Cadalyst Labs Review: Autodesk Revit Structure 2-Digital Models for Structural Design

1 Dec, 2005 By: Cyril Verley Cadalyst

Autodesk extends building information modeling to

LET ME BEGIN with the disclaimer that though I am not a structural engineer, I've been working with them as a registered architect on projects of all sizes for the past 25 years. I understand and empathize with the challenge of producing a set of construction documents. Architects and engineers have been coping with this challenge without much assistance from technology, until now.

In 1997, Revit Technology released a new parametric software package to help the AEC community design and document architectural projects within a true single building information model. The challenge facing the industry in producing a set of construction documents also happens to be the lifeblood of what Autodesk Revit offers—a coordinated document set. Today, Autodesk Revit continues to expand this BIM (building information modeling) solution for the AEC industry.

 Autodesk Revit Structure 2
Autodesk Revit Structure 2

Revit is now available for the structural engineering community with Autodesk's most recently released product, Revit Structure 2. Like Revit Building, Revit Structure is a BIM application that reduces time, errors and costly design changes within the single model project. Using Revit Structure is not about building the entire model in 3D. 2D drafting is part of the BIM model that Revit Structure helps users manage.

The Parametric BIM Model

The key to Revit's building model is its single project database. All the views of the model, such as plans, sections, elevations, 3D isometric, schedules and sheet layouts, are queries of that database and are managed within the project. All the views in the database are live and can edit the model (figure 1).

Figure 1. Within Revit, all views of the model are live and can be changed.
Figure 1. Within Revit, all views of the model are live and can be changed.

Revit Structure ships with a robust line of 3D structural elements such as walls, beams, columns, bracing and slab elements for concrete, steel and wood construction. As these 3D elements are loaded into the project, users can select the member types wanted or create a custom list of members that share common values such as section modulus and flange depth.

Figure 2a. Users can load 3D components, such as these steel columns, from a list of sizes.
Figure 2a. Users can load 3D components, such as these steel columns, from a list of sizes.

These 3D components are then inserted into the model and controlled by 2D centerline grids and floor-level lines. As the grid or levels change, the structural elements adjust to the new locations.

Analytical data such as connection types, phase of the member and structural analysis data can be assigned to these same structural elements. As these elements are placed into the project, they are drawn point-to-point in a 2D plan or section view with the 3D model developing in the background (figures 2a and 2b).

Figure 2b. The sectional callout detail view of the project shows in-depth information.
Figure 2b. The sectional callout detail view of the project shows in-depth information.

Structural Engineering Process

The typical work environment within a structural engineering office is divided in two groups: engineers and the CAD department. Engineers choose a structural system for the project and verify that system using various industry-standard structural analysis software programs, including ETABS, RAM, ROBOT Millennium and RISA-3D. Typically, their results are single-line stick structural graphic drawings of the project's structural members. This data is then passed to the production staff or CAD department that translates the stick data into full-member construction documents by redrawing the members in multiple views using a drafting program such as AutoCAD.

During the design process, iterations go back and forth between the engineer and the CAD drafters. The engineer submits corrections to the production staff, which then translates those changes into the construction document set. This process is very inefficient and demands a great deal of time and effort. Coordination of the changes and updates is done manually as documents are passed back and forth from one group (and software type) to the other.

Revit Structure streamlines this entire process. All the data produced by the engineers and the production staff can be created, edited, managed and printed as final construction documents simultaneously within the same single project model. As the model is edited by the engineer, the production drawings automatically update.

Structural Analysis

Engineers begin a project from scratch in Revit Structure by drawing the base 2D centerline grids and floor-level lines of the project. Then they add 3D structural walls, columns, beams and bracing members as an initial schematic design. The data they draw can display as analytical data, as fully displayed members or as both simultaneously.

To do the structural analysis for cases such as lateral loads, seismic loads and dead loads, the digital model can be exported into three different industry-leading structural analysis programs, as stated earlier: ETABS, RISA-3D and ROBOT Millennium. The export is a single command and can be done at any time during the design process.

Once exported, the project is studied in the analysis software and the structural members are properly sized with correct connections. The new data, including loading changes and member sizing, can then be filtered back into the Revit Structure model. All views such as structural plans, elevations, sections and detail sheets are updated in Revit Structure according to changes determined by the analysis package. Because Revit Structure allows a bidirectional flow of the filtered data to the various analysis software packages, Revit Structure becomes a translator and manager of all the final design data.

Documenting the Design

As data produced by the engineer is updated in the Revit Structure model, the CAD production staff doesn't have to translate any data to different software. Instead, all the members designed as stick graphics also display as full 3D members that can be dimensioned and annotated by the drafters.

Data can be viewed in multiple ways, for example as a stick design with the view set to an analytical display and as normal display for a CAD production document view.

Not only can the production staff tag and dimension the 3D model within a detail view, they can also add 2D lines and select from thousands of 2D detail components that ship with Revit Structure.

Another output from Revit Structure is a Graphical Column Schedule that automatically monitors all the columns. This schedule labels all the columns at the centerline grid intersections and is another live view of the model.

Within a structural bay, the engineer can specify a certain beam type, size, direction and frequency. The production staff can avoid having to draw each member separately and instead create a structural beam system at each structural bay. The sketch data is added to the bay and the layout rules are set.

Project Reviews

When the engineer needs to review the production staff's work, it can be reviewed within the same digital model. The data viewed in a stick diagram is the same data in a 3D CAD view, which is yet again the same data in a detail view.

True BIM Collaboration

Revit Structure is a complete structural package that allows the engineers and the production staff to edit, manage and document a structural design project within the same digital model.

Typically, architects send their drawings to engineers for design and documentation of the project's structure. Using Revit Structure, the engineer can import an architect's designs and export them back using two major groups of file formats.

The engineer can use 2D files such as AutoCAD DWG and DXF files or MicroStation DGN files as architectural backgrounds in a Revit Structure model. These files can be linked into the model and like the AutoCAD xreference file, they update as the AutoCAD file is revised. Revit Structure can also export the 3D model to Architectural Desktop. The architect can see structural properties of beams, columns and braces as true Architectural Desktop objects.

Once the file is linked to the model, the engineer can lay out the Revit Structure centerline grids by picking the 2D grids from the linked files. As the structural drawings develop, the engineer can export the work to the architect as 2D and 3D DWG, DXF and DGN files. Exports can be done as batch exports using a single Revit Structure command.

Revit Structure can also import a 3D Architectural Desktop file so the engineer can query the ARX objects. To copy elements, the engineer would use the Architectural Desktop 2D file.

The ideal design process has both the architects and the engineers using Revit simultaneously. One possible scenario is to combine the data produced by both groups into a single Revit model and manage that data using the program's Multiuser Workset feature.

However, most projects have architects and structural engineers who work for separate companies and therefore require a more typical scenario of two separate Revit models of the same project. The architects using Revit Building and the engineers using Revit Structure can link to each other's model. As changes are made by one design professional, the other monitors and reacts to those changes.

When an engineer receives an architect's Revit Building model, the model is first linked into an empty Revit Structure project (figures 3).

Figure 3. When an engineer receives a Revit Building model (left), the first step is to link it to a Revit Structure model (right).
Figure 3. When an engineer receives a Revit Building model (left), the first step is to link it to a Revit Structure model (right).

Once linked, the engineer can view all the architectural components of the building model. Using the Copy/Monitor command, the engineer copies the architect's original centerline grids, level lines and columns that are then translated into new structural grids, levels and structural columns. The types of columns, grids and levels can be preset by the engineer.

After the Copy/Monitor command is complete, the new structural grids, levels and columns display a graphic eye when selected, showing that the elements are being monitored for any changes made to the architect's original linked file.

At this point, the engineer can now begin to design the project structure by editing the existing members, adding new structural elements such as walls, floor slabs, beams and bracing, and doing the analysis. At the same time, the architect may make further design changes. When the architect resubmits the changes to the engineer, the linked file is reloaded and Revit Structure then displays the design change.

If the engineer wants to see a list of all the design changes the architect made project-wide, the Coordination Review feature is activated, listing all design changes item by item. This allows the engineer to view and reply to the architect's changes. The engineer can choose to Do Nothing, Accept, Reject or Postpone and make more edits or comments to the component in question. The engineer's decision then becomes part of the project and can be changed at a later time.

The benefit here is that the engineer can now manage all changes submitted by the architect, follow up at any time and make decisions that update the project model. The Revit Structure project becomes an audit trail and manager of the project's design changes, practically eliminating the need for redlining the drawing set.

Many Benefits

These features are some of the many benefits provided by Revit Structure 2. Even though this is a relatively new product, it's built on the foundation of Revit's core parametric model, which has proven success in the architectural industry. Revit Structure streamlines collaboration within an engineering firm and with the project architects while keeping its essence of a coordinated document set.

The only remaining piece needed is a version of Revit for HVAC. I look forward to the day when the entire building is a single building model. Highly Recommended.

Cyril Verley, RA, and president of CDV Systems, has been a registered architect for 17 years, with 13 years' experience consulting with and training AEC firms on Autodesk products.

About the Author: Cyril Verley

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