Post-Tensioned (PT) Slabs#
Introduction#
This quick guide shows the workflow and commands available to model post-tensioned (PT) concrete slabs with Revit and SOFiSTiK Analysis + Design. After modelling in Revit, the slab’s analysis is conducted in the SOFiSTiK Structural Desktop (SSD). There, the most important steps necessary for the analysis and evaluation of results are explained.
PT Slab Workflow#
The Model#
Hint
Remember to download the project file at the top of this page. It already includes the analytical elements and boundary conditions, so you can focus on modelling the tendons and analysing the system.
The tendons will follow the grid alignment.
Geometry, materials and cross sections.#
Loads and boundary conditions.#
Adding a Prestressing System#
Make sure to select a design code before adding a prestressing system.
Setting the design code in the project’s settings, tab Analysis.#
In the Prestressing System tab, also located in the project settings, the following steps must be completed.
Create a new prestressing system.
Adjust the name of the prestressing system.
Select or create a new material for the prestressing steel.
Check and adjust the number of strands and the system’s properties.
Creating a new prestressing system and adjusting the system’s properties.#
Further details are provided in the section More about Prestressing Systems.
Go now to the tab Duct Geometry of the prestressing system. There, adjust the outer duct diameter and the minimum permissible curvature radius.
Note
In this example, we intentionally chose values not for their analytical relevance but to achieve a clear graphical display of the ducts and their tendons.
Properties of the duct geometry.#
With this, the creation of the prestressing system for the model is complete.
Project with the newly defined prestressing system.#
Drawing Tendon Projections#
In Revit, the geometry of a tendon is defined by its duct geometry. To model the ducts, we use the SOFiSTiK Analysis + Design command Tendon Projection.
It allows you to draw the pink lines as shown below. These lines define the geometry of the duct axes projected in the 2D plane of the slab. Tendon projections must:
be placed on an analytical panel,
and must align with the plane of the analytical panel.
Select a tendon projection to show its properties in the Structural Properties panel. There the tendon settings, like the prestressing system, the name of the tendon, as well as the jacking procedure, can be modified.
Plan view layout showing the tendon ducts and Structural Properties panel.#
Further details about the drawing of tendon projection are in the section More about Drawing Tendons > Tendon Projection.
Drawing Station Lines and Checking the Elevation#
To finish drawing the tendons, we define the vertical elevation of the duct using constraint points called “stations”. To create stations, draw station lines using the SOFiSTiK Analysis + Design command Tendon Station.
The station lines are shown in orange below. A station is created each time a tendon projection line intersects with a tendon station line.
Tendon station lines.#
To check the elevation of a tendon at its stations, select the tendon, then check the property Stations in the Structural Properties panel.
Access to the stations defined for a tendon projection.#
Hint
By default, the tendon’s layouts are set to Automatic. In automatic layouts, stations are always considered as high points in the tendon’s elevation. By setting the tendon layout to User Defined, you can determine the elevation of the tendon at each station.
Further details are provided in More about Drawing Tendons > Tendon Elevation.
Modelling the tendon in Revit is now complete.
Generating the Finite Element Model#
Set the current view to “Main System” and click Calculate to generate the Finite Element model of the system, including tendons.
The tasks, which are performed by clicking Calculate, can be adjusted in the Structural Properties panel of the system. At the task Generate FE Mesh, the exact geometry of the tendons and their ducts is calculated based on the tendon projections and station lines.
Calculate the main system to generate the Finite Element model.#
Hint
Ensure that both tendon and duct lines are visible in the current view. If they are not visible in the active Revit view, the tendons will not be considered in the Finite Element model.
Visibility of tendon projection lines and tendon station lines in Revit’s Visibility/Graphics dialogue.#
Staged Analysis of the PT Slab#
Open the Finite Element model of the main system by clicking on SSD (SOFiSTiK Structural Desktop). When clicking on SSD for a system for the first time, SOFiSTiK Analysis + Design asks to choose a template file. Here the template “SOFiSTiK Building” is appropriate.
In SOFiSTiK Structural Desktop, calculation results are already available for the load cases defined in Revit. However, they do not yet include the results from tendons. Tendons require a construction stage analysis.
Add a task Construction Stages in the project navigation tree.
Inserting a task Construction Stages in the project’s navigation tree.#
Double-click the construction stages task to open it. By default, tendons are introduced in construction stage 11. This matches the default value in Revit’s Structural Properties panel. If tendons were modelled in Revit using other stage numbers for their construction sequence, adjust the stage numbers in the task Construction Stage accordingly.
Check and adjust the stage numbers in the task Construction Stages.#
Important
The numbers of the construction stages that are assigned to the tendons must match the ones entered in SOFiSTiK Structural Desktop in the task Construction Stage.
The Construction Stage Manager, allows you to modify the Tendon relaxation.
Setting to control the tendon relaxation in the task Construction Stages.#
PT Slab Results#
The commands highlighted below provide access to output information. This section presents how to access from Revit some of the outputs which are typical for PT slab projects.
Visualization, Graphical Output and Report to access PT slab outputs.#
Report#
The results related to PT slabs are displayed in the TENDON section of the Report. In the tab Results, the complete report is accessible. The following figure shows an overview of the tendon stresses as a table. This information is also available graphically in the Graphical Output.
Overview of stresses in a tendon.#
By clicking on the tab Graphics, only the images of the report are listed. The following figure shows the calculated geometry of the tendon within the duct. It is placed within the QUAD-elements. Also, the elevation of the tendon is indicated.
Graphical overview of the calculated tendon geometry.#
Graphical Output#
With Graphical Output, general information about the tendon is available graphically. They are in the group System Values.
Example of tendon-related system information in GRAPHIC: display of the tendon’s number.#
After analysing the construction stages, information about tendon stresses and forces is available. This information can be found in the group Results, in the subgroup listing results for the QUAD elements.
Example of tendon-related result in GRAPHIC: display of stresses in the tendon.#
Visualization#
Visualisation opens the Viewer, which displays the Finite Element mode, including its tendons. Hide the QUAD Finite Elements, or set them to transparent, to see the tendons/ducts. Dedicated settings allow controlling the visibility of the tendons, their ducts and their anchors.
Viewer settings for the display of tendons.#
More about Prestressing Systems#
Prestressing systems can be managed in the Project Settings. The options in the red box allow you to:
Edit existing prestressing systems.
Create a new prestressing system.
Import prestressing systems from an external SOFiSTiK database.
Delete existing prestressing systems.
Prestressing systems in the Project Settings.#
All properties of a prestressing system can be defined in the tabs Reference Force, Immediate Losses and Duct Geometry. For instance, the material, the number of strands, and their total area can be defined in the tab Reference Force.
Tab Reference Force of a prestressing system.# |
Tab Immediate Losses of a prestressing system.# |
Tab Duct Geometry of a prestressing system.# |
Note
These properties should be defined and adjusted in Revit. They are visible in the SOFiSTiK Structural Desktop but should be managed consistently and for the complete project in the Revit platform.
More about Prestressing Steels#
Prestressing steel can be created in the material dialogue. The properties of the steels automatically comply with the design code of the project. Use the checkbox on the left of a property to manually override a value.
SOFiSTiK material dialogue.#
Note
These properties should be defined and adjusted in Revit. They are visible in the SOFiSTiK Structural Desktop but should be managed consistently and for the complete project in the Revit platform.
More about Drawing Tendons#
The geometry of a tendon is modelled by its duct geometry. In Revit, a simplified geometry is provided for the duct. It describes:
the “tendon projection”: the trajectory of the duct axis projected in the 2D plane of the slab.
as well as the tendon “elevation”: the position of the duct axis in the height of the slab’s thickness.
The final geometry of the tendon is calculated from this input when creating the Finite Element model and also accounts for the eccentricity of the tendon in the duct.
Tendon Projection#
SOFiSTiK Analysis + Design command Tendon Projection defines the trajectory of the tendon’s duct axis projected in the 2D plane of the slab.
Therefore, you should draw Tendon Projection lines to represent the horizontal trajectory the duct would have when looking at the PT slab from above.
Command to draw tendon projection lines.#
A tendon projection can be created in the same way as a standard Model Line.
Tip
A tendon projection line must be drawn in the slab’s analytical panel. By setting the analytical panel as the work plane before starting the command Tendon Projection, you ensure your line is in the correct plane.
Note
Tendon Projections are model lines assigned with a specific line style: “SOF_TendonProjection”.
Tendon Properties#
In the Structural Properties panel, you can assign the following information to individual tendons:
Prestressing system and tendon name
Elevation (geometry)
Construction Stages
Additional prestressing information
Tendon Elevation#
The elevation of a tendon’s duct axis is its trajectory in the slab’s thickness (height and inclination).
The elevation is defined by creating constraint points called “stations” along the tendon’s projection. At these stations, the elevation of the tendon is fixed. Everywhere else, the elevation is calculated according to the chosen tendon layout.
SOFiSTiK Analysis + Design command Tendon Station allows you to draw model lines to define stations. A station is created each time a tendon projection line intersects with a tendon station line. Using station lines instead of single points to define stations increases modelling efficiency in PT slab projects containing several tendons with similar elevations.
Command to draw tendon station lines.#
A station line to generate stations for tendons.#
Tip
A tendon station line only intersects with tendon projections when they are in the same plane. By setting the analytical panel as the work plane before starting the command Tendon Station, you ensure your line is in the correct plane.
Note
Tendon Stations are model lines assigned with a specific line style: “SOF_TendonStation”. They do not have SOFiSTiK properties.
There are two options to derive the tendon elevation: you can either use an automatic tendon layout or define it manually.
The automatic tendon layout considers stations as high points and creates a free tendon layout between them.
The user-defined tendon layout considers stations as control points of a spline.
Automatic Tendon Layout#
The automatic tendon layout uses four input values a, b, c and d, to determine the shape of the tendon.
Stations are considered as high points, and the tendon otherwise runs in the lower layer. Midspan points are automatically generated internally. Distance from top/bottom refers to the position of the duct’s center line relative to the top/bottom of the slab.
User-Defined Tendon Layout#
The user-defined tendon layout considers Stations as control points. The elevation of the tendon at these points is controlled in a dedicated dialogue box.
In the User-Defined tendon layout, the height and inclination at each station are managed in the dedicated dialogue box.# |
Properties to control a user-defined tendon layout: on the tendon’s geometry. Axis Reference: Distance from top (a), Distance from bottom (b and c). Inclination: free inclination (d and e), fixed straight length of 1000 mm on the right side (f).# |
Hint
Definitions of tendons in Revit do not show the final geometries. To obtain this geometry (both duct and tendon), it is necessary to calculate the Finite Element model. In the Finite Element model, the final geometry of the duct and the tendon is visible. See PT Slab Results for more information about tendons in the Finite Element model.
Note
After generating the Finite Element model, you can read the final geometry of the tendons with Dynamo nodes. With this, you can create tendon drawings and plans in Revit.
Tips for Modelling Multiple Tendons#
Tendon projections and tendon stations are Revit Model Lines assigned with a specific line style. It is recommended to use standard Revit commands to model them efficiently. E.g., use Copy to duplicate them.
Alternatively, existing Model Lines can be easily turned into tendon projections or tendon stations by changing their line style.
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