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Punching Shear

Punching Shear

Punching Shear

I am designing a post-tensioned flat plate slab

We have used StudRails in the past, so I am using the software developed by Decon Studrail to check the punching shear at each column and the size the reinforcing when required.

Unfortunately, I need shear reinforcing at almost every column.

The Decon software uses the joint moments in both directions simultaniously.  ACI 318 only address punching shear with moments in one direction at a time, just like every concrete textbook I have.

Doesn't this seem rather conservative?

Any suggestions our there?

RE: Punching Shear

ACI has a new techincal document that discusses alternative shear reinfocement (like Decon studrail).
I don't know the name of the document, but you should be able to find through ACI publications order desk.

RE: Punching Shear

I have also used Studrails, and spoke at length with the owner of Decon, and his Masters' thesis supervisor at University of Calgary, who "wrote the book" on studrails.

Their main advice was that load cases should not be unnecessarily combined. For example, in calculating the punching shear caused the unbalanced moment condition, only the area load causing the the moment should be used in design, and not the total "axial only" column load. Alternatively, in the "axial only" case, the unbalanced moment should be small, depending on the difference in your spans.

Designing the punching shear based on the worst of both conditions will lead to shears that are unrealistically high.

Breaking it down further, you may be able to design the punching shear in each direction seperately. BUT keep in mind that pattern loading (ie, concentrated loads, wall loads, etc) may need special attention as more load combinations may be possible.

Studrails are a good innovative product that will certainly gain more recognition in North America in the future.

Hope this helps.

RE: Punching Shear

This is a followup to MATHMAN

I appreciate your post regarding your conversations with the owner of DECON.  Do you know if what you discussed with him is documented anywhere?  Unfortunatley, the portion of the code that they re-produce in the software literature specifically indicates the summing of the unbalanced moments in each direction as the appropriate design method.  I do agree with you however that this appears very conservative.


RE: Punching Shear

It really depends on how you have analyzed your slab. If you have analyzed it using the equivalent frame method, then your unbalanced moment in each direction is the result of taking 100% of the load in that direction. Clearly, it would be rather conservative to check punching shear combining two moments, each of which are based on taking 100% of the load in that direction. On the other hand, if you have used a FEM approach to determine your unbalanced column moments, the moments are "real" and should be combined when checking for punching shear.

What does ACI 318 say about this? After re-reading it this morning, I could find no explicit direction one about the combination of moments from two directions. This is hardly surprising since ACI tries (most of the time) to avoid telling us how to analyze our structures. It's up to us to determine our design demands through rational analysis; it's up to them to prescribe the methods to calculate member capacities.

ACI 352 does indicate that biaxial bending should be considered, but again doesn't discuss how one arrives at the moments that should be considered when performing this check. I would point out, however, that the equations are empirically based. The moments for which the equations have been calibrated are the actual moments from the physical test. These moments are probably similar to what an FEM model might show, but are likely much less than what equivalent frame model would show.

RE: Punching Shear

This is a followup to HochwaltPE

Thank you for your insight.  

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