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Post-tensioned cable (PTC)

Last modified by Akos Rechtorisz on 2023/04/24 16:03

Post-tensioned cable propertyDescription
Modules where available1585056654158-405.png  1585056661203-274.png
FunctionIt is a structural component, modelled by equivalent load system.
Axis positionArbitrary (horizontal, vertical and skew)
ShapeWavy with Base points and Inflection places
  • Radius of curvature
  • Angular deviation
  • Stress function with friction losses
  • Stress function with anchorage set slip losses
  • Stress function with elastic shortening losses
  • Stress function with all time dependent losses
  • Equivalent force for 1 strand
  • Equivalent force for PTC
Manufacturingthe Manufacture drawing can be set with proper x’ and z’ shift
Default Short CommandBPTC

Table: Post-tensioned cable properties


Post-tensioned cable object (hereinafter PTC) is a structural component, modelled by equivalent load system.

warning.pngCurrently the unbonded structural design modelling is available.

The object contains of the shape (continuous line), the reference line (dashed line) and an arrow marking the active end.


The following figure shows a post-tensioned beam after jacking: the actions on the cable a), the actions on the structure b) and the modelled forces acting on the reference line c).


Besides the equivalent forces in the local z direction from the angular deviation, the neglected  x’ component of the angular deviation, the friction force, and moment caused by the friction force - cable eccentricity (relative to the reference line) could be significant along the reference line.


light.pngThe plane of the shape can be modified by using “Change direction” or “Rotation” functions.

The definition of a PTC automatically creates two load cases:

  • PTC T0: Initial stress state (after the jacking process, it contains the short term stress losses)
  • PTC T8: At the end of the design lifetime (it contains the time-depenedent stress losses)

Definition steps

  1. Start 1585056675018-687.png Post-tensioned cable for bars/Post-tensioned cable for shells command from 1585056682723-706.png tabmenu and pick the parent object (a bar or a shell), then define the line of the PTC. The two Tool window work in slightly different ways: for shells, the user can define more cables by using Line by selection option, but for bars only one cable can be defined.


    It is strongly recommended to use Axes: all aid functions (Macros, Shape wizard, Layout wizard) highly rely on them. It could greatly speed up the definition process.
  2. Click 1585056791005-884.png  Default settings and set the properties of the PTC


    • Identifier
      The program automatically generates it, but you can define custom value. Identifier (ID and Position) number can be displayed in model view (Display settings).
    • Strands
      User can set the type from the strand library and the number of the strands by typing.

    • Jacking
      User can select the Jacking side in the drop-down menu next to Jacking side. Jacking side options: Start, End, Start then End, End then Start. The last two are Both-sided jacking with same stress, but it was necessary to distinct, because the effect of draw-in could result in different stress functions using shorter cables.
      Jacking stress is calculated as 0.8 * fpk by default after the Strand type selection.
    • Short term losses
      • friction: It is estimated by EN 1992-1-1 (1) (formula 5.45) using the Wobble (k) and Curvature coefficients (µ):
      • 1585056921812-564.png
      • anchorage set slip
      • elastic shortening
    • Long term losses
      • shrinkage of structure
      • creep of structure
      • relaxation of post-tensioned cable

        The Elastic shortening loss and Long term losses can be estimated by specific dialogs:

        image-20230424122018-1.png   image-20230424122018-2.png

        Elastic shortening loss can be estimated by Estimate ES… button. This dialog is automatically filled with the parent object’s data. Calculate stress values are the result of the estimation and the Elastic shortening stress loss field is applied on the General tab if the User accepts. It uses modified EN 1992-1-1 (2) (formula 5.44) to handle sparsely placed cables:



        Where n is the Number of strand.Average stress in the structure (σc) is informative only.

        Long term losses can be estimated by the Estimate T8… This dialog works as the previous one: fields are filled with the parent object’s data and the calculated results are at the bottom (Relaxation information is in the Strand library). It uses EN 1992-1-1 5.10.6 (2) (formula 5.46). The summation of the calculated long term losses is equal to the result of this interaction formula.

        Estimation dialogs calculate cross section data for 1m wide stripes if plate object is the parent.

        Both estimation dialog is available during the Default settings editing, edit fields are filled with zeros.

    • Shape
      There are settings related to the geometry of the cable (Display settings) here.


      The Shape table can contain Base points and Inflection places: these determine whether linear or parabolic shape is applied.

      Base point: A point with exact position, user known x’ – z’ coordinates and the angle of the tangent. Usually minimum and maximum places and end points. (Black rectangles in the preview.)

      Inflection place: A function connection place (xinf), where user defines x coordinate only. Using it between Base points determines two parabolic function: fn and fn+1 where C1 continuity is fullfilled: fn(xinf) = fn+1(xinf), f’n(xinf) = f’n+1(xinf). (Blue circles in the preview.)

      warning.pngInflection place” is not an accurate mathematical expression in this feature: the change of function convexity is not ensured. However this term makes recognizable the underlying function.

      For x’ and z’ fields some  macros are available:

      • x’ macro: Start, End, Axis distances
      • z’ macro: Top, Middle, Bottom

      The Top and Bottom values mean the distance between the axis of the cable and the edge of the element. These values are used by the z’ macro and Shape wizard.


      Using Shape wizard (Shape wizard… button) parabolic shapes can be easily created. This tool highly relies on previously defined axes. The Axes define outer (marked by o) and inner spans (marked by i) along the PTC. At the span ends Base point will be created with maximal z’ coordinate, the minimal (B, E) place of the Base points and Inflection places (A, C, D) can be defined by ratio of spans (Lo and Li). B/Lo and E/Li minimum positions can be calculated automatically by checking the box next to Auto calc. min. position. At that time these textboxes are disabled.


      The same settings can be found in the Tool window as well for easier definition:

      warning.pngIt is not recommended to use Beam/Apply default physical alignment option for beam containing PTC, since it can cause unnecessary eccentricity in the shape because Shape wizard uses the physical element.


       By allowing the Display physical element optionthe section of the cross-sectioned element is shown.


      light.pngDisplay physical element option shows all cross-sectioned element, the parent object is signed by dark-grey, the non-parents are signed by light-grey.

      The Equilibrium status and Minimal radius of curvature are also displayed.


    • Results
      There are several results -listed in the order of calculation - which can be chosen from Result tab/Result drop-down menu.


      Stress functions can be displayed in fpk ratio by Display stress functions in f pk ratio option.
    image-20230424124309-13.pngThe example shows a typical Stress function in the T8 state and an Equivalent force at T0 state.



    • Manufacturing
      On this tab the Manufacture drawing can be set with proper x’ and z’ shift.

      The Manufacture drawing can be exported into AutoCAD by clicking Export…


      To fine the division section points can be inserted along the cable. Click Generate points to open the dialog. At the third option the division-distance can be set by the User.


      The position of each point can be edited with Edit points button. By clicking Sort  button the points will be sorted by their x’ value in increasing order.

Optional steps:

  1. Modify the PTC properties with the 1585057015546-777.png  Properties tool of the Post-tensioned cable bar/shell tool palette.
  2. Set the display settings of PTC elements at Settings > All > Display > Post-tensioned cables.

  3. The PTCs are stored on “Post-tensioned cable” Object layers. At layer settings, the default color and pen width can be set for all PTC elements. The color and pen width settings by selected PTC elements can be modified by Edit > Properties > Change appearance.


By clicking  Add to documentation… User can decide about the content of the documentation.


image-20230424125427-14.pngThe example shows the first page of a typical PTC documentation.


There are three connected lists available:

  • post-tensioned cables: Major and General tab data
  • post-tensioned cables secondary data: Stress status and quantity estimation
  • post-tensioned cables manufacture table: Manufacture drawing coordinates



Handling multiple PTCs properties

If the PTC-s have the same settings, length and shape, all features of properties dialog are available, otherwise only the common properties appear.


light.pngAdd to documentation… and Manufacture tab/Export… functions are available at multiple selection and they document all selected cables at the same time.


Post-tensioned cable option is added to the Colour schema. Available colour modes: ID, Strand type, Strand number, Jacking side.

PTCs can be selected by several option of the Filter: Structural element, Identifier, Strands.

PTCs have detailed tooltips supplemented by length and Stress status (connected chapters from the EN 1992-1-1: (1) and 5.10.3 (2)).


Layout wizard

It is a tool to create a set of PTCs in a specific layout. It can be launched from Structure/Shell component/Post-tensioned cable/Layout wizard.


The Layout wizard searches for the closest solution to the Unequal loading settings from the PTC layout variations which fulfilled the following conditions:

  • difference between the sum of the PTC’s forces acting in local z+ direction and the product of Considered load and Balance ratio is less than the Maximal deviation
  • geometrical requirements

The Layout calculation process shows the currently calculated layout parameters and a few indicator values. After the calculation it shows the parameters of the best found solution.


The function uses Axes to divide the selected structural element to stripes (Mid span and Column span) assuming columns in the axes cross points.

1585057125352-192.png  1585057129579-924.png

The algorythm can handle the holes of the plate regions.

warning.pngAt least two Axes crossing the plate are needed to use this tool.
light.pngChecking Shape wizard using height [mm] instead of physical model option can highly reduce the runtime if the selected plate’s thickness is constant.
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