What Makes Revit Structure Different
I have two young boys, 4 and 2, and I love watching them learn. However, there are so many times when they get frustrated because they do not understand how something works. For example, I played games with both of them on the iPad before they fully understood the basic functionality. Later, I would find them trying to play the same games, but they would be stuck at a certain step.
The frustration and anger from an 18-month-old can be quite comical. But once the boys mastered the basic functionality, it wasn’t long before they were exploring their iPad games on their own.
Mastery learning is the same as the age-old saying, “You have to learn to walk before you can run.” This same concept of mastery learning also applies to Autodesk® Revit®. You have to learn how to model before you can analyze and document. Otherwise, what are you really creating—a model or drawings?
The structural tools in Revit are similar to all the other tools. For example, you can still place or sketch elements. However, structural elements and families behave a little differently than other families. My goal is to point out some of those differences by focusing on some areas in Revit that are different on the structural side and are important to understand during the design phase.
There are four features within the structural tools in Revit that have some unique behaviors: the structural analytical model, the Material for Model Behavior parameter, the Section Shape parameter, and trusses.
Structural Analytical Model
To begin, you should understand that an analytical model is created along with the physical structural model. The structural analytical model is created in Revit so that the physical model can be analyzed. However, your company may not use this analytical model for analysis. The good thing is that the analytical model can be turned off, if needed. Nonetheless, you should realize that it is being created in the background unless you disable it.
The structural analytical model can also be used for other purposes such as advanced modeling and QA/QC of the physical model. You do not have to concern yourself with these advanced topics when you are getting started with Revit—just realize that the structural analytical model can have purposes other than strictly analysis.
An analytical model is made up of a set of structural member analytical models. Each structural element you place in a project with Enable Analytical Model selected in the Properties palette will contain a structural member analytical model. Structural columns, structural framing elements such as beams and braces, structural floors, structural walls, and structural foundation elements can have structural member analytical models.
To get a better sense of the analytical model, start a new project using a default Revit structural project template. Once the project is open, you can see the default views in the Project Browser. In addition to the standard Level 1 and Level 2 views, there are Analytical views for these levels, as well as an Analytical Model 3D view.
When you start one of the tools to place an element that can contain a structural member analytical model, you can control whether or not the analytical model is created with the element's instance properties. In the Properties palette, under Structural, pay attention to Enable Analytical Model. This parameter is selected by default.
Just to see how it works, create a few columns, beams, and structural walls with Enable Analytical Model selected. Then switch to the Level 2 – Analytical structural plan. For the Analytical views, the display of the analytical model is turned on. The display of the analytical model can easily be toggled on and off. There is a control in the View Control bar to toggle the display: Hide Analytical Model/Show Analytical Model. You can also turn on the visibility of the analytical model along with the physical model if you wish.
Switch to the Analytical Model 3D view. Once again, the visibility of the structural analytical model is turned on in this view. An analytical model is a simplified 3D representation of the full engineering description of a structural physical model. The analytical model consists of those structural components, geometry, material properties, and loads that together form an engineering system.
Figure 1: Sample view of structural member analytical models
As for the actual structural member analytical models, you can select them like any other Revit element. When selected, you can view the instance parameters in the Properties palette. However, be aware that if you move one of these analytical models, the physical structural element will be moved as well. And if you move a structural element, the analytical model will move because they are linked.
You must be aware of the structural analytical model when creating structural elements. After a structural analytical model has been created, you can use it to analyze the structural design. You can run consistency and member support checks, apply boundary conditions, and add loads. Autodesk also provides additional structural analysis tools that you can use to analyze and check your structure from within the Revit environment. These tools are not part of the standard Revit installation, though. The analytical model can also be linked to an external structural analysis program such as RISA or ETABS.
Material for Model Behavior
When I first started learning how to use the structural tools in Revit, I was confused with the different parameters for the different materials of beams and columns. In other words, I could not figure out what controlled the different parameters for the various materials of beams and columns. For example, steel beams and concrete beams are in the same category, they are created with the same family template, they are placed with the same tool, and so on and so forth. Looking back now, the answer is simple! The Material for Model Behavior parameter in the family controls how the elements will behave in the project environment.
First of all, you must realize that beam and column families are loadable families. Typically, elements that are defined by a path are in system families such as walls, ducts, pipes, etc. This is not always the case, but for the most part it is. Beams and columns are also defined by a path. In other words, the geometry is determined by the length of the path. The cross-sectional geometry, on the other hand, is determined by a loadable family for beams and columns.
Figure 2: The Material for Model Behavior parameter can be controlled in the Family Editor
When you open a beam or column family in the Family Editor, pay attention to the parameters in the Properties palette. One is Material for Model Behavior. The available options are Steel, Concrete, Precast Concrete, Wood, and Other.
The option you select then controls several behaviors for elements of that family that are placed in the project. It controls the parameters that are available, cutbacks from other structural elements, joining behaviors, shape handles that will be available, and whether or not the element will be able to host reinforcement. So, you can see that this is a very important parameter to set correctly from the beginning of creating a family.
While you may not copy a steel beam family to create a concrete beam family, you may be tempted to copy a precast concrete beam family. While this may not be best practice, just ensure that you change the Material for Model Behavior parameter. Or if a beam or column family is not behaving properly, check this parameter first.
The Section Shape parameter was a new feature added to structural framing elements in 2015. If I had to describe what this parameter added to structural design with one word, it would be "consistency."
This parameter provides additional dimension and structural analysis properties that you can use in external analysis and code checking applications. These additional parameters ensure accurate data exchange with structural analysis, code checking, and steel detailing applications.
The reason is because the additional dimension and structural analysis parameters that they add to the family are standard across each family that the shape is added to. The parameter values can be modified as needed, but the parameters remain the same.
As far as controlling the geometry of the family, you can create as many parameters as necessary. These parameters may vary from family to family, which is a problem when it comes to analyzing the design. So instead of trying to standardize every single parameter that controls the geometry of structural framing families, you can simply add a section shape type.
Think of it this way: If two people start from scratch to build a wide flange beam family, are they going to use the same dimensions and parameters to control the cross-sectional geometry? Will they even use the same general layout of reference planes? With the answer being "probably not," then how will there be consistency in analysis, detailing, and code checking when various structural framing elements are used? Autodesk’s solution to this issue was Section Shapes.
Figure 3: The Section Shape parameter can be controlled in the Family Editor
As with the Material for Model Behavior parameter, the Section Shape parameter can be controlled in the actual family. In the Family Editor, pay attention to the Section Shape parameter in the Properties palette. When you click in this field, you can click the More button in order to open the Structural Section Properties dialog. In this dialog, simply choose the appropriate section shape.
Figure 4: Apply a Section Shape by selecting the appropriate shape in the Structural Section Properties dialog
Once the family has a section shape applied, additional type parameters will be available in the project environment. These parameters do not control the actual geometry of the family. They are just available for external analysis, code checking, and detailing, so that each family with the specified Section Shape applied will have the same parameters available for calculations. Inside of Revit, the parameters can be used in schedules.
Trusses in Revit are made up of structural framing elements. Essentially, truss elements host structural framing elements to model actual truss designs. The difference here is that truss families are not structural framing elements—they are profiles. Once loaded into a project, the profiles will host structural framing elements.
Trusses are still model elements. However, when you open or create a truss family, you will notice that the tools are a little different. You do not create actual forms to model the geometry. Instead, you create profile lines to model the top chord, bottom chord, and webs. The actual geometry of the structural framing elements hosted on the truss are controlled in the structural framing families.
The design of trusses in Revit actually simplify things. Due to trusses hosting structural framing elements, you do not have to create all this geometry in the family. Just think of the difficulties that would occur if you had to.
Figure 5: Truss families consist of profiles that define the top chord, bottom chord, and webs
Additionally, when you create truss elements in a project, there is one thing you must understand. In the Type Properties for a truss, you can control the Top Chords, Vertical Webs, Diagonal Webs, and Bottom Chords. While you should specify all the parameters, the Structural Framing Type is critical. This is where you control the structural framing type that will be hosted to the associated truss profile.
Figure 6: The Structural Framing Type used in the truss can be controlled in the Type Properties
When set to the default Set Framing Type option, the last used structural framing element in Revit will be the member that is hosted to the truss. In other words, if you just placed a concrete beam, then that concrete beam type will be placed in the truss. In order to avoid the wrong framing elements being used, you can explicitly define them in the truss type parameters. You may want to create multiple types in order to have various options. Just be aware of this functionality with trusses.
Revit is a very powerful building information modeling tool. As with any tool, it is critical to understand its full potential so you can maximize the output you get from that tool. These four features within the structural tools do not make up an exhaustive list of all the differences from traditional Revit tools, but they are important things to understand. Frustration and anger from grown adults is not always comical… ;)