Punching shear check fundamentals

Tekla Structural Designer
2021
Tekla Structural Designer

Punching shear check fundamentals

Before you check punching shear in Tekla Structural Designer you should familiarize yourself with some concepts that apply to the punching checks generally and others that apply when the checks are being used to design shear rail reinforcement.

Overview

When punching check items are designed/checked an applied load on the slab/mat foundation is calculated accounting for the difference in column/wall axial load and bending. The slab shear stresses are then checked, accounting for the reinforcement present (background and column/general patches).

All columns and walls that are not perpendicular to slabs are treated as if they are for the punching areas developed. The relevant load is resolved to act perpendicular to the slab for punching checks. This is conservative as the punching area/perimeter will be smaller than that for the angled column or wall.

Column head drops and the presence of openings within a certain distance of the punching shear boundary are taken into account in the punching shear calculations.

Where punching checks are closely spaced and the perimeters overlap the checks are beyond scope.

If working to the Eurocode or ACI head code, the punching reinforcement can optionally be designed at certain locations as an orthogonal or circular arrangement of stud rails. For details of the locations, see: Applicability.

Note: View punching checks applied to walls with caution. Their applicability to long walls is particularly questionable, as the check does not consider the potential for stress concentrations at the ends of the wall.
 

Applicability

Punching shear can be checked at the following locations:

  • In flat slabs
    • Concrete columns supporting a flat slab with and without drops

    • Concrete columns supported by a flat slab

    • Concrete columns through a flat slab with or without drops

    • Concrete walls supporting a flat slab

    • Concrete walls supported by a flat slab

    • Concrete walls through a flat slab

    • Columns in other materials supported by a flat slab

    • Point loads at either face of a flat slab

  • In foundation mats
    • Concrete columns supported by a foundation mat

    • Concrete walls supported by a foundation mat

    • Columns in other materials supported by a foundation mat

    • Point loads on a foundation mat

    • individual piles supporting a foundation mat (but only if designing to Eurocodes, or ACI 318)

 

If designing to Eurocodes, or ACI 318, punching shear reinforcement can be designed at the following locations:
  • In flat slabs
    • Concrete columns supporting a flat slab without drops

    • Concrete columns supported by a flat slab

    • Concrete columns through a flat slab without drops

    • Columns in other materials supported by a flat slab

    • Point loads at either face of a flat slab

  • In foundation mats
    • Concrete columns supported by a foundation mat

    • Columns in other materials supported by a foundation mat

    • Point loads on a foundation mat

Punching shear check axis system

For all punching checks, the YZ plane is in the slab plane. Using the YZ system in the slab plane coherently assumes that the vertical force is positive upwards following the right hand rule.
  • Punching checks applied to point loads - by default the check Y and Z axes default to align with global X and Y. The Loaded Area Orientation angle can then be used to rotate these in relation to the global Y axis.
  • Punching checks applied to columns - as the following image illustrates, the check Y and Z axes are aligned and locked to the local axis system for the column elements so it is easier to relate forces in both objects.
     
    Global coordinate system
    Punching shear check axis system
    Note: The punching shear check axis system and the column 1D member local coordinate axis system are locked together, so if the column is rotated, the punching check axes also rotate.
    Column 1D member local coordinate system

Punching shear perimeters

Punching shear perimeters are peripheral lines around the section of the column/wall/point load being checked for punching shear.
  • Can have different shapes based on the check location, column/wall/point load section and/or type of reinforcement if applicable

  • Can be interrupted by openings or slab edges

  • Length can be adjusted by the user through the punching check properties grid

Every punching shear perimeter is positioned at a distance from column face which is a multiple of the effective depth found at the first punching shear perimeter (control/critical). This results in evenly spaced perimeters either inside and outside slab patches where reinforcement can vary.

The types of perimeter vary according to the design code being worked to. For more information, see:

The effect of slab edges on punching shear perimeters

  • Edges: perimeters have three sides or less
  • Corners: have two sides or less
  • Perimeters use the slab edges as a boundary until the length of the full internal perimeter is found to be smaller than the edge/corner perimeter

The effect of openings on punching shear perimeters


A dead-zone inside two tangents drawn from column center to opening outline is created where perimeters are considered ineffective (total perimeter length gets reduced) if an opening edge is found at less than a specified distance from the loaded area. The distance varies according to the design code being worked to. For more information, see:

User perimeter reductions

If you want to consider the effect of openings, but do not want to place them in the slab, this can be done by specifying a user reduction in the punching check Properties window. When applied, the length of the perimeter is reduced by the specified amount.

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

Reinforcement type and properties

If designing slabs or foundation mats to Eurocodes, or ACI 318, punching shear reinforcement is allowed in the form of stud rails. No punching reinforcement is allowed if designing isolated foundations.

Stud reinforcement uses the same material database as ordinary concrete reinforcement. New steel grades and bar/stud sizes can be defined by clicking Home > Materials > Reinforcement

Note: Type and Rib Type are not used in design but are relevant to filter the classes and sizes

Reinforcement design settings

If you are using the Eurocode or ACI head code, and have specified stud rail reinforcement when creating the punching checks, you should review the reinforcement design settings by selecting Design > Settings > Concrete > Punching Shear > Reinforcement Layout.

These settings are used to control:

  • Minimum and maximum bar sizes
  • Minimum and maximum spacing values
  • Auto-design method, and the “Starting number of studs per face” in auto-design

These settings only apply to slab punching checks and not the punching checks applied to isolated foundations.

For further details, see:Punching shear design settings

Reinforcement auto-design

When auto-design is selected you are can choose to select reinforcement from Minima or from the Current reinforcement layout.

Depending on how you have configured your Punching shear design settings auto-design will attempt to either:
  • Minimize Number of studs on the first reinforcement line used in the design (equivalent to number of design rails), or
  • Minimize Size of the studs used in design
When starting auto-design from minima:
  • Number of stud rails per column face starts at 1;
  • Number of studs per rail starts at 2;
  • Number of rails or stud size - depending on the design option - increases along a peripheral line until a pass is obtained;
  • Number of studs per rail increases until reinforcement is no longer required.

Reinforcement layout

Note: Punching reinforcement can only be specified when designing to Eurocodes, or ACI 318, and it only applies to slab punching checks - i.e., no punching reinforcement is allowed in isolated foundations.
  • Reinforcement arrangements - the following arrangements are available:
    • Circular arrangement (predominantly used in EC design)
    • Orthogonal arrangement (predominantly used in ACI design)
  • Reinforcement lines are:
    • peripheral lines around the loaded area passing through stud positions of the same index along all “design rails”

    • reinforcement lines are not shear perimeters
  • Rail positions - Within the circular and orthogonal arrangements the rail positions are:
    • Face rails
      • Positioned between column bounding rectangle projected boundaries and aligned with the check local axis

      • Arranged symmetrically in relation to the section bounding rectangle centerline, except at edges where the edge rail can be offset to the inside of the slab by a minimum value equal to the slab concrete cover.

      • The minimum number of rails is 1

      • The maximum number of rails is 20

    • Diagonal rails

      • Positioned at corners and at an angle with the check local axis

      • Placed at equal angles from each other and the two orthogonal directions starting at 45 degrees for a single corner rail

      • Angles are measured with center at the origin of the two closest face rails

      • The minimum number of rails is 0

      • The maximum number of rails is 10

  • Rail positions within arrangements
    • Circular arrangement
      • Face rails and corner rails

      • Can have edge rails as well

      • Provided uniformly around the shear perimeter

      • Shorter rails might be used to fulfill spacing requirements

    • Orthogonal arrangement
      • Face rails but no corner rails

      • Can have edge rails as well

      • Non-uniformly around the shear perimeter

  • Number of studs per rail
    • Minimum number of studs = 2

    • Maximum number of studs = 20

    • The actual number of studs per rail depends on the design, user editing and punching check characteristics (position, presence of openings, spacing of rails)

  • Rail types
    • Design rails
      • Identified in red in user interface
      • Full length rails
      • Rail properties are user editable
      • Considered in the design
    • Detailing rails

      • Identified in blue in the user interface
      • Used to fulfill spacing requirements (circular arrangements only)
      • Used to fill distances to edges
      • Shorter rails interrupted by opening dead-zones
      • Shorter rails interrupted by edges
      • User has no direct control over these but they respond to changes to design rails
      • Not considered in the design
  • Types of detailing rail
    • Rails used to fulfill spacing requirements (circular arrangement)
      • Can be shorter or have similar size to “design rails”

      • Will occur in circular arrangements only

      • Automatically added if the spacing of the “design rails” is larger than the maximum allowed along the first reinforcement line

      • Automatically added if the spacing of the “design rails” is larger than the maximum allowed at reinforcement perimeters beyond the first reinforcement line (shorter rails).
    • Rails used to fill distances to edges
      • Will in most usual cases have the same length as “design rails”

      • Can have different length if affected by opening dead-zones or angled edges

      • Can occur in both circular and orthogonal arrangements

      • Placed between the column section boundary and the slab edge

      • Only get used if “Design rails” are added

      • Automatically added if the column is eccentric to the slab edge by more than the maximum allowed rail spacing

      • Spacing between edge “detailing” rails is equal to the maximum allowed rail spacing in the given direction
    • Shorter rails due to interruption by an opening dead-zone
      • Will be shorter than “design rails”

      • Can occur with “design rails” and “detailing rails”

      • Can occur in both circular or orthogonal arrangements
    • Shorter rails due to interruption by edges
      • Will be shorter than “design rails”

      • Can occur with “design rails” and “detailing rails”

      • Can occur in both circular or orthogonal arrangements

Use of property sets to standardize checks and reinforcement

The benefits of using property sets with punching checks are:

  • Quick reinforcement standardization - allows to have a standard reinforcement arrangement (rails/spacings) checked at various positions throughout the model
  • Ensuring the same user property overrides are used at identical positions
  • Applying point load loaded area properties quickly to any point loads
Punching check property sets can be created in two ways:
  • from the ribbon,
    • by clicking Home > Manage Property Sets > New... > Punching Check
  • from a scene view,
    • by right clicking on an existing punching check item and selecting Create property set

Creating a punching check property set will save the following properties:

  • Tension reinforcement surface (top/bottom);
  • Point Load Breadth;
  • Point Load Depth;
  • Point Load Orientation;
  • Beta - user limit for EC;
  • User factor for Vt for BS design;
  • u0 user reduction for EC and BS design;
  • u1 user reduction for EC and BS design;
  • bo user reduction for ACI design;
  • Slab override and the associated flags for slab in y+/y-/z+/z-;
  • Slab override (drop) and the associated flags for slab in y+/y-/z+/z-;
  • All reinforcement design data (everything under "Design Input" in the properties window)

Once created, a punching check property set can then be applied to other punching checks from the Properties window.

Applying a punching check property set will update:
  • Tension reinforcement surface (top/bottom);
  • Point Load Breadth - point loads only;
  • Point Load Depth - point loads only;
  • Point Load Orientation - point loads only;
  • Beta - user limit in EC design;
  • User factor for Vt in BS design;
  • u0 user reduction in EC and BS design;
  • u1 user reduction in EC and BS design;
  • bo user reduction in ACI design;
  • Slab override and the associated flags for slab in y+/y-/z+/z-. If not overridden the slab presence flag statuses are not passed;
  • Slab override (drop) and the associated flags for slab in y+/y-/z+/z-. If not overridden the slab presence flag statuses are not passed;
  • Reinforcement design data - Will apply only where reinforcement arrangements can be designed.

Note the following:

  • Punching check property sets can be edited at any stage between creation and application;
  • Only property fields available in the destination check get populated by the property set;
  • It is not a reinforcement copy feature. As an example, applying property sets from edge or corner checks to internal checks will result in a different number of rails with the same properties being used;
  • Adding a column drop hides but does not delete the reinforcement data if it exists and this will reflect in any property sets saved at this position afterwards;
  • Inconsistent slab presence flag statuses may occur between slab and drop when a property set from a punching check without a drop is applied to another at a different position and with a slab drop;
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