Autodesk Revit Structure

As promised in the last AECbytes Product Review which looked at the new features in the main Autodesk Revit Building 8 release and the updated 8.1 version, here is a look at Autodesk Revit Structure, a new BIM application for structural engineering built on the Revit platform. The first version was released in June and was followed by an updated release last month, Revit Structure 2, which features interoperability with Revit Building 8.1 and Autodesk Architectural Desktop 2006, allowing architects and engineers to work in a more integrated manner. This review takes an indepth look at how Revit Structure works for structural design and explores the new integrating capability in Revit Structure 2.

Before getting into the review, let us look at the professional context within which Revit Structure is being introduced.

Structural Engineering in a Post 9/11 World

Coincidentally, the publication of this review comes a few days after the fourth anniversary of the terrorist attacks on the World Trade Center and the Pentagon. Four years ago, shortly after 9/11, I found that the only meaningful topic I could write about in Cadence AEC Tech News (which I was authoring at that time) was structural design software, as the collapse of the twin towers of the World Trade Center brought into sharp focus the critical importance of structural design. We discovered that the buildings had actually been designed to withstand the impact of an aircraft colliding into them, and what caused the twin towers to collapse was not this impact but the large amount of fuel in the aircraft. These created fires of enormous magnitude that could not be withstood by the fireproof coating on the steel structure, and ultimately caused the structure to melt and collapse. There was, however, a time lapse between the impact of the aircraft and the collapse of the building, a structural design aspect that probably saved many lives.

In the three issues of Cadence AEC Tech News following 9/11/01, I provided an overview of the state of the art of structural design software at that time. This primarily comprised of several applications providing full-fledged structural design and analysis capabilities, such as STAAD.Pro, RAM Structural System, GT STRUDL, ETABS, and so on. They had an interface for graphically modeling the structure of the building for analysis, but this was relatively primitive and focused primarily on the symbolic representation (showing analytical components such as nodes and finite elements) rather than the physical representation (showing structural components such as beams, columns, slabs, etc.) of the building. The focus of these applications was really on the analysis and design of the structure. Thus, while these sophisticated applications could be used by structural engineers to design complex structures in 3D, they still had to resort to 2D CAD after the design was complete to document the physical structure of the building. Most architects at that time were also using 2D CAD, and therefore, collaboration continued to be based on dumbed-out 2D drawings. There were only a handful of tools which actually modeled the physical structure of buildings in 3D, including Intergraph's Frameworks Plus and Bentley's Structural for MicroStation TriForma (now known as Bentley Structural), and there was no integration of these with architectural modeling tools at that time. The term BIM (building information modeling) had not yet been coined, and while the model-based concept was embodied in Graphisoft's, Bentley's, and Revit Technologies' building design solutions, it had yet to capture a significant mindshare among architects and engineers.

A lot has changed in four years, both in terms of the technology as well as in our requirements from it. BIM is now a well-known concept, and many architectural firms have already begun transitioning to working with this technology. For structural engineering firms, however, the change has been less dramatic, partially because of the paucity of structural BIM applications. Up until June, Bentley Structural was the only solution available for structural engineers who wanted to adopt the BIM approach and collaborate with architects using a model-based representation. The introduction of Autodesk Revit Structure opens up another BIM channel for structural engineers and should help to speed up adoption, particularly for engineering firms collaborating with architectural firms that are using Revit Building or Autodesk Architectural Desktop. And even though structural engineers continue to rely on dedicated structural design and analysis applications such as ETABS, STAAD.Pro, RAM Structural System, and RISA-3D for the core of their work, BIM adoption is important because it will enable these analytical tools to integrate seamlessly with structural models, which in turn will integrate better with architectural models. It will allow buildings to be designed to conform to more rigorous structural standards, such as withstanding the impact of a large aircraft (a critical requirement for tall buildings after 9/11), hurricanes of the magnitude of Katrina which just devastated New Orleans, earthquakes (such as the "big one" California is bracing itself for, expected within the next few decades), and so on. Our buildings need to be fortified against an ever-increasing list of natural and man-made disasters, and BIM technology will be critical to structural engineers in tackling the increasing challenges and demands of their profession.

With this glimpse into the professional landscape within which Revit Structure is being introduced, let us move on to look in more detail at the application itself.

Overview of Autodesk Revit Structure

This section provides an overview of Revit Structure for those who are not familiar with the Revit platform. Built from the ground up with a centralized building data model, intelligent building objects, sophisticated parametric technology, and the latest programming concepts, the Revit platform imbues Revit Structure with power as well as ease of use. Instead of drawing 2D structural plans, sections, and elevations, engineers can use Revit Structure to create a single digital model of the building structure, which combines both the physical model as well as the analytical model. It can thus be used for the different types of structural analyses engineers need to perform to design their structures, as well as to derive the construction documentation needed to build the structure.

Following modern user interface design concepts, the interface of Revit Structure is simple, minimal, and clean, as shown in Figure 1, with most of the screen space devoted to the drawing area. All the tools for modeling the various structural components, drawing drafting elements, setting up viewing parameters, and so on are contained in a single Design Bar located on the left of the window. There is only one floating palette, the Project Browser, which provides quick access to all the views, reports, schedules, sheets, and so on that are defined for the project. It is docked next to the Design Bar in the default layout. Additional components of the interface are the Toolbar located below the Menu Bar, which contains some common Revit Structure commands; the Options Bar located below the Toolbar, which contains different options for the currently selected tool; the Type Selector located on the left of the Options Bar, which allows the selection of a specific component type from the component libraries (referred to as "families" in Revit) for the current tool; the Properties Button located next to the Type Selector, which allows you to specify the different attribute values of the selected component type; and the Status Bar at the base of the window, which displays the prompts associated with the selected tool.

While we will look at the process of creating a structural model in detail in the next section, some other noteworthy aspects of Revit Structure's interface are highlighted here. One of these is the interactive dimension display demonstrated in Figure 1. As you model an object, its dimensions are displayed interactively, along with its distances from neighboring components, allowing you to position it more accurately. If you type a dimension using the keyboard, this overwrites the displayed dimension and sizes the object according to the value you have entered. When an object is selected, its dimensional relationships with neighboring objects are conveniently displayed and can again be interactively edited. Snapping is always on, and quickly locates endpoints, endpoints, and center lines of existing components. With this innovative style of dimension display and input, accurate models can be created without the rigmarole of absolute and relative coordinates. Revit, in fact, does not even have a coordinate system with an origin and the X, Y, and Z axes—it doesn't need one.

Another innovative aspect of Revit Structure's interface is that, unlike traditional CAD applications, it does not have layers. The Revit platform neatly eliminates the need for layers by automatically organizing building elements in the model according to their type—or "category"—such as columns, walls, beams, floors, and so on. To selectively see only the required element types in any view, you can turn off the visibility of the other categories for that view. There is also a tool that lets you temporarily hide or isolate selected elements, irrespective of category. While long-time CAD users may miss the flexibility of organizing any element types by layer as desired and controlling their visibility and modifiability, the absence of layers will be a welcome relief for those who have had to deal with the management hassles involved in enforcing layering standards.

The structural model can be created and viewed in any planar or 3D view. Section lines can be created to automatically derive building sections, 2D detail views can be created that are associated with the main view by callouts, schedules of any component type can be generated, and views can be placed to assemble a set of correctly referenced drawing sheets (see Figure 2). All these disparate views are of the same structural database or model, which means they are consistent. This consistency is maintained even when modifications are made—any change made in any view has a rippling effect, with all plans, sections, elevations, 3D views, and schedules automatically reflecting the change. Moreover, Revit's built-in relationships between components are maintained despite the changes, such as the connection between a beam and the columns that support it. This makes editing of the model a lot easier. Custom relationships such as alignments, spacing, and attachments between components can also be defined, and are maintained when modifications are made.

With this overview capturing the basics of how Revit Structure works, let us move on to look at the key processes in more detail.

Developing a Structural Model

In most cases, you would start in Revit Structure with an architectural plan or model. To this end, Revit Structure allows the import of DWG, DXF, and DGN files, the ability to import and reference 3D objects from an Autodesk Architectural Desktop (ADT) model, as well as the ability to cross-link with a Revit Building model. Of course, you can also create the structural model from scratch. If you are using a 2D CAD file, there is also an option to link instead of importing, in which case, it remains within Revit Structure as a backdrop and can be refreshed if the architect supplies an updated version. The imported or linked geometry can be referenced or converted directly to create new structural components. For example, the lines representing an architectural grid on a DWG file can be used to create a new structural grid either by digitally tracing the lines (referencing endpoints, midpoints, etc.) or by selecting the appropriate lines and converting them directly to Revit Structure grid lines. 2D wall lines can be converted directly to 3D structural walls in a similar fashion, using preset floor elevations to specify height. Figure 3 shows a structural model being created with reference to a 2D plan drawing in DWG format that was imported into Revit Structure and then halftoned. The gridlines of the imported drawing were used to define the grid for the structural model by simply clicking on them. Subsequently, the Column tool was activated from the Design Bar, a column type was chosen from the Type Selector list, and columns were placed on all the grid intersections in one step by selecting all the grids. You can now select all instances of the column, again in one step, and modify different properties such as the base offset, top offset, type of connection, type of material, various attributes needed for structural analysis, and so on. It is also possible to specify these properties before creating the columns.

You can add other structural components such as beams, beam systems, structural walls, bracing, slabs, and so on in a similar fashion—activate the tool, select the type, modify the default properties if necessary, and place the element as required in relation to other components (see Figure 4). There is automatic snapping based on the type of component being modeled. So, for instance, girders can be quickly created by snapping to column centers or by selecting gridlines. For the latter option, separate girders are automatically created in each segment of the gridline where Revit Structure identifies supports such as structural columns and walls, eliminating the need to model each girder separately. Components such as beams systems are created in a sketch mode by selecting the supporting beams and specifying the direction, spacing, and other properties. Slabs are similarly created in a sketch mode by drawing lines or defining the perimeter using walls, beams, etc. Adding structural walls is identical to how walls are created in Revit Building: depending upon the selected wall type and associated thickness, walls are modeled with the correct width, and the joints are automatically cleaned up. They can be drawn from scratch or converted from imported or linked geometry. Vertical bracing can be added in section or elevation views and can be snapped to midpoints and endpoints of beams. Other common structural elements such as footings, joists, trusses, and so on, round up the modeling repertoire of the program. Revit Structure also includes architectural and site modeling tools in separate tabs of the Design Bar, which allows elements such as stairs, walls, doors, windows, terrain, and so on to be modeled if required. If the modeling is being done in a planar view, it can be viewed in 3D at any time to verify that it is being created accurately (see Figure 4).

Once you have created the structural components on one level, you can copy and paste them on multiple levels of the building in one step, allowing a multi-level structural model to be created relatively quickly, such as the one shown in Figure 4.

Linking to Structural Analysis tools

Analysis is the core of the structural engineer's task, and the greatest strength of Revit Structure lies in its ability to create a fully associated analytical model of the structure that corresponds with the physical model, and then link it bi-directionally to popular structural analysis tools for performing different kinds of analyses. You can choose to send the entire analytical model or only a portion of it for analysis. Revit Structure also allows you to tweak the analytical model for correct input to an analysis program, without affecting the physical model. This is a very important capability as the analytical model will often have to be manipulated for obtaining the correct results from the analysis programs. Additionally, you can add loads and member releases, change material properties, and so on before linking to a structural analysis tool.

Currently, the analysis tools that link directly to Revit Structure using its API (application programming interface) are ETABS, RISA-3D, and ROBOT Millennium. If these applications are installed, the analytical model can be sent to any one of them by selecting them from within Revit Structure. The application is launched with the Revit analytical model open in the window, ready for analysis (see Figure 5). When the analysis is complete and the engineer accepts the results, this information is brought back into Revit Structure and used to dynamically update the entire physical model and any documentation that may have been derived from it. Thus, the analysis results can be coordinated more reliably with design, and if any structural components are resized, the application ensures consistency and accuracy between the updated building model and the final drawings.

Engineers using in-house applications or spreadsheets can directly use the API to link the analytical model created inAutodesk Revit Structure to their structural tools.

Additional Functionality

Figure 3 showed an example where the structural model was created by referencing the elements of the 2D CAD drawing of the architectural plan. Revit Structure also supports the workflow in which an architectural model is created in ADT. The engineers can import DWG building objects from ADT into Revit Structure for visualization of the 3D building model in any view, without having to import individual plan views and elevations one by one. These imported objects can be queried for information such as their material properties and dimensions, and can be used as the basis for modeling the structural components. The structural model created in Revit Structure, can, in turn, be exported back to ADT as true building objects, allowing better coordination between the architectural and structural models. The same process works for coordinating with MEP engineers who are using Autodesk Building Systems (ABS). The structural model created in Revit Structure can be exported into ABS, and duct and pipe objects created in ABS can be imported into Revit Structure as DWG ACIS solids and can be checked for interferences with the structural model.

If the architectural model is being created in Revit Building, the integration is further improved by cross-linking the architectural and structural models. The structural model can then be created more quickly—for example, structural columns can be defined simply by clicking on the architectural columns. All the structural drawings, details, and schedules are directly available on the Revit platform for the architect to view. Interference checking can be performed between architectural and structural elements to detect coordination problems and conflicts. (Interference checking was described in more detail in my review of Revit Building 8.) Architects and engineers can also use the new coordination monitor feature in Revit Building 8.1 and Revit Structure 2 to get electronic notification of any changes made during the design process, and thereby synchronize their workflow better. It currently works only for key elements such as grids, columns, and levels and allows you to copy these elements from a linked project, say an architectural model, into the current (host) project. The copied elements are automatically related to the original elements and are tracked by the Coordination monitor (see Figure 6). If you attempt to modify a copied element, a warning dialog box is displayed. Similarly, if any of these elements are modified in the original linked project, say by the architect, the Coordination Monitor issues a warning about it in the host project. You have the ability to approve or reject changes and attach comments for review.

Revit Structure has many additional capabilities that come from its underlying Revit platform including the ability to create multiple design alternatives for a portion of the design within the same file, modifying component families and creating new ones in the Family Editor, the use of groups to create repeating units, revision management and tracking, using worksharing and worksets to allow multiple team members to work on the same project, round-tripping of markups with Autodesk DWF Composer for project review, integration with Autodesk Buzzsaw for online project management, export to DWG for communicating with those using 2D CAD, and various other features. All of these operate in the same way as they do for Revit Building and have been described in more detail in my reviews of Revit 6, Revit 7, and Revit Building 8/8.1.

One final feature worthy of special mention is details, since it accounts for a substantial amount of structural engineering design and documentation. Revit Structure allows you to create 2D views of the model and add 2D parametric components such as anchor bolts in footings, fasteners, welded symbols, plates, rebars, annotations such as text, dimensions, and so forth. It also allows you to generate 3D details from the model such as 3D rebars in concrete elements, 3D steel connections, and so on. It comes with an extensive library of structural symbols including moment frame connections for beams, plate and splices for columns, brace representations in plan view, welded symbols, etc.

Analysis and Conclusions

Autodesk Revit Structure provides an integrated modeling solution for structural design, analysis, and documentation that can bring the many benefits of BIM to the structural engineering profession: reducing the tedium of manually generating drawings and schedules and of having to enter the same data multiple times into different analysis tools; reducing the inconsistencies and errors of 2D CAD-based processes and producing a more accurate drawing set; allowing greater focus on design by the minimization of drafting; enabling better coordination with the other building disciplines, particularly those using the model-based approach as well; and using interference checking within the model or between different disciplinary models to detect potential conflicts at design time rather than at construction time. It is estimated that a structural engineer in the US, on an average, uses between three to four different kinds of structural analysis tools. With a tool like Revit Stucture for modeling which links to all the necessary analysis tools, engineers no longer need to learn how to model the structure in all those individual tools and can instead focus on their most critical task—the design and analysis of the structure.

As an application, Revit Structure has the same core strengths as Revit Building that were summarized in last week's review: relative ease of use; automatic generation and coordination of all views and documents and the instant update of all views when any change is made to the model, allowing design changes made later in the project to be easily accommodated; parametric building components; built-in associativity that intelligently propagates changes to all associated elements; the ability to define custom relationships between elements; the display of temporary dimensions that can be written over to resize a component while it is being created; the immediate availability of 3D views that provides instant and critical feedback on design decisions and helps expedite client approvals; and the ability to automate many tasks related to drawing setup and coordination.

The biggest strength of Revit Structure, however, is unique to itself, one it doesn't share with Revit Building. This is the bidirectional link it has to structural analysis tools, which automatically inputs the analytical model of the structure to those tools and then uses the results of their analysis to automatically update the physical model and all related documentation derived from it. We have no such direct link to an analysis tool in Revit Building yet, so from that perspective, Revit Structure is way ahead of Revit Building.

Revit Structure also shares some of the limitations I pointed out for Revit Building: the quality of the documentation that accompanies the application is less than stellar; and it does allow some illegal operations to be performed (such as beams intersecting with other beams, columns overlapping with other columns, and so on) which does not guarantee a fully accurate building model for input to analysis tools. However, in the case of Revit Structure, these can currently be overlooked since the application is so new. Revit Structure does have some limitations of its own. For instance, its interface for tweaking the analytical model is quite primitive, making the process a lot harder than it should be. Also, the Copy/Monitor feature works in a rather convoluted manner and is far from intuitive. It needs a lot more development before it can be routinely used by architects and engineers on the Revit platform to collaborate with each other.

But it is simply so terrific to have a dedicated BIM application for structural design that links bidirectionally to popular structural analysis tools that, at least for now, Revit Structure can get away with being less than perfect!

About the Author

Lachmi Khemlani is founder and editor of AECbytes. She has a Ph.D. in Architecture from UC Berkeley, specializing in intelligent building modeling, and consults and writes on AEC technology. She can be reached at lachmi@aecbytes.com.

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