the effort as the code is already doing punching shear this way and is saying it fails!