Punching Shear Question

[Nick6781]

Need someone to validate my thoughts.

The diagrams on the left show a continuous slab with an idealized fixed joint at the interior column. The unbalanced moment Mu in the slab will be transferred to the top of the column. According to ACI 318-11 Section 11.11.7.1 (Transfer of Moment in Slab-Column Connections), part of the unbalanced moment is transferred through flexure, while the remainder is transferred through shear stress. This part is clear to me.


The diagrams on the right show the same continuous slab, but with a hinge at the interior column connection. This condition could arise in practice if the slab is supported by a slender steel post or an unreinforced concrete column, or a masonry pier. Since the slab within the critical shear perimeter does not bend about point O, there is no moment transfer at the joint. Additionally, the combined shear stress distribution along the critical shear perimeter should be relatively uniform. Given this, should we only check for pure punching shear in the scenarios I mentioned? Am I missing anything?

 

[jhnblgr]

Transfer of Moments in Slab-Column Connections

www.structuremag.org

 

[HTURKAK]

I do not agree with the figure for second case. The slab negative moments at an interior column can be unbalanced that is, different on either side of the
column if the column can resist to the moment. In your case , the column providing pin support to the slab so some limited unbalanced moment can be discussed to redistribute with torsion.

I will suggest you ;
- Develop a FEM and analize the slab to see the moments.
- Pls look ACI SP-17 THE REINFORCED CONCRETE DESIGN HANDBOOK for worked examples.

 

[Nick6781]

I do not agree with the figure for second case. The slab negative moments at an interior column can be unbalanced that is, different on either side of the
column if the column can resist to the moment. In your case , the column providing pin support to the slab so some limited unbalanced moment can be discussed to redistribute with torsion.

I will suggest you ;
- Develop a FEM and analize the slab to see the moments.
- Pls look ACI SP-17 THE REINFORCED CONCRETE DESIGN HANDBOOK for worked examples.

In the second case, the moment is unbalanced; however, the sum of moments about point O — M1+M2+V1⋅L+V2⋅L— is zero, as point O is a hinge. This can be demonstrated using a 2D beam model. Basically, my argument is that no unbalanced moment needs to be transferred through the slab-column joint if the column is not detailed to resist significant moment.

 

[KootK]

Given this, should we only check for pure punching shear in the scenarios I mentioned? Am I missing anything?

With regard to steel posts and such, it is my experience that most engineers will treat that as uniform, vertical punching shear as you suggested.

Theoretically, there is some nuance to be considered that might render that approach dubious.

Very few practical situations will provide anything close to true pin behavior. What you're left with in reality is basically "fixed until it tears apart". Maybe you're okay with it tearing apart... maybe you're not.

Realistic situations where the columns offer some degree of rotational restraint to the slab may have the following for unbalanced moments affecting punching shear:

1) The moment associated with full fixity if nothing fractures or yields.

2) The moment associated with yield if something in the connection does yield.

3) The moment associated with overcoming the degree to which the connection is prestressed by the supported axial load.

Even in the case of a fairly true-ish pin connection, one then has to wonder whether there will be anything uniform about the shear delivery. Most often the slab will have a non-zero slope as it crosses the column connection which implies kind of a line load / one way shear condition rather than uniform two way shear.

You can't have your cake and eat it too. Either the column matches the curvature of the slab and there is conventional moment to be considered a the joint. Or the column does not match the slab rotation and you have eccentric, non two-way shear delivery. And all points in between.