Post-Tensioned Concrete Slabs

February 28th, 2013

As we all know, Autodesk® Revit® doesn’t provide post-tensioned concrete modeling. Inserting tendons in the concrete is a very tedious and time-consuming procedure, even if we do it just as 2D annotation.

I have worked out an easier procedure for adding and modifying tendons (2D) by creating a number of new families we can use over and over.

Modeling the Slab

We start by modeling a cast-in-place concrete slab. For most large PT slabs, we also have to model the drop panels at the column supports. First model a flat slab for the entire area, then model the drop panels as separate slabs from the bottom of the flat slab. Merge the flat slab and drop panels into one slab.

Figure 1

Figure 2

When we have finished modeling the slab itself, we duplicate the plan view (DO NOT use Duplicate with Detailing). This way we have one view for the Mild Steel reinforcement and another for the Post-Tension tendons. Having two views will make the drawings easier to read and less crowded with information.

Modeling Mild Steel in the Slab

After modeling the slab, the drop panels, and openings, we can begin modeling the mild steel.

Above each column (even if we don’t have drop panels) is a pocket of top bars. We model these bars as Area Reinforcement.

In the properties dialog box, do the following.

Figure 2a

  • Change the layout rule from Maximum Spacing to Fixed Number.
  • Turn off the bottom bars and change the number of Major and Minor Direction rebar as needed.
  • Add the reinforcing
  • Remove the Area System
  • Change the length of the resulting bar sets to the required length by dragging the ends or changing the “B” dimension in the properties dialog box.
  • Tag the reinforcing with the Rebar tag and the Quantity and Type tag.

Figure 3

On the slab edge at walls or other supports where there is reinforcing in only one direction, we use Path Reinforcing. For Path Reinforcing we can either leave the system intact or we can remove the system to see all the reinforcing detail. In the Properties dialog box change the spacing, length, and diameter as required. If the system is removed, the reinforcing set needs to be tagged as before.

Figure 4

Figure 5

Annotating the Mild Steel

If the company’s preference is to annotate with 2D elements the mild steel and not to model the reinforcing, one additional family needs to be created for the rebar shapes.

Family Creation

90 and 180 Hooks

  • Start a new Detail Component family.
  • Create a 180hook component.
  • Save.

Figure 6

  • Start a new Detail Component family.
  • Create a 90hook component.
  • Save.

Figure 7

Reinforcing

  • Start a new family using the Detail Component Line-Based template.
  • Load the 90 and 180 Hook families.
  • Add a reference plane about 3’-0” from the base.
  • Dimension the plane location.
  • Insert the two detail components at the origin.
  • Add a line between the base and reference planes.
  • Mirror the two detail components about the center of the line you just added.
  • Separately dimension the line and the two detail components at the reference plane.
  • Create the same label for all the dimensions.
  • Create Instance Parameters for all the detail components.
  • Add controls in both directions for flipping.
  • Save.

Figure 8

The call-out on the reinforcing is simple text, and the extent is a special dimension style defined for this purpose. In the dimension, just change the real number to the letter “o” and move it to the intersection of the reinforcing and dimension line.

Figure 9

In Figure 6 each reinforcing is the same family with the hooks turned on and off as required. The bars also can be stretched to the desired length.

Figure 10

Post-Tensioned Reinforcing

At first I tried to model the sections of PT tendons as a straight bar and then edited the sketch and added the curves. This ended up being extremely time consuming and cumbersome, and most of the tendons would have to be individually modeled. This would not be an advantage because you still would have to add all the necessary text for tendon height and pulling force. After a lot of debate I decided that for now the only way is to just draw 2D annotation.
Reaching this conclusion required me to take an inventory of the symbols and detail components I would need to make this task easier.
My list for the families is not too long: Special Arrow, Terminator, Curved Tail, Curved Head, and a Straight Tendon with Arrow and Terminator. The Arrow and Terminator will be Annotation families and the rest Detail Components.

Family Creation

Arrow symbol

  • Start a new Family with the Generic Annotation family template.
  • Create a triangular filler region.
  • Add lines around the filled region. The lines should be the same line weight you choose for the PT symbols, below.
  • Save the Family as your Arrow symbol.

Terminator symbol

  • Start another new Family with the Generic Annotation family template.
  • Draw a short line for the tail (make the length match your AutoCAD® standard).
  • Save the Family as your Terminator symbol.

PT Head and PT Tail symbols

  • Start a new Family with the Generic Annotation family template.
  • Add an arc about ¾ to 1” long from the origin tangent to the horizontal reference plan.
  • At the end of the arc add the Arrow symbol perpendicular to the arc.
  • Save as your PT Head symbol.
  • Repeat using the Terminator symbol to create your PT Tail symbol.

Figure 11

PT Head and PT Terminator Detail Components

  • Open a new Family using the Detail Items family template.
  • Load the PT Head symbol you created and add vertical and horizontal controls for flipping.
  • Save as PT Head Detail Components.
  • Open another new Family using the Detail Items family template and repeat the above using your PT Terminator symbol.
  • Save as PT Terminator Detail Component
  • Load both families into the project.

Figure 12

Tendon

To make a tendon we will use the new Arrow and Terminator symbols.

  • Open a new family with the Detail Item Line-Based family template.
  • Load the families Arrow and Terminator.
  • Insert these two at the origin.
  • Add a line from the origin to the reference plane.
  • At the other of this line again insert an Arrow and a Terminator symbol.
  • Dimension the end of the Arrow and the Terminator separately.
  • Create the same label for all dimensions (this way they will move together when you stretch).

Figure 13

  • Select the Right Arrow and at the label create an Instance parameter.

 

Figure 14

  • Select the Right Terminator and at the label create an Instance parameter
  • Do the same for the Left Arrow and Terminator. You will now have four Instance parameters.

Figure 15

  • Save as your Tendon Family.
  • Load the Tendon Family into the project.

The detail components will be stretchable, and you can turn the arrows and terminators on or off at each end. This Tendon Family will be used for the majority of the PT representations in your model.

Figure 16

When you have a curved portion between the straight lines, just add a spline line between the tendon detail components. When you have intermediate fix points, just add the tail family as shown in Figure 13.

The extent of the tendons can be defined with the same special dimension style used for the mild steel.

Figure 17

The tendon Height and Pull Force is simple text added as needed.

Conclusion

Setting up all these families, symbols, and detail components will seem like a lot of work at first, but using them instead of drawing each 2D tendon individually will save you an enormous amount of time and frustration as you create the initial project model and make the many inevitable modifications.

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About the Authors

Anamaria Brown

Anamaria Brown

Anamaria Brown is the owner of AMB Structural CAD, an AutoCAD and Revit consulting firm that specializes in drafting complex AutoCAD projects and Revit modeling. A structural engineer in her native Romania, she escaped to the United States in 1985 and almost immediately began working as a drafter for Skidmore Owings Merrill at the beginning of the CAD revolution. Deeply involved with Draft, Microstation, and the development of SOM’s AES, she quickly became the major projects CAD team leader for SOM, and later for Epstein and Sons and Thornton Thomasetti. As a member of the National CAD Standards Committee she was involved with the creation of Version 3 of the NCS. Most recently she worked for TRC Worldwide in Sarasota, Florida. You may contact Anamaria through her website, AMBStructuralCAD.com, via email to AMBrown@ambstructuralcad.com or by phone at 941.735.6186

 

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