1-2-3 Revit: BIM for Structural Design

Integrated modeling for structural analysis and documentation can improve workflow

This month I'll discuss the value of using BIM for structural design by comparing and contrasting the traditional structural workflow and a workflow supported by BIM. Next month we'll examine how the structural engineering firm can use their structural model together with a model from the architect, if available, for especially powerful collaboration and coordination.

Traditional Structural Workflow
Traditional structural workflows have two main branches: the iterative design/analysis process and the documentation process. Both begin with the architect's design, which is communicated through drawings.

The structural engineers interpret the architectural design to create an overall structural design, and then they create specialized analytical models in different software applications for the different types of analyses required (gravity, seismic and so forth). Time constraints usually dictate that the documentation effort parallels the design effort, so as the structural engineers begin their analyses, the structural drafters begin developing the documentation set -- framing plans, bracing elevations, typical details, etc.

Last month's article explained the inherent problem with this workflow: using multiple models that are not coordinated with each other or the documentation requires manual effort to keep them and the documentation package synchronized, to the detriment of a firm's efficiency, quality and flexibility.

Now let's examine how the use of a purpose-built BIM solution to drive analysis, coordination and documentation eliminates these problems.

The Starting Point: Architectural Drawings
Without BIM, the architectural document set is the launch pad for the structural design process. If the drawings are paper-based, designers interpret the drawings and create the structural design and analytical models (note the plural) from scratch. If the drawings are CAD-based, some architectural files (plan drawings for example) may be imported and digitally traced to create structural elements.

With BIM, referencing the architectural plans is still the first step (unless the architect can provide a building model from their own software -- the subject of next month's article). But instead of creating several models, there's just one model -- a single integrated structural model that includes both a physical model that drives documentation and coordination, and multiple analytical models used for analysis.

(Note on terminology: a structural model used for documentation is often referred to as the physical model because it represents the physical layout of the structure in the building -- beams, columns, walls, footings, etc. However, it is a digital model -- you can't touch it or carry it to a client meeting, as you might a small-scale architectural model sometimes built for presentation purposes. An analytical model is an abstract and usually simplified 3D digital model used for structural analysis.)

Analysis
Without BIM, you must produce individual models to front-end each type of analysis. One common complaint of structural firms is that their highly educated staffs spend too much time transcribing information from one software package to another, configuring various analytical models for input into different analysis software applications, and then manually coordinating the analysis and design results with documentation.

With BIM, the analytical model and the physical model are created simultaneously, and they are just different views of the same computable building model, containing the necessary information needed for third-party analysis applications. Revit Structure doesn't replace the analysis applications; it provides a common modeling interface to them and a common model to document the results. Revit Structure is currently linked to several leading industry applications for building analysis including RISA-3D from RISA Technologies, ETABS from CSI and ROBOT Millennium from RoboBAT. Using the Revit application programming interface (API), data moves directly from the Revit Structure building information model to the analysis software, and the analysis results are delivered back into the model -- keeping analysis, design and documentation all synchronized.

Roundtrip Analysis
For example, an engineer needs to run a general gravity analysis. First the engineer adds analysis-specific data to the Revit Structure model -- such as external loads (dead, live, snow, etc), load combinations, releases and boundary conditions -- and then selects Send Model to RISA from the Revit Structure user interface. The RISA-3D application is launched, and all the necessary information needed for the analysis is extracted from the Revit Structure model -- location of members, cross sections, release conditions, material properties, loads, etc. The analysis runs, and the results are displayed within RISA-3D. Based upon the results, the engineer can modify the structure (for instance, beef up a column size from W30x108 to W33x130) and recalculate the building as needed. When the engineer is satisfied with the analysis results, the RISA-3D application is closed, and the Revit Structure model is automatically updated to reflect the changes. (In our example, the column is automatically changed to a W33x130.)

Next the engineer wants to run a seismic analysis using ETABS. The Revit Structure model automatically updates with the results of the RISA-3D analysis, so there's no need to manually update the analytical model required for the ETABS analysis because it's all the same model. The engineer sends the Revit Structure model to ETABS, and this new analysis iteration confirms that the redesigned W33x130 column is sized appropriately.

Coordination and Documentation
Continuing our example above, without BIM someone would have to manually update all the documents that show that column and all the reports that include that column, and hopefully remember to change the column in the analytical model used for the ETABS analysis. Needless to say, this coordination effort can be immense. Junior staff and/or engineers in training are routinely consigned to this drudgery.

With BIM, a single model drives documentation and analysis. The column that was changed in the RISA-3D run is automatically updated in the Revit Structure model (physical and analytical views) and all the affected documentation.

Summary
Revit Structure enables firms to work more efficiently and make better use of their time with integrated modeling for structural analysis and documentation. Revit's parametric change management coordinates analysis results more reliably with design to produce higher quality, more accurate and more consistent work.

Thomas Weir at Brandow & Johnston Associates -- a California-based structural engineering firm that has successfully engineered over 15,000 building projects throughout California and the United States (figure 1) -- reports, "With the advent of Revit software and utilizing the techniques of building information modeling, we find significant advantages in our ability to navigate through the design process. Having the 3D computer model enhances our understanding of the structure considerably, so our final product is done more efficiently and with better overall coordination."

Figure 1. Brandow & Johnston Associates use the Revit Structure BIM to increase their efficiency and overall coordination on projects like this one.