How to calculate punching shear reinforcement

1. What is punching?

Punching is the failure of a plate submitted to a concentrated load. The element causing the load is punched through the plate.
The failure surface is conical (= punching cone) . The element that causes the concentrated load on the plate does not shoot through paper like a hole punch.

2. How to calculate the punching shear reinforcement

2.1 In case of a footing

• The punching shear reinforcement is automatically calculated in the general reinforcement design .

2.2 In all other cases

• Calculate the reinforcement  or F2.
• Go to the reinforcement results and activate the button for the punching reinforcement .
• First select the point, then select all surrounding plates.
  If you select the surrounding plates first, it becomes hard to see if the point is selected or not.
  Click once with the right mouse button. The dialog below opens.

  

• Punching verification around point within rectangle:
  • Define the zone in which punching should be verified
    • Dimensions Lx’ & Lz’
    • Orientation
    • Eccentricity

    Make sure the dimensions are larger than the first punching cone. If not, Diamonds will just show a warning and you can try again with bigger dimensions.

• Imposed parameters:
  The parameters in this section contain values taken from your model by default.
  They are also easy to adjust and play around with. When the punching shear results are not to your liking, you can redo the calculation with and adjusted geometry without having to redo the elastic analysis and reinforcement calcualtion first. BUT if the results are to your liking with the adjust geometry, you have to make that adjustment permanent and redo all calculations.
  
  • Define the contact surface
    • If the punching is caused by a column, you’ll see the area of the cross-section here.
    • If the punching is caused by a point load or support, you’ll see a default contact surface (=0,028m²). You have to modify this to the correct value.

    In both cases, the contact surface can be adjusted to verify the effect of a thicker column/support, without having to reanalyse the whole model.

  • Define the plate thickness
    By default you see the plate thickness from the geomety.
    The plate thickness can be modified to verify the effect of a local thickening, without having to reanalyse the whole model.
  • See the main reinforcement
    Diamonds uses the maximum of the calculated reinforcement and the practical reinforcement.
    The value you see is calculated as weighted average of either the top or bottom reinforcement in both directions: or .
    Depending on the orientation of the punching shear cone, it will either be the top or bottom reinforcement.

    Keeping in mind that punching shear is mostly verified in points that are sensitive to peak(s), this weighted average can be a peak value… For which Diamonds will assume it’s present until the last punching shear circumference! That is a conservative approach.

    It is advised to modify the main reinforcement to a more smeared-out practical value using the formula. You can use a cutline to determine this smeared-out value in both directions x’ and z’. Then you use the formula to calculate the weighted average.
    If you are in doubt between top or bottom reinforcement, you can always check what Diamonds has taken.

• Click ‘OK’ to start the calculations.

3. Interpretation of the results

Let’s say you need punching shear reinforcement, and this is the result Diamonds gives you.

Option 1 (easy but conservative)

• The area in which punching shear reinforcement is required is drawn in pink.
  Using the mouse and the distances at the bottom of the screen, you can determine the diameter of this area.
  
• Assume that in the entire pink zone, 826mm²/m² is required.
• Both radial and orthogonal lay-out patterns for the punching shear reinforcement are possible, but let’s go with this radial pattern (source image: Max Frank).
  
• Using the diameter of the pattern (orange line) determine the area of the pattern (m²).
  Determine the area (mm²) of one vertical rod (red line) .
  The number of rods multiplied by , divided by , gives the mount of punching shear reinforcement per square meter in the chosen reinforcement pattern.

     

  This values must be larger than the 826mm²/m² calculated by Diamonds and must be place all over the pink zone.

Option 2 (more work but more economic design)

• You can destinguish 2 zones in this model:
  • White zone with radius and required punching shear reinforcement of 826mm²/m².
  • Pink zone (donut shaped – outer radius , inner radius ) and required (mean) puching shear reinfrocement of 500mm²/m².

  Using the mouse and the distances at the bottom of the screen, you can determine radius of these zones.
  

• Assume a radial reinforcement pattern again.
  
• Calculate the area (m²) of the white zone using .
  Determine the area (mm²)of one vertical rod (red line) .
  The number of rods in the WHITE AREA multiplied by , divided by , gives the mount of punching shear reinforcement per square meter in the white zone.

     

  This values must be larger than the 826mm²/m² calculated by Diamonds and must be place all over the white zone.

• Calculate the area (m²) of the donut zone ().
  Determine the area of one vertical rod (red line) a (mm²).
  The number of rods in the DONUT zone multiplied by a, divided by A, gives the mount of punching shear reinforcement per square meter in the donut zone.

     

  This values must be larger than the 500mm²/m² calculated by Diamonds and must be place all over the donut zone.

4. FAQ