1 Introduction
An HVAC department have to design using rectangular elbows with a specified inner bend radius (the manufacturer machinery dictates the inner bend radius). The question arises how they would create a family representing this component. This blog deals with creating such a family.

2 New Family
Create a new family using the Metric Generic Model family template file and save it out under an appropriate name.

Set the family category to “Duct Fittings”

Set the OmniClass instance property to 23.75.70.14.14 (Couplings for Air Ductwork). This can be useful for scheduling purposes.

3 Parameters
Create shared instance parameters for the width (common, length), height (common, length) and angle (common, angle) of the duct. This allows us to schedule these properties should a manufacturer require them. Give the shared parameters representative values.

Create a family type parameter called Inside Radius (common, length) and give it value (in the value field) of 150mm. This allows a user of the family to define their own family type per inside bend radius if required.

If the creator of the family wanted to make it a little more difficult for a user to change the inner bend radius, they could enter the value of 150mm in the formula field instead. This type property would then be unavailable for edit in the type properties when the duct bend is inserted into a project (the family would have to be edited before the inner bend radius could be changed). Another consequence of entering the value I the formula field is that family types of differing inner bend radii cannot be created.

I assume that there may be various duct bend family types varying per inner bend radius (150mm, 200mm and 250mm). I therefore create these family types and set the radii in the value field accordingly).

Create a family instance parameter called “Center Radius” (common, length) and enter the formula:

“= Inside Radius + Duct Width / 2”.

Create a family instance parameter called “Duct Length 1” (common, length) and enter the formula:

“Center Radius * tan(Duct Elbow Angle / 2)”

4 References
Work in Ref. Level plan view. Create a lateral reference plane called “Sweep Centre Lateral” above the “Center (Front/Back)” reference plane. Align dimension between the “Center (Front/Back)” and “Sweep Centre Lateral” reference planes and set the dimension label to the “Center Radius” parameter.

Work in Ref. Level plan view. Create a longitudinal reference plane called “Sweep Centre Longitudinal” to the left of the “Center (Left/Right)” reference plane. Align dimension between the “Center (Left/Right)” and “Sweep Centre Longitudinal” reference planes and set the dimension label to the “Duct Length 1” parameter.

Create a longitudinal reference line from the sweep center down to the Center (Front/Back) reference plane. Copy it and then rotate it by 45 degrees anti-clockwise with the sweep center as origin.

Draw a reference line (arc) between the two reference lines (at the ends), centered at the sweep center. This arc is the path that the sweep will use.

Create an angular dimension between the “Sweep Centre Longitudinal” reference plane and the reference line that was rotated by 45° and label the dimension with the “Duct Elbow Angle” parameter.

Draw reference lines depicting the centerline path the duct would have taken if the elbow were not there.

5 Solid Geometry
Load the profile family that is to be swept along the arc path into the family. It may be found within the appropriate library:

“C:\ProgramData\Autodesk\RVT 2015\Libraries\SA Metric\Duct\Fittings\Profiles\ M_Duct – Rectangular.rfa”

Browse to the duct profile family instance in the project browser and RMC (right mouse click) on it to edit the type properties.

Associate the width and height of the profile with the appropriate parameters.

Create a sweep along the arc path using the duct profile.

Pic the reference arc path

Select the loaded duct profile family for the profile from the drop-down.

Switch to a 3D view and change the Visual Style to “Shaded” to see the sweep more clearly.

6 Add Connectors
Place a duct connector on one of the faces (by face method) of the duct bend. This method centers the connector on the face, preventing the need to position it using dimensions and parameters.

Select the connector. Change the instance “System Classification” property to “Fitting”. Notice the instance properties availability changes.

Associate the connector height, width and angle properties with the duct height, width and angle parameters.

Place a connector on the other face of the duct elbow bend and once again associate its height and width properties with the duct width and height parameters.

In the Ref. Level plan view, select one of the connectors then link the two connectors to each other.

Change the Visual Style to wireframe to see the link when one of the connectors are selected.

7 Use in Project
Start a new project with a mechanical template and load the elbow family into it.

In the new project, browse in the project browser to the system duct family types, duplicate “Radius Elbows / Tees” and rename it appropriately. Note: any of the duct family types can be duplicated and the type properties set as required.

Select the new duct type and RMC to edit type properties, specifically the Routing Preferences.

Select a type of the new family of duct elbow for the default elbow type.

Draw a duct using the newly defined duct type and notice how the elbows are now of the inner radius type.

In this case the duct dimensions are similar to how the elbow family parameters were defined, but the elbow fitting adjusts itself to the size of duct that is used.

In this case we designate some other dimensions for the height and width of the duct and notice how the elbow fitting adjusts accordingly.

Here the duct elbow is used in a main duct. As can be seen the flow propagates correctly through the elbow fitting (as seen using the system inspector).

8 Visibility/Graphics Overrides
Currently the elbow will show in the projects even when the detail level is set to “Coarse”. We need to alter the family so that the elbow is hidden in coarse detail as with the duct.

In the family editor, select the solid geometry and edit the instance property, “Visibility/Graphics Overrides”. Uncheck the checkbox labeled “Coarse”.

This means the solid geometry will now not be shown when the detail level is set to “Coarse” in the project that contains the elbow family.

Note the geometry will be displayed in the family editor even if the detail level is set to “Coarse”!

9 Schedules
Create a new schedule.

Select the “Duct Fittings” category and give the schedule an appropriate name.

Select the following parameters to display in the schedule: Family; Type; Count; OmniClass Number; OmniClass Title. These are available for the category by default. Notice however that we also schedule the shared parameters we created, namely: Duct Elbow Angle; Duct Height and Duct Width.

Here the schedule is displayed.