1-2-3 Revit: BIM and Structural Engineering

Ensure your BIM makes the best use of digital data

This month's article will focus on how the newly released Autodesk Revit Structure extends the Revit building information model into structural design using a common, computable building model for structural modeling, for both construction documentation and driving analysis.

Computable Building Model
Digital data is not necessarily computable data -- a distinction that at first glance may seem insignificant but in actuality wreaks havoc for the user of the data. For example, you can use a word processor to create rows and columns of financial data, but most of the numeric calculations and modifications must be done manually. The data is digital, but not very useful.

In contrast, a spreadsheet version of the same financial data might look identical to the word processor version, but the spreadsheet model contains numerical values, relationships and sophisticated calculations. When a number changes, the rest of the spreadsheet updates automatically. The spreadsheet model is computable whereas the word processor representation is not, even though both are digital.

The building industry, for the most part, has adopted the word processor approach to documenting building designs over the past 20 years. CAD tools are primarily used to create electronic drawings of buildings. Even some 3D models are little more than 3D drawings. Although the output of these systems may resemble the output of a BIM solution -- just as the financial table in the word processor looks the same as the spreadsheet table -- it is not computable information.

Figure 1. Autodesk Revit Structure extends the Revit building information model into structural design, using a common, computable building model for structural modeling, for both construction documentation and driving analysis.

Computable Building Information for Analysis
It's quite common to try to use this "uncomputable" type of building design data for analysis and find that the data, although seemingly computable, is actually an empty shell -- a collection of graphic elements with no implicit knowledge of building elements such as walls, beams or ducts. For the most part, humans look at the data, interpret it and transfer it to new applications for additional analysis.

Architects make occasional use of analysis packages, lighting studies or baseline energy calculations, for example, which are typically outsourced to specialized engineering firms. Whereas the structural engineer is heavily dependent on analysis, which is an integral part of the structural design process. As a result, a computable building model is a key ingredient for efficient structural design processes.

Redundant Structural Models
Traditional structural processes begin with the architectural document set (be it paper or CAD-based). The structural engineering team interprets the architectural design to create an overall structural design, then create specialized analytical models, using different software applications for the multiple types of structural analyses required for the project -- gravity, dynamic (e.g., seismic), and wind analyses. Later, the structural drafters create yet another representation of the building in the construction documentation process?another form of a building model.

This traditional workflow results in multiple models (including the drawing set) that are not coordinated -- requiring manual efforts to keep them in sync. Opportunity for errors abounds. One of the analysis programs prompts a change to a structural column, but the structural drafter misses the change, so the analytical representation doesn't match the physical representation. The documentation falls out of sync. The other analytical models become outdated, the downstream analyses are compromised, and the validity of the design suffers.

Integrated Modeling for Structural Engineering
Revit Structure allows engineers to create a single building model that is used for the complete production of construction documents and (since it is computable) can be used for different types of analyses.

The physical representation drives the construction documentation. As the physical representation develops, the analytical representations are created automatically, containing the necessary data needed for third-party analysis applications. The engineer adds specific loads, material properties and so forth -- and runs the analysis. Currently, Revit Structure is linked via an API to several leading industry applications for building analysis: ETABS from CSI, RISA-3D from RISA Technologies and ROBOT Millennium from RoboBAT.

The analysis program can then return information that dynamically updates the building model and therefore the documentation as well. This capability eliminates much of the redundant work done by structural engineers to model and analyze single- or multi-material building frames (steel, wood, concrete) using many different applications.

Structural Modeling and BIM
Like the Revit architectural product (Revit Building), Revit Structure uses building information modeling -- where every view, drawing sheet and schedule is a direct representation of the same underlying database. Users can explore different structural systems and alternate design options from within the same model. As the design team makes changes to the building structure, the parametric change technology within Revit Structure automatically coordinates the changes across all other representations of the project. As a result, the design model (physical and analytical representations), design options and documentation stay coordinated, consistent and complete.

For example, if any structural member's section is changed, all the views where this element is displayed are updated automatically, including name tags and other labels referring to the element properties. If another analysis is run, the analytical representation reflects the change to the structural member.

Summary
Next month we'll explore the value of integrated structural modeling by diving deeper into the iterative structural design workflow -- beginning with the architectural model (whatever the source may be) and moving to the structural design and documentation process -- and look at how the design analysis results are incorporated back into the model and how the ripple of changes automatically occurs.

The computable building model at the heart of Revit Structure integrates essential information into one place. And it's finally letting structural engineering firms delight in the benefits of building information modeling that architects have been savoring for several years.