Creating Trusses with Revit® Structure Sharon Fricke – Advanced Solutions Inc. Kenneth Marsh - Autodesk

Code SE304-2

Learn how to create trusses with the new Truss tool in Revit Structure. After creating different types of trusses, we must modify them. We’ll learn about the new Toolbar icons for attaching, editing profiles, and resetting the truss, then dive into the family for some in-depth look at the truss and how we can modify it to our specific needs. This class is for those engineers and detailers who use trusses other than the standard Pratt Gabled truss. Attendees should have general Revit Structure knowledge.

About the Speaker: Sharon is currently an implementation specialist focusing on Revit Architecture and Revit Structure interoperability. She has 15 years experience working with Autodesk products in Construction, Facility Management, Engineering Design, Interior Design and Design/Build. She currently works as an Autodesk Authorized Reseller in an Autodesk Authorized Training Center and is an Autodesk Certified Implementation Expert in both Revit and Building Systems sfricke@advsolinc.com

Creating Trusses with Revit® Structure

INTRODUCTION:

Revit Structure 2008 has purpose built truss tools within the software to assist in truss creation and modification. We will look at these tools to define standard truss configurations. Then we will explore truss modification utilizing these new tools.

THE OLD TOOLS

(or how can we move forward, if we don’t know where we have been)

In Revit Structure 4 – we had a family template for Structural Framing – Complex and Trusses.

In this family template you had the typical reference planes of front/back, left/right, and the Reference Level. You had the ability to create solids and voids to represent the truss family. You could also bring in components from your library to define the trusses. The catch was that if you brought in the components, the analytical information was not embedded in these elements.

You also had pre-built family parameters of the identity data. These parameters should look familiar; Keynote, Model, Manufacture, Type Comments, and well you know the rest. Other items available in the previous release for truss family creation were the family parameters as shown in the following illustration:

These parameters gave enough information to allow the elements to be exported to AutoCAD for Architects as structural members and material definitions for proper behavior.

THE NEW WAY

(or look what I can do)

Today with Revit Structure 2008 you have purpose built tools for truss creation. When we investigate the tools for truss creation we find now that there are tools for specifying the top and bottom cord as well as the webs.

The family template is also predefined with reference planes and some family parameters.

Here we can see the named reference planes as well as two of the dimensional family parameters.

LEFT

BOTTOM

RIGHT

TOP

CENTER

So what’s next? Start to define the truss family.

BUT remember, if you need a modified Howe Flat truss, OPEN the Howe Flat truss and modify. Don’t start from scratch.

So let’s go back to the truss creation mode. By selecting on the bottom cord tool in the Design Bar, you will be able to sketch the line work representing the bottom cord of the truss. Maintain it along the bottom reference plane. Then do the same with the top cord tool in the Design Bar maintaining it along the top reference plane. Make sure that you are aware of the left and right reference planes for the edges.

Once you have these two in place, then it is time to start to add the webs. But you should never randomly place webs. Nor should you place webs by dimensioning. Rather you should use the typical Revit methodology of placing REFERENCE PLANES – dimensioning and then aligning the webs to the reference planes.

In true Revit family creation format, even before placing the webs, you would flex the model, checking to be sure that the reference planes behave appropriately.

At this point, you might have something that looks similar to this:

Although it is difficult to see, there is a top cord aligned to the top reference plane (magenta in color) and there is a bottom cord aligned to the bottom reference plane (blue in color).

Notice that the webs are black when vertical and green when diagonal.

Now would be a good time to take this family – save it – then load it into a project to be sure that what you have created, is what you anticipated.

You should notice that when this family is sketched in the project, the members are defined by the last used members, in this case a wide flange. If you have just completed using a concrete beam, you will find that the members of your truss have now transitioned into concrete members. (not a good idea)

How did this happen? When the truss was created, no specific member was selected. This allows for fewer truss types to be created and modifications can occur to each truss individually.

It is interesting to note that one truss family may be created. This family may be copied in a project and the members changed. But any of the type parameters when changed will affect all of that family type. Although this is typical Revit behavior, it may throw the new Revit Structure user off. Be aware that if you need to create specific trusses with specific members – go ahead and assign the elements when creating the truss.

TRUSS MODIFICATION

When modifying the trusses, there are new tools within the program. The first method of modification would be to attach the top cord to the roof.

When hovering over the truss, you will see the blue dashed lines indicating the entire truss element. Once selected there will be no visible difference in the truss. You can tell you have the truss selected by the Options Bar. This bar will now show icons indicating the modifications that may be done. Selecting

the Attach/Detach Top Cord icon, will allow this truss to modified in relationship to the roof element. Similar to attaching walls to the roof, once this command is invoked, the next step is to select the roof element to allow the join. The caution is that this is a constraint and over use of constraints will lead to increased opportunity for project issues.

The next option for truss modification would be the Edit Profile icon which is next to the Attach/Detail icon in the Options Bar. This tool will allow either the Top Chord, Bottom Cord or both to be modified.

When this tool is selected, the truss will show as sketch mode. While in this mode, the top chord will show as magenta and the bottom cord as blue. Notice there are two reference planes, one on each side. These are guides that will assist in keeping the truss in alignment.

Again, utilizing the Edit Profile tool, you may make changes. When creating a scissor truss, select the truss and use the Edit Profile button which will put you in sketch mode. Using the Split tool, the bottom cord may be made into two separate pieces. These pieces may then be manipulated to show the lower portion of the scissor.

Then the top cord may be modified in a similar manner.

These cords do not have to be linear. With the new curve tools the top cord can be bowed. So by using a three point arc, you will now have a new configuration of the same truss.

This brings us to a very important point on the modifications to the trusses. This is the same truss that has been configured into different shapes for different purposes. It is easy to modify the truss shape but you need to be very careful. If this truss is copied in plan without looking to find further information, you can see that it might have a negative impact on the design.

Now the other option is to modify the truss family.

Once in the family editor things should start to look familiar. We had a similar truss in the beginning with our truss creation tools. Again the top chord is magenta and the bottom cord is blue while the diagonal webs are green and the verticals are black. When modifying the truss family, extreme care should be taken to achieve the anticipated results. These trusses use groups to achieve their look. These groups have formulas behind them so as the truss expands and contracts in length; the appropriate number of webs are created.

Finally – what about those trusses that don’t have the physical appearance that some people are looking for.. how can we handle that?

IF it is important that the truss be shown with the physical members in a more appropriate fashion, it can be done. This is not something that I would do for all trusses but again if it was something that was very

important, I would use the cut geometry tool to clean up all the ends of the truss. The method to doing this is to create reference planes across the areas that will be cut. So in this instance, I placed reference plane across the bottom of the top cord, the top of the bottom cord and along the areas of the webs where I needed a cut. Then select on the element (not the entire truss), unpin it and using the cut geometry tool followed by the reference plane and the element will trim to that reference plane. NOTE: if you remove the reference plane the element will go back to its original state. I have “hidden’ the reference planes in the image below.

a

The other thing you might notice is that the ends have been modified on the truss. By unpinning the bottom cord and extending it, I have created a butt end for my truss. This method takes time to get the correct results. It may not be in the best interest of the project to trim all trusses. If time and budget will not allow all trusses to be modified, show those that are key and do not show the rest.

The new tools within Revit Structure 2008 allow for easier truss creation and modifications. Take advantage of these tools and truss away!

(Tips and Tricks session to follow as time permits)