Autodesk Revit Structure 4

It was exactly a year ago last September when I first reviewed Autodesk Revit Structure, the BIM application for structural engineering built on the Revit platform that, along with Revit Building and Revit Systems, forms a complete and integrated BIM suite for supporting collaborative multi-disciplinary building design. Revit Structure has been going through a highly accelerated development cycle since its first release 15 months ago. The version I reviewed last September was Revit Structure 2; this was followed by Revit Structure 3 in April. Towards the end of last month, yet another new version, Revit Structure 4, was released. This review takes a detailed look at the improvements introduced in both these recent versions of Revit Structure.

For a comprehensive overview of Revit Structure, please see my last review of Revit Structure 2, where I also devoted some time to exploring the professional context—the post 9/11 world—within which the application was being introduced. For a detailed overview of Revit's architectural application, please see my review of Revit Building 9 published earlier this summer. An updated version, Revit Building 9.1, is now available for compatibility with the new version of Revit Structure.

Improved Modeling Tools

Both Revit Structure 3 and Revit Structure 4 have introduced several improvements to make the modeling of structural elements more efficient and intuitive. One of the biggest pain points that was addressed in Revit Structure 3 was the modeling of sloped elements such as beams and braces, which was tedious to do earlier, requiring additional reference planes to be defined. A 3D Snapping option is now available, which, when checked, allows a sloped beam, for example, to be directly created in 3D by snapping to the required points of other structural elements, irrespective of the current work plane. You can also specify precise starting and ending heights of a beam by adjusting two new instance parameters: the Start Level Offset and End Level Offset respectively (see Figure 1-a).

Modeling an entire system of sloped beams has been made even easier in Revit Structure 4 with a new one-click beam system and an associated 3D option. To quickly add a beam system to a closed loop of structural members such as the sloped beams shown in Figure 1-a, all you need to do is activate the plan or ceiling view, select the Beam System tool , specify options such as beam type, layout, etc., and move the mouse arrow inside the bay. You will see a preview of the beam system aligned based on the nearest structural member, as shown in Figure 1-b, so moving the mouse around the different members will show you different previews. To add the beam system, simply click when the alignment of the system is as required. If the 3D option has been checked, the beam system will automatically become non-planar to accommodate different support heights, as shown in Figure 1-c. Another example of a non-planar beam system created using the one-click method with the 3D option is shown in Figure 1-d.

The ability to create sloped and non-planar elements does not, however, extend to other structural components yet. Thus, you cannot create a single non-planar slab to span across the non-planar beam systems shown in Figure 1-d; you would have to model each span individually as a separate planar slab. Similarly, you cannot tilt columns or incline walls along the vertical direction by simply manipulating a parameter, as you can do to change the slope of a beam.

Another significant modeling enhancement in Revit Structure 4 is a new TrussWizard tool, created as an add-on application using the Revit API and available to subscription customers only. It provides a convenient short-cut to the typically laborious process of modeling a truss, allowing users to easily create trusses using industry-standard truss profiles as templates. You model a beam, select it, and then launch the TrussWizard application, where you can specify the type of truss and define the number of panels, peak locations, truss heights, as well as the member sizes for the different parts of the truss (see Figure 2). A preview window allows you to easily experiment with several truss configurations before inserting the desired truss into the model. The TrussWizard replaces the original beam with the newly designed truss, giving you also the choice of aligning either the top chord or the bottom chord of the truss with the location of the original beam.

Revit Structure 4 also includes some improvements for simplifying the manual modeling of trusses. You can now copy, move, mirror, array, and rotate braces out of the vertical plane they were created in, both in plan as well as 3D views. The Trim/Extend tool can be applied to braces and beams for trimming or extending them. It is now possible to move multiple elements of the same type—such as walls, lines, beams, and braces—that share a common end joint by a single control. Other modeling enhancements from Revit Structure 3 include the ability to create slab foundations in addition to isolated and wall foundations, and the ability to attach/trim column tops and bottoms to roofs, framing, and slabs. Some new structural families for modeling have been provided, including pre-cast solid and hollow core concrete planks, and castellated and cellular steel beams.

Better Support for Concrete Construction and Documentation

Recognizing that outside of the U.S., most of the construction is typically in reinforced concrete, a significant development effort has been invested in providing new and enhanced features for reinforced concrete modeling and detailing. Concrete beams, columns, and foundations now automatically join when created. Using this auto-join capability in conjunction with the new one-click beam system described in the last section makes it very easy to model, for example, a pan joist floor system, also referred to as a ribbed or waffle slab (see Figure 3). In addition to automatic joins, improvements have also been made for concrete cleanup between concrete beams joined to the same concrete column, where the beams are wider than the column. Additional options have been provided to control how hidden concrete elements should be displayed in plan view.

A Rebar toolset introduced in Revit Structure 3 and expanded in Revit Structure 4 provides several ways to add steel reinforcement to concrete beams, columns, walls, and slabs in section views, while symbolically representing the reinforcement in plan views. It comes with a new Rebar System Family that includes industry standard bar diameters for use in placement and sketching of bars, ties, and stirrups. There are four ways in which rebars can be added to structural components. For components with smaller cross-sectional areas such as a column, beam, footing, and so on, you can use the Place Rebar tool to add rebars in section view where needed. By default, the rebar length runs along the entire length of the component in which it is being placed. The length can be edited as needed by switching to a plan or elevation view. Back in the section view, you can place additional rebars or create a rebar set by specifying the quantity and spacing. Rebar ties and stirrups can be added to the section by using the Sketch Rebar tool. You can apply various pre-defined hooks to the start and end and also fillet hooks and bends by specifying a radius (see Figure 4-a). This rebar is created only at the location of the section, but you can activate its visibility in plan or 3D view and convert it into a set of rebars running along the length of the component by specifying the number and/or spacing (see Figure 4-b). One limitation to note with regard to a rebar set is that it appears listed in the schedule only as one rebar; however, its length is correctly calculated by adding up the lengths of all the individual rebars in the set.

For larger areas such as walls or slabs that need reinforcement, two additional Rebar creation methods are available. The Sketch Area Reinforcement tool allows you to simply select the wall or slab in a planar or 3D view, define the area to be reinforced by sketching lines or picking the edges, and change the major direction edge if it is different from the default. The reinforcement is automatically created with major and minor bars for the selected area according to the parameters of the selected Area Reinforcement type, which can be fine-tuned as desired. The reinforcement can be checked by viewing it in a section view of the component. The final Rebar tool, added in Revit Structure 4, is Sketch Path Reinforcement. It allows a large number of reinforcements to be placed by specifying the spacing or quantity along a specific path, which can includes lines as well as arcs. The rebar is laid out perpendicular to the path, with the hooked end near the path, and the bars extending to one side of the path. You can choose to place a single bar, or a different alternating bar. Sketch Path Reinforcement is useful for laying out a large amount of rebar around the boundary of a region when the bars have the same length but are not all parallel to each other. The reinforcement is automatically adjusted if the wall or slab is resized or if an opening is created in it.

Improvements in Analysis

Analysis is the core of the structural engineer's task informing all the major design decisions, and as I pointed out in my last review, the greatest strength of Revit Structure lies in its ability to create a fully associated, independently editable, analytical model of the structure that corresponds with the physical model, add loads and member releases, and then link it bi-directionally to popular structural analysis tools for performing different kinds of analyses. Both Revit Structure 3 and Revit Structure 4 have several enhancements that make its analysis capability even more powerful. To start with, checks have been provided for member support and analytical model consistency, which provide warnings in the early stages of design about the stability of the structure and give engineers greater insight into their designs prior to submitting them for complete analysis to an external application. The checking can be automatic, displaying warnings as structural members are added to the project, or by running the check whenever needed. When checking member supports, warnings are posted for all unsupported structural elements (see Figure 5-a). When checking analytical model consistency, all inconsistencies within the analytical model or between the analytical and physical models are detected. You can also control how the checking works by only enabling selective checking criteria and setting Tolerance distances to meet your specifications (see Figure 5-b). Complementing this checking capability is another new feature in Revit Structure 4, which can automatically detect and adjust the analytical models of adjacent structural elements such as columns, beams, walls, and slabs, based on user-defined tolerances, so that they align more accurately and yield consistent analytical end point connections throughout the model (see Figure 5-c). It is enabled by turning on a new "Auto-detect" parameter for these elements.

While previous versions of Revit Structure had the ability to add loads to the model, Revit Structure 3 expanded this capability in several ways. Coordinate systems were introduced for load modeling, allowing loads to be modeled more easily in the desired orientation. Three types of coordinate systems can be used to model loads: the project coordinate system, the current work plane, or the host work plane. Loads can be tagged to indicate load type and magnitude; they can be scheduled from one project to another; load cases and combinations can also be transferred from one project to another. The modeling of non-physical attributes for analysis has been further enhanced in Revit Structure 4 by allowing the boundary conditions of a structural element to be defined without actually requiring users to model its supports. Boundary conditions, also known as supports or restraints, are used to communicate engineering assumptions about support conditions to analysis software packages, and previously, these could only to be manipulated in the analysis applications. The new Boundary Conditions tool in Revit Structure allows point, line, and area support conditions to be assigned that can be pinned, fixed, roller, or user defined (see Figure 6). Springs and coils can also be assigned. A point boundary condition can be defined for only for beam, column and brace ends; the line boundary condition is limited to the individual analytical lines for structural beams, walls, wall foundations, slabs and slab foundations; and finally, the area boundary condition applies to analytical lines for structural slabs, walls and foundation slabs.

Recall from my review of Revit Building 9 that a useful new feature was the ability to create user-defined filters to override the graphic appearance and visibility of all elements that share common properties in a particular view. This same capability was also introduced in Revit Structure 3 and has been additionally enhanced so that it is useful not just for display but also for creating, saving, and retrieving selection sets of elements for sending to an analysis application. Engineers often need to analyze just a portion of the model, so the ability to quickly retrieve a selection set based on specific criteria can be very handy.

And finally, with regard to the actual analysis itself, Revit Structure has improved its two-way interoperability with analysis tools like ETABS from CSI Inc., RISA-3D from RISA Technologies, and ROBOT Millennium from Robobat that it was already linking to in previous versions through its API (application programming interface). It has since expanded its analysis repertoire with links to RISAFloor, also from RISA Technologies, and RAM Structural System from RAM International. Other developers of structural analysis software vendors around the world who are linking their applications to Revit Structure include CSC (UK) Ltd., developer of Fastrak Building Designer, a comprehensive steel building design package; Graitec, developer of Arche, software for structural analysis and design, and Advanse Steel, software for steel construction; Ace Hellas, developer of Scada Pro, software for static and dynamic analysis; ADAPT, developer of ADAPT-Builder Platform, a comprehensive suite of integrated design software for concrete floor systems, beams, frames, mat, and slab-on-grade foundations, reinforced or post-tensioned; and MIDAS, developer of MIDAS/Gen, an integrated design system for building and general structures.

Also, just earlier this month, Autodesk announced its intention to acquire Robobat, the developer of the ROBOT Millennium structural analysis and design application. This should eventually result in a much tighter integration between the structural modeling capabilities of Revit Structure and the analysis and design capabilities of ROBOT Millennium. It is possible that eventually the design and analysis will be integrated within Revit Structure itself, so that the model no longer has to be "sent" to an external application for analysis, and the built-in analysis can better guide the design as it is being developed. Robobat also develops steel and concrete detailing software, and Revit Structure can stand to benefit greatly from the integration of this technology as well, providing a more complete solution for structural engineering and construction.

Other Enhancements

The new version of Revit Structure also includes many improvements made to the Revit platform that we saw in the review of Revit Building 9. These include the ability to save drafting views, schedule views, and sheets with drafting views as new files for reuse in other projects; the ability to save 2D and annotation information added to a live Revit detail to a new RVT file for reuse in other views or projects; a new Show Hidden Lines tool for displaying obscured lines of detail elements fully or partially hidden by others; and several improvements related to tags including tags for detail components, keynote tags, easy toggling of a tag between horizontal and vertical orientations by using the Spacebar, and tag leaders with free ends that allows them to be placed on any point of the tagged element.

Other enhancements include more detail component families and an improved graphical column schedule display that can be customized by the user. The Coordination Monitor, which can be used to integrate the different disciplinary models created in Revit Building, Revit Structure, and Revit Systems, has been enhanced so that modifications to walls, floors, and openings can be monitored in addition to grids, columns, and levels. Workset improvements include improved visibility for linked files and the ability to create schedules across linked files. Interoperability-related platform enhancements that are significant in Revit Structure include more export layers and merging of collinear lines when exporting to DWG; more options for linetype scaling in DWG/DXF exports; the ability to export object data to 2D DWF files instead of just lines; support for Autodesk i-drop, which allows i-drop objects from an i-drop enabled web page to simply be dragged and dropped into Revit Structure; and the ability to import IFC files in addition to exporting them. In addition, Revit Structure is now capable of exporting a CIS/2 file for improved workflow with downstream detailing applications. (For more information on CIS/2, an open standard for structural steel engineering, please see the AECbytes article, The CIS/2 Format: Another AEC Interoperability Standard.)

Strengths and Limitations

Autodesk Revit Structure, now in its second year, continues to gain momentum as 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. Revit Structure not only automatically generates the analytical model from the physical model of the building, but also keeps it in sync with the physical model while still allowing it to be independently editable by the engineer. The bidirectional links to an increasing list of industry-leading structural analysis applications leads to greater engineering insight and allows the analysis results to be readily incorporated back into the design.

The two recent releases of Revit Structure feature a solid set of improvements and upgrades in all the key areas of the application: modeling, analysis, documentation, workflow, and interoperability. Many time- and effort-saving smarts have been introduced such as the one-click beam system, truss wizard, rebar layout system that automatically adjusts to a change in shape in the host element, the checks for member support and analytical model consistency, and the automatic adjustment of the analytical model for more accurate input to the analysis tools. The ability to export CIS/2 files is a significant improvement, allowing the model to be used downstream for steel detailing. The new and enhanced features for reinforced concrete modeling and documentation afford more control and flexibility in display, and easier reinforcement detailing. They will help to speed up BIM deployment in countries where concrete construction is the norm.

The last time I reviewed Revit Structure, it was still a relatively new application, so while analyzing its limitations, I commented that Revit Structure could get away with being less than perfect, at least at that time, because it was so terrific to finally have a dedicated BIM application for structural design that could link bidirectionally to popular structural analysis tools. While Revit Structure can still be rated as an excellent BIM application for structural engineering, we can no longer gloss over its limitations. I have already alluded to some of these in the course of this review, such as the inability to model inclined columns and non-planar slabs, which limits the ability of the application to easily model complex geometry and freeform structures. Being able to quickly model sloping beams and non-planar beam systems is a good start, however, and future releases of Revit Structure should continue to ease the modeling constraints on structural components. Also, while many improvements have been made for the documentation of concrete structures, more support is needed for their design as well as analysis. For example, as of now, there is no ability to bring the reinforcement analysis results back into the design. The quality of the documentation in Revit Structure, as in Revit Building, continues to need significant improvement, with an overabundance of text that is not sufficiently illustrated and the complete lack of any video tutorials, making it difficult to learn the program without recourse to external resources. Even the customary "What's New" section in the online Help is missing in this release. There are a few PDF documents on the product CD, including release notes and a slide presentation, which help to provide an overview of the new features, but to actually understand how they work, you need to go and search for them in the online Help.

Conclusions

The impending acquisition of Robobat by Autodesk should lead to a much tighter integration between the structural modeling capabilities of Revit Structure and the analysis, design, and detailing capabilities of ROBOT Millennium, and perhaps in the long run, a seamless integration of modeling, analysis, and detailing within one application. The scenario could be repeated with other existing analysis and detailing applications or with new ones developed in-house by Autodesk. The outlook is definitely promising and should lead to many exciting advances ahead in the field of structural engineering.

During my training as an architect in the pre-BIM era, I was required to take some structural engineering courses and I recall finding it difficult to relate seemingly abstract engineering concepts such as moment, shear, support conditions, fixed versus pin joints, load transfer, and so on to the actual buildings we were learning to design. But after working with Revit Structure, particularly with the analytical model, these concepts are starting to become more understandable. In conclusion, therefore, I would say that one of the most significant but hitherto unrecognized benefits of having a BIM application for structural engineering like Revit Structure that can be used in conjunction with an architectural BIM application like Revit Building lies in its educational value—in helping both students as well as practicing architects and engineers to develop a better understanding of structural design in relation to architectural design and vice versa. And it is this human factor—more than glorified technological aids such as Revit's Coordination Monitor or IFC exchange or interoperability—which will ultimately lead to better designed buildings that harmoniously integrate space and structure.

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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