prestress in punching shear slab 1

[bratty]

In PT slab, prestressing helps to increase the punching shear resistance. If the column is elongated and out of balance moment is about narrow width which direction of prestressing stress should one consider to assist in the punching shear? Or the prestressing stresses come in 2 directions as the slab is prestressed orthogonally?

Will the elongated side of the column subject to longitudinal shear as the axial stress in the slab varies as the force in the tendons varies too? If this is true, then the axial stresses due to PT will help in one direction but reduces in the orthogonal direction?

 

[cliff234]

In my experience designing post-tensioned flat plates (mostly 7" or 8" thick), the p.t. does not significantly improve punching shear strength. As a somewhat conservative old engineer, I do not rely on any additional punching shear strength from the tendons. If you have a punching shear problem, reliance on the tendons to get you out of trouble will not work.

 

[bratty]

Cliff234

Thanks for your. Based on my workout, for every 0.5 MPa compressive stress, the punching shear resistance increase by 10%. So for 1MPa prestress, the punching shear resistance increase by 20%. To avoid drop panels, understanding of direction of the axial stress is critical to avoid non ductile shear failure

 

[hokie66]

What code or theory is your basis for that much increase in capacity?

 

[bratty]

Hokie66

It is AS3600. In any concrete code, any compressive stress will help to increase the shear even for beams

 

[hokie66]

Must be a relatively new addition to AS3600. I knew that ACI had something like that, but preferred to ignore it. My philosophy is to take the most conservative approach for punching shear.

 

[rapt]

The punching shear calculation in AS3600 has always had a P/A component.

If the P/A in each direction is very different, I would use the average. If the sides lengths are very different and the P/A different in the 2 directions, I would weight the average based on the side lengths.

Seeing it is compression on the shear perimeter we are looking at, the P/A transverse to a side would act on that side.

At edges, you need to consider the location of the tendons relative to the columns to decide if there will be P/A on the shear perimeter.

 

[bratty]

Rapt
What about the P/A along the edge of the column or which is parallel to the column? Along the face, will it be a longitudinal shear?