Punching shear checks to EC2

Tekla Structural Designer
2021
Tekla Structural Designer

Punching shear checks to EC2

Aspects of Tekla Structural Designer's punching shear design that are specific to EC2.

Limitations and assumptions

Applicability of wall punching checks

Punching checks of walls on slabs are made but should be viewed with particular caution.

In particular there is some debate regarding the applicability of a punching check from a long wall - the check doesn’t consider the potential for stress concentrations at the ends of the wall.

Columns and walls not perpendicular to slabs

EC2 only provides specific design guidance for rectangular columns which are perpendicular to slabs. The program treats all columns and walls that are not perpendicular to slabs as if they are for the punching areas developed.

This is conservative as the punching area/perimeter will be smaller than that for the angled column or wall.

Overlapping control perimeters

The calculations are beyond scope in the following situations:
  • If two control areas touch then both areas are set to Beyond Scope.
  • If an edge or corner area contains another column or wall then both areas are set to Beyond Scope.

The exception to this is the pile pair check in the design of pile caps.

Loaded perimeter near slab edges

EC2 only provides specific design guidance for rectangular columns with this guidance being further limited, for the case of edge and corner columns, to those cases where the edge(s) of the slab coincide with the edge(s) of the column. In the program the equations for other column shapes and scenarios are therefore obtained by modifying those equations presented in EC2. It is considered that the modified equations result in either the correct perimeter length being obtained or a conservative value i.e. an underestimate of the perimeter length.

Punching shear perimeters


There are a number of perimeters associated with Punching Shear
  • Loaded perimeter, u0 - perimeter around the loaded area - e.g. face of the wall or column
  • Basic control perimeter, u1 - is the check punching shear perimeter 2d from the loaded perimeter
  • Outer perimeter - the first perimeter at which the punching check passes with no need for shear reinforcement - equal to or outside the basic control perimeter (equal to if the check passes at the basic control perimeter).

d is the effective depth to tension reinforcement in the slab. If a slab around the check position changes depth, the thinnest slab and its d values are used. (Note this definition changes in the presence of a drop panel. )

Loaded, control and outer perimeters

  • Loaded perimeter
    • The Loaded perimeter, u0 is the minimum length perimeter enclosing the loaded area, which for a regular column will be the column section perimeter.


    • In some cases of irregular column shapes, such as the L, T, and elbow shapes below, it will be different (shorter) than the actual column section perimeter.


    • In the case of columns with drops there are two loaded perimeters - one for the column and one at the edge of the drop, (uses each effective depth).
    • The maximum allowable stress limit is checked at the loaded perimeter.

  • Control perimeter
    • There will only be one control perimeter, u1 at check positions without drops.
    • At positions with drops there will be one control perimeter outside the drop and possibly one inside if the drop is large enough for it to fit.

    • Is of circular shape and located at two times the effective depth from column face for columns with circular cross section.

    • Is of rectangular shape with rounded corners and located at two times the effective depth outside the perimeter equivalent rectangular section for all other shapes.

  • Outer Perimeter
    • Is the punching shear perimeter with enough length to allow for concrete to resist shear stress on it’s own.

    • Reinforcement is no longer required at this perimeter.

    • No other perimeters are required to be checked beyond the outer perimeter.

    • The distance from bounding rectangle section to the outer perimeter is used to calculate the required length of reinforcement.

    • Its shape follows the control/critical perimeter shapes but changes with the use of non-uniform reinforcement arrangements (i.e.: orthogonal).

Length of the loaded perimeter u0

Loaded perimeter for columns

The length of the loaded perimeter, u0 at the column face is calculated in accordance with clause 6.4.5(3) of EC2.

Each possible column shape also has a bounding rectangle or circle calculated to aid in the design calculations.

For a rectangular column:


u0 = 2 x (D + B)

Bounding rectangle properties:

DBound = D

BBound = B

Bounding Perimeter, u0Bound = 2 x (DBound + BBound)

For circular columns


Loaded perimeter, u0 = π x D

Bounding circle, DBound = D

Bounding circle perimeter, u0Bound = π x DBound

For columns which have a re-entrant corner, i.e. where an internal angle is greater than 180 degrees, the length of a side and the slab/column interface is adjusted as indicated in the sketches below with the perimeter taken as the shortest distance around the column.

u0 = shortest distance around the column, as shown above.

Bounding rectangle properties:

DBound = D

BBound = B

Bounding Perimeter, u0Bound = 2 x (DBound + BBound)

Loaded perimeter for walls

The length of the loaded perimeter at the wall face may be calculated in accordance with clause 6.4.5(3) of EC2 as determined below.


u0 = 2 x (D + B)

Bounding rectangle properties:

DBound = D

BBound = B

Bounding Perimeter, u0Bound = 2 x (DBound + BBound)

Loaded perimeter for point loads

The length of the loaded perimeter at the point load may be calculated in accordance with clause 6.4.5(3) of EC2 as determined below.

u0 = 2 x (Dload + Bload)

Bounding rectangle properties:

DBound = Dload

BBound = Bload

Bounding Perimeter, u0Bound = 2 x (DBound + BBound)

Additional loaded perimeter drops

The additional loaded perimeter for a column/wall with a drop is defined by the perimeter of the rectangular drop

u0drop = 2 x Bdrop x Ddrop

The equivalent perimeter

For “circular” shapes of column (circle and polygon with n sides), the equivalent perimeter -

  • DEquiv = DBound x u0 / u0Bound

  • BEquiv = BBound x u0 / u0Bound


For “rectangular” shapes of column (all except circle and polygon of n sides) and walls, the equivalent loaded perimeter -

  • DEquiv = DBound x u0 / u0Bound

  • BEquiv = BBound x u0 / u0Bound


The equivalent perimeter is used in three situations

  • adjustment of the loaded perimeter length/shape u0 for edge and corner columns/walls

  • in calculation of β for edge and corner columns/walls.

  • Reduction in VEd

Length of the basic control perimeter u1

For all internal column/wall shapes and point loads the length of the basic control perimeter is

u1 = u0 + 4 π d

Where d is the effective depth to tension reinforcement in the slab.

Note: If a slab around the check position changes depth, the thinnest slab and its d values are used.

Modification of control perimeters to take account of slab openings

If any openings have been defined in the slab and if the nearest opening edge is not greater than 6d from the face of the column then the length of the loaded perimeter at the column face, u0, and the length of the basic control perimeter, u1, are both reduced to take account of the presence of the opening(s) in accordance with fig. 6.14 of EC2.

Note: When a perimeter length has been reduced to cater for openings - as the exact position of the opening in relation to the reinforcement strips is not known, the calculations conservatively ignore any patch reinforcement in the punching checks - only the slab reinforcement is used.

User Modification of control perimeters

If you want to consider the effect of openings, but do not want to place them in the slab, this can be done by defining the following properties:
  • u0 - user reduction
  • u1 - user reduction

When applied, the length of the respective shear perimeters (except that at the column/wall face) are reduced by the specified amount.

User reductions can also be applied in order to:
  • adjust the length of perimeters of irregular section shapes
  • obtain the reduced loaded perimeter for edge and corner cases as suggested by EN1992-1-1:2004 clause 6.4.5(3)

Magnification factor, beta

The magnification factor β is used to increase the basic transfer shear force VEd to take account of the increase in shear stress across part of the control perimeter due to the moment transferred into the column. It is calculated differently depending on whether the column is internal, at an edge or at a corner.

For internal columns,

  • EN 1992-1-1 clause applicability: 6.4.3(3) and 6.4.3(6),

    • β is always calculated by the rigorous method using equation (6.38) modified for a stress distribution from biaxial bending,

    • The value for internal columns in braced structures with approximately equal spans from figure 6.21N can be optionally used as a minimum,

For edge columns,

  • EN 1992-1-1 clause applicability: 6.4.3(3) and 6.4.3(6),

    • β is calculated as the minimum between the values obtained by the rigorous method using equation (6.39) modified for biaxial bending and the simplified value for edge columns from figure 6.21N with an added allowance for unequal spans equivalent to the portion of stress calculated by the rigorous method from moments acting about an axis perpendicular to the slab edge. The minimum is used because exclusively using the rigorous method becomes overly- -conservative where both experience and guidance from BS 8110 shows that regardless of column contribution to lateral stability using the simplified method produces satisfactory results,

  • EN 1992-1-1 clause applicability: 6.4.3(3) and 6.4.3(4),

    • EN1992-1-1 6.4.3(4) introduces the special case of moment eccentricity being towards the exterior of the slab. And in this special case β is calculated solely by the rigorous method using equation (6.39) modified for biaxial bending,

For corner columns,

  • EN 1992-1-1 clause applicability: 6.4.3(3) and 6.4.3(6),

    • β is calculated as the minimum between the values obtained by the rigorous method using equation (6.39) modified for biaxial bending and the simplified value for corner columns from figure 6.21N. The minimum is used because exclusively using the rigorous method becomes overly-conservative where both experience and guidance from BS 8110 shows that regardless of column contribution to lateral stability using the simplified method produces satisfactory results,

  • EN 1992-1-1 clause applicability: 6.4.3(3) and 6.4.3(5),

    • EN1992-1-1 6.4.3(5) introduces the special case of moment eccentricity being towards the exterior of the slab. And in this special case β is calculated solely by the rigorous method using equation (6.39) modified for biaxial bending,

Pad base punching shear checks

When working to EC2, punching shear checks are carried out for pad foundations using STR load combinations.

Punching shear should be checked at the face of the column and clause 6.4.4(2) of EC2 states that punching shear should also be checked at perimeters within 2d from the column face where d is the average effective depth of the tension reinforcement in the two orthogonal directions.

In Tekla Structural Designer punching shear is checked at 9 locations i.e. at the column face and at the control perimeter located at 0.25d, 0.5d, 0.75d, d, 1.25d, 1.5d, 1.75d and 2d from the face of the column. Design checks being reported for all perimeters that fall within the dimensions of the foundation.

The punching shear checks for pad bases follow the same basic principle as used for mats, the main differences being:
  • Checks at multiple perimeters up to 2d are required in pad base punching checks.
  • Column Local axes are always parallel with the pad base edges in the pad base punching checks.
  • Loads from the column are always above the pad base (one direction).
  • No openings can be placed in pad bases.
  • No shear reinforcement is used in pad bases.

Pile cap punching shear checks

When working to EC2, punching shear checks are performed for the column and the individual and paired piles.

The punching shear check for the column is similar to that for pad bases, but with the following differences:
  • the shear force at a perimeter uses the value from the column reduced by pile loads within the perimeter
  • a single additional perimeter based on the location of the pile closest to the column will be checked, only if located in the region between the loaded perimeter and the perimeter at 2d
The punching shear check for the pile is similar to that for pad bases, but with the following differences:
  • variable d is replaced with dred where dred =min (h – “pile penetration depth”, average reinforcement effective depth)
  • no moments act on top of the pile, only axial load considered
  • shear stress at the column face is checked only for the pile with the largest pile load:
    • vEd,0 = β * Pn,max / (u0 x d)
Was this helpful?
Previous
Next