AECbytes Product Review (September 30, 2007)
Revit Structure 2008
Structure 2008 is the new release of Autodesk’s integrated BIM solution for structural design and documentation built on the Revit platform.
Pros: All the strengths of the Revit platform including relative ease of use, parametric building components, and automatic synchronization of all graphical and tabular views; bi-directional link to popular structural analysis tools allowing the analysis results to be readily incorporated back into the design; allows collaborative multi-disciplinary building design when used with Revit Architecture and Revit MEP; several modeling improvements in the new release including curved beams, dedicated truss element, slab modification tools, the ability to cut holes in beams and columns and add stiffeners, and rebar improvements for concrete construction; critical improvements in groups and linked files, and the ability to split views, which make it easier to model and document large projects; several display enhancements that allow views to be customized as required; ongoing development and availability of extensions from Robobat that extend the capabilities of the application.
Cons:Limited ability to model complex geometry and freeform shapes due to lack of support for inclined walls, non-planar slabs, curved beam systems, etc.; some modeling operations cannot be done in 3D and require work planes and framing elevations to be set up to work in 2D views; quality of documentation is functional and could be significantly improved to enhance the learning experience.
Price: List price is $4,995.
Earlier this summer, I reviewed the new version of Autodesk’s Revit-based BIM application for architectural design, Revit Architecture 2008, which featured several improvements to groups and linked files, the ability to split views to make it easier to model and document large projects, several display enhancements that allow views to be customized as required, and various other modeling, documentation, visualization, and interoperability enhancements. The corresponding new release of Revit Structure, version 2008, includes most of these improvements and additionally, several other enhancements that are specific to structural modeling, analysis, and detailing. Let’s take a detailed look to see what these are.
To compare Revit Structure 2008 with previous releases, please see my review of Revit Structure 4 that was published last year, and my earlier review of Revit Structure 2, which also provides a comprehensive overview of how the application works. A brief description of how Revit Structure is being implemented in an engineering firm can be found in my recent article on the AEC Technologies Strategies Conference 2007, under the section, “Implementation of BIM at Walter P. Moore and Associates.”
New Ability to Model Curved Beams
New options added to the Beam tool make it easy to create beams that are curved, both horizontally in plan as well as vertically in elevations and sections. The curve can be an arc, spline, or partial ellipse, and you can create the beam either by drawing it with the selected type of geometry or by picking a line such as a wall or slab line. The example shown in Figure 1-a was created by using the Draw option with the 3 point Arc geometry option selected in the Options Bar. On the other hand, the example shown in Figure 1-b was created by using the Pick option and selecting the spline-shaped cantilevered edge of the slab. It is also possible to create curved beams in elevation views, as shown in Figure 2.
Figure 1. (a) Creating a curved beam by using the Draw option. (b) Creating a curved beam by using the Pick option.
Figure 2. Creating a vertical curved beam by using the Draw option in an elevation view.
Recall that the last release of Revit Structure had introduced a new one-click beam system with an associated 3D option that made it very easy to model an entire system of sloped beams, even if they are non-planar to accommodate different support heights. This works to a certain extent with curved beams as well. An example is shown in Figure 3-a, where a beam system was quickly created with respect to the horizontal curved beam shown earlier in Figure 1-a. Another example of a 3D beam system created using the new curved beams as supports is shown in Figure 3-b. On the other hand, the Beam System tool does not work with the spline-shaped beam that was shown in Figure 1-b, and I found that it also did not work with other complex configurations of curved beams.
Figure 3. Creating beam systems supported by curved beams.
Along with the ability to model curved beams, options are also available to change their analytical model to suit the requirements of specific analysis tools. The default analytical model of a curved beam follows the physical curve, as shown in Figure 4-a. For those analysis tools that don’t recognize curves, the curve can be approximated with segments, as shown in Figure 4-b. This was done by selecting the Approximate Curve option in the Element Properties dialog of the beam. As you can see, a maximum discretized offset value can be specified to control the segmentation. Additionally, in cases where a curved beam has additional framing that it supports, as in the given example where the curved beam is supporting a beam system, the Use hard-points option can be selected. As shown in Figure 4-c, the analytical model will now be reconfigured such that these points are used in the approximation of the curve.
Figure 4. Different options for manipulating the analytical model of a curved beam.
New Dedicated Truss Element
In the last release of Revit Structure, a new TrussWizard tool was introduced that allowed you to convert a beam to a truss by specifying its type and defining the number of panels, peak locations, truss heights, and member sizes. This tool was created as an add-on application using the Revit API and was available to subscription customers only. Revit Structure 2008 improves upon this ability by introducing a dedicated Truss tool that can be selected from the Design Bar. Figure 5-a shows the use of this tool to model a truss in a section view. The truss type was selected from the Options Bar and the truss was created simply by specifying the start and end points. Revit Structure provides many predefined truss types, as shown in Figure 5-b, that can be loaded into the project. Trusses also have their own dedicated Family Editor with specific layout tools for truss chords and web members, which allow you to create different truss configurations with functionality that is directly linked to the specific part of the truss. Each truss type has parameters to specify the truss top and bottom chord members as well as vertical and diagonal web members.
In addition, after a truss has been created, you can use the Modify tool to adjust its configuration automatically according to a roof or floor slab, as shown in Figure 5-c. You can also directly edit its profile in a Sketch mode. This automatic reconfiguration of a truss works with curved roof profiles as well.
Figure 5. Creating a truss and then modifying it to match the roof configuration.
If multiple trusses have been created and they have a common roof, they can be selected and reconfigured in one step. A more complex example showing multiple trusses created and edited using the new Truss tool is shown in Figure 6.
Figure 6. Multiple trusses modeled and edited with the new Truss tool.
While engineers generally try to avoid cutting openings in beams and columns, it is sometimes required in order to accommodate mechanical components such as ducts. Revit Structure 2008 now allows the Opening tool to be used for this purpose. A new Opening by Face option allows you to preselect a face of a beam or column and subsequently sketch out the shape of the opening that you need. For rectangular openings, it also possible to add a fillet at the corners by specifying a radius. Figure 7 shows this new capability being used to add a circular and a square opening to a beam to accommodate mechanical and structural components that need to pass through it.
Figure 7. Using the Opening by Face tool to cut holes in a beam.
The new ability to cut holes in beams and columns has been accompanied by a new Stiffener component type which can be used to add stiffeners around these openings, as shown in Figure 8. Both linear and circular stiffener types are available, and they can be created using the Component tool’s Place on Face option. Other modeling enhancements in Revit Structure 2008 include an additional parameter for slab materials that allow their thickness to be variable instead of constant. When this is active, the non-variable material layer will remain the same thickness throughout the slab while the variable material layer will adjust according to its slope. This ability makes a useful complement to the new slab modification tools that were demonstrated in the review of Revit Architecture 2008. Some of the parameters of beam elements have been renamed to be in sync with engineering terminology; so, for instance, “Angle” is now called “Cross-Section Rotation,” “Horizontal Justification” is now called “Lateral Justification,” and so on. Some more structural elements that have been introduced in addition to trusses and stiffeners include pan joists, pan joists with ledges, and pile caps.
Figure 8. Adding stiffeners to the openings created in Figure 7.
The last version of Revit Structure had expanded its Rebar toolset to provide several ways to add steel reinforcement to concrete beams, columns, walls, and slabs in section views, while symbolically representing the reinforcement in plan views. Revit Structure 2008 includes several more improvements in Rebar representation and modeling, some of which are illustrated in Figure 9. Rebars can now be seen in double-line mode by selecting the “Fine” detail level in 2D views. This allows more accurate placement and dimensioning of the rebars. There is a new parameter to define a rebar element as a standard bar or a stirrup/tie bar, which determines which bend diameter to use when constructing the bar. An additional new parameter called Schedule Mark has been introduced which allows related bars to be categorized into collections and grouped in schedules. Other rebar improvements include a new “Minimum Clear Spacing” layout rule for Rebar sets, an “Estimated Rebar Volume” parameter for the concrete host, a Rehost tool that allows a rebar to be extended through multiple hosts, new Path Reinforcement settings that allow start and end hooks for primary and alternating bars, and customizable abbreviations for area reinforcement such as Slab Top – Major Direction, Slab Top – Minor Direction, Each Way, Each Face, etc.
Figure 9. Rebars can now be seen more accurately in double-line mode in 2D views, and have an expanded set of parameters.
Revit Structure Extensions from Robobat
Last year, Autodesk had announced its intention to acquire Robobat, the developer of the ROBOT Millennium structural analysis and design application, which was anticipated to bring about a tighter integration between the structural modeling capabilities of Revit Structure and the analysis and design capabilities of ROBOT Millennium. While this acquisition did not eventually go through, there is still a close partnership between the two companies that has resulted in the availability of several extensions for Revit Structure developed by Robobat. Labeled as “Revit Structure Extensions,” these are available at http://www.extensions4revit.com (see Figure 10). They are a series of easy-to-use applications, developed using the Revit API, that extend the capabilities of Revit Structure 2008 in various key areas, including structural analysis and reinforced concrete drafting. Many Extensions are available for free, while some that offer more detailed functionality are available for a small fee. The Extensions website is updated regularly, with at least one new module posted every month. The intent is to eventually have a comprehensive library of Extensions covering a wide range of tasks.
Using the Extensions involves first downloading and installing an “Extensions Engine,” a platform that Robobat have developed to host each Extension, providing easy access in the Revit Structure pull down menus to the downloaded applications. Some of the currently available Extensions are shown in Figure 10. They include a free variant of ROBOT Millennium that bi-directionally integrates with Revit Structure, allowing dynamically import and export of data between the two products; an Excel based model generation Extension, which allows users to define Excel data to automatically generate beams, columns, levels, walls, and footings in Revit Structure; a Freeze drawings Extension that allows a drawing to be unlinked from the object model so that it will remain unchanged despite any changes in the model; and a Grids generator Extension that facilitates the definition and generation of axes, grids, and levels with the added capability to automatically generate columns, beams, walls and footings on grid intersections.
Figure 10. The Extensions for Revit website, and the list of currently available extensions.
Analysis and Conclusions
In addition to the structural engineering specific enhancements that were described in this review, Revit Structure 2008 also includes, as mentioned earlier, all the major enhancements of the Revit platform that were described in detail in my review of Revit Architecture 2008, including improvements in the editing of groups and working with linked files; the ability to split large plan, section, and elevation views into smaller dependent views that can then be placed on sheets at a more readable scale; improved display options that allow different views to be customized to look exactly as desired, with the ability to permanently hide some elements in a view and apply graphic overrides to other elements to display them differently from the graphic settings associated with their categories; automatic positioning of dimension arrows for readability; the ghosting of spot dimensions before placement; and being able to control which element obscures another element through masking. All of these add up to make Revit Structure 2008 another compelling release and strengthen its position as an integrated modeling solution for structural design, analysis, and documentation that can bring the many benefits of BIM to the structural engineering profession. Its bidirectional links with many industry-leading structural analysis applications can facilitate better engineering insight and allow the analysis results to be readily incorporated back into the design.
The only real limitation of Revit Structure I could discern at this point was the lack of ease and fluidity in modeling more complex geometry and freeform structures. While the new ability to model curved beams is a definite improvement, beams that curve vertically can only be created in an elevation view that matches the plane of the beam, requiring special framing elevations or work planes to be created before a beam can be modeled. It would be so much easier to be able to create these in 3D. Beam systems also only work with basic curved beams, not with more complex configurations. Also, at the moment, beam systems can only create linear beams. Modeling inclined walls and columns is still difficult in Revit Structure. You also cannot quickly create, for instance, a non-planar roof system that spans across a series of non-planar beams. Let’s hope that future releases of Revit Structure will focus on making it easier to model more complex and varied structural forms.
While the aborted Robobat acquisition may be disappointing to those who envisioned that it would eventually lead to a seamless integration of modeling, analysis, and detailing within Revit Structure in the future, it is good to see that Autodesk and Robobat are continuing to work together. The Extensions concept is very promising, and actually reinforces the concept of BIM as an intelligent platform on top of which many supporting technologies can be built, both by the developer of the BIM platform as well as by third-party solution providers. As Revit’s API continues to develop, we can expect to see many more such third-party tools being developed that extend the core capabilities of its BIM applications.
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
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