Modeling Linear Structures: Tunnels and Viaducts

Aligning AutoCAD Civil 3D and Revit with Dynamo

It is complex to model structures such as tunnels and viaducts that are controlled by an alignment and profile. Put that in an environment where the alignment changes often, and the structural engineer, using a tool like Revit software, finds it hard to accommodate that change. AutoCAD Civil 3D software is great for designing the alignment and profile — with complex curve elements and subassemblies for the mass modeling of complex structures — but we need Revit software for the detailed concrete modeling.

Solutions to the challenge involving export/import of data between software have helped, but they present their own new challenges. This is where writing Dynamo scripts comes into play, providing integration and coordination between AutoCAD Civil 3D software and Revit software to give an interactive and dynamic modeling environment that can integrate structures like stairs and the MEP (mechanical, electrical, and plumbing)-required equipment.

Understanding the Linear Structures Modeling Challenge

Dar have a long history in BIM for buildings and have executed many projects for hospitals, commercial premises, hotels, and sports facilities: architectural, structural, and MEP disciplines author their project BIM models and use the models for downstream processes such as design review and coordination, drawing production, visualization, and cost estimation.

However, BIM in the infrastructure environment has proven to be more of a challenge, with BIM standards less well defined than for buildings, while clients are increasingly demanding BIM deliverables (e.g., coordinated models) in addition to drawings. Linear structures are a particular case in point: known workflows were a hybrid of 2D CAD and 3D BIM, which would no longer be acceptable, and the discipline design teams were at breaking point to manage modeling tasks that supported the mandated BIM workflows.

Dar decided to initiate a project with Autodesk to develop processes for the coordinated design of tunnels, viaducts, and retaining walls across the design disciplines of transportation, structures, architecture, and MEP. This article presents some of the results of that project.

Requirements Gathering and Prioritization

The project that was selected for this linear structure proof of concept is a large new airport in the Middle East, in concept design stage and moving into detailed design. Hundreds of kilometers of tunnels are planned in this airport to enable passenger, baggage, and service vehicle movement. The adopted platforms are Civil 3D for transportation modeling (runways and taxiways, roads, surface modeling) and Revit for the other disciplines related to linear structures.

A two-day workshop was held between Autodesk experts and Dar BIM team members to gather and prioritize the detailed requirements for our project. Dar’s BIM team consists of experts from all design trades who act as a development, training, and support group for project teams in all design offices and are thus familiar with the challenges of current methods. Requirements, challenges, and priorities were scrutinized and grouped to give a clearer picture of where focus during the project needed to be.

Linear structures project requirements.
Linear structures project requirements.

As a consequence of this we determined to spend the bulk of our effort on tunnels, many of the challenges we were discussing being common across the structure types anyway, then test the process on bridges and retaining walls, before dealing with their specifics (piers, etc.).

This article will illustrate the challenges and solutions with a section of a box tunnel from the airport project. We hope that the examples used here will inspire you to extrapolate the techniques used for other structure types.

Key Technical Challenges

The following key technical challenges were highlighted:

— Civil 3D surfaces are not well represented in Revit.

— Revit models rapidly become too large to manage effectively when importing DWG data.

— Revit models created from imported DWG data do not allow to <host> Revit families.

— If the transportation model updates and needs to be imported into Revit again, structures, architecture, MEP trades all have major modeling updates to carry out. In some cases, they need to delete the model and start again.

— Revit does not cater for some of the structures type sectional views; e.g., developed profiles through tunnels, dimensions in cross-sections that are not orthogonal to modelled elements.

— Revit doesn’t handle some of the corridor modeling features required for infrastructure easily; e.g., spirals and super elevation.

— Structures team had investigated using Civil 3D corridors to model concrete structures, but had run into some roadblocks:

  • Model data structure became too complex to handle using their methods for some of the box tunnel structures.
  • They didn’t have a solution to model retaining wall counterforts/buttresses dynamically with the linear concrete element of the wall.
  • They didn’t have a solution to incorporate voids through wall and ceiling elements of the concrete structure that would be dynamic with the corridor.

Key Workflow Challenges

The workflow challenges identified by the team were:

— How to integrate Civil 3D based transportation modeling workflows with other disciplines that use Revit as the main modeling tool.

— How to precisely reference the Civil 3D produced elements when designing and locating Revit-based elements (e.g., equipment rooms, emergency stairs, tunnel MEP).

— How to maintain the cross-discipline model relationships as design is iterated and updated.

— How to ensure that design deliverables (e.g., drawing sheets) stay up to date.

Adopted Approach

It became clear that the structures team were pivotal to creating a solution, as they provide the physical model that will be used as a basis and reference for the MEP trades and for nonlinear elements such as rooms: if we found a modeling solution for structures then that could enable all the other teams.

The proposed approach consisted of the following:

— Structures teams should model linear elements in Civil 3D since that is the tool that understands a linear reference system of baseline and chainage.

— Structures should begin to reference transportation corridor model elements, not just the surface.

— Structures team should coordinate a corridor code naming standard.

— Early investigations showed that linking an IFC file to Revit provided a number of benefits to importing DWG files directly:

  • IFC retains properties set data exported from Civil 3D. (Note that this can be useful for other downstream BIM uses such as quantification, material schedules and adding phase data.)
  • IFC gives ability to select, hide, and isolate sub-elements (solids regions) from the link, essentially behaving as a native Revit element. Using DWG, you can only use the link visibility to switch off and on layers not elements.
  • Elements created from IFC can be graphically overridden.
  • IFC imported geometry allows Revit to create section views on the model. DWG import does not.
  • Autodesk tests show Revit file size reduction of up to 50% using IFC over DWG import.

— Structures should provide IFC file of corridor model elements to Revit to continue the modeling process.

— Create a custom Dynamo package to introduce a linear reference system of baseline, chainage, offset and elevation (COE) to linear structures projects

— Produce a library of standard Dynamo graphs to allow trades to model:

  • Place linked Revit files at specific COE along the structure.
  • Place equipment Revit families at specific COE along the structure.
  • Place equipment Revit families at specific COE and at then at chainage intervals along the structure.
  • Use Dynamo to take Revit geometry to punch holes in Civil 3D corridor solids.

The below sections walk through the key steps to engage the cross-discipline team in the modeling workflow.

Tunnel Process

Workflow Overview
As part of the requirements gathering process the interaction points between the trades were assessed and the process steps mapped out.

In the graph below,

  • Horizontal swim lanes represent the trade teams.
  • Vertical columns separate the new linear process from the existing unchanged buildings process that need to interact at set points.
  • Blue boxes are processes.
  • White boxes model production.
  • Yellow boxes 2D deliverables.
  • Grey and green boxes are intermediate model based files that enable a process.

High-level view of workflow.
High-level view of workflow.

Model Authoring

Transportation Team Create Road Alignment/Corridor
Transportation team provide a corridor model that contains the key geometry of alignment; profile, roadway, and datum surfaces. Coding of other key feature lines from the corridor such as edge of carriageway or shoulder can also be agreed.

Transportation reference alignments.


Key to this step in the process is transportation agreeing with other trades, especially structure and architecture the naming standards of elements that will provide reference later in the process. This is standard Civil 3D corridor modeling techniques.

Transportation reference coding for key feature lines.

Transportation corridor model and solids used for reference by Dynamo and Revit.


Structure Team Create Concrete Tunnel
By referencing the transportation generated elements, the structure team now create the corridor data for the concrete tunnel. Additional Civil 3D alignments and profiles are created as required.

Structures team perform additional Civil 3D modeling relative to transportation before modeling concrete.


In coordination with the architecture and MEP teams, codes and locations are agreed for feature line names that they will need as reference, to be generated by the subassemblies.

Structure tunnel feature naming.


Structure team generates the corridor model and publish a corridor solids DWG from selected regions.

Connection with Civil 3D for the Revit structure model.

Civil 3D structure model coordinated with transportation.


Architecture Team Place Rooms Relative to Corridor
The architect opens the Civil 3D transportation corridor model. There is a Dynamo node creates IFC file and links into Revit master model and manages process to achieve shared coordinates so that the corridor is placed in the world coordinate system.

Shared Coordinate System Management: Notes

This is a big deal managing coordinate systems between Revit and Civil 3D. We want to manage the project coordinates for the local site in Revit and to be able to locate Revit models against the larger project in world coordinate systems back in Civil 3D and in other tools like InfraWorks 360.

At this point in our process, we need a Revit model with shared coordinates set up match the world coordinate system. We also need to handle import of solids from AutoCAD to Revit that are already in a WCS and Revit does not handle coordinates that are far from the origin. The Revit link IFC option does not give a shared coordinate option at this time.

This means that we need to invert the shared coordinates and move the model close to the origin and then use Revit shared coordinates to align the model again. We have configured a Dynamo node to manage this process.

Placing Revit links relative to structure corridor model.


In our example, we are going to place an equipment room on the side of the tunnel. The modeling of these rooms is standard BIM practice at Dar.

In the project that Dar are working on there are hundreds of these rooms or other structural elements to place on the hundreds of kilometers of box tunnels on this huge airport.

Equipment room positioned correctly against tunnel using reference point.


Initial placement of these rooms, precisely aligned to the tunnel, manually would be a tedious and error-prone process. However, management of change in the design becomes almost impossible to control manually.

To provide automation we establish a relationship between the room and the Civil 3D corridor model. The architects use a feature line on the back of sidewalk to locate the room. The room model has its origin positioned in 3D at the center of the threshold of a door into the tunnel, allowing for a small upstand into the room that allows for a slope in the profile of the tunnel so the finished floor level of room is never below the sidewalk level.

Ghassan Zein is the design application manager/BIM manager for Dar Al-Handasah. An architect by education, with over 25 years of experience in architectural design, urban planning, project management, BIM, and geographic information systems, Ghassan has put his multifaceted experience to the service of developing, adapting, and optimizing BIM collaborative processes.

Ian McGregor is senior implementation consultant within the BIM service line at Autodesk, Inc. He is focused on assisting customers deliver BIM transformation within the infrastructure industry. He has extensive experience managing requirements for large and complex projects, and delivering across multiple engineering standards and languages.

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It is complex to model structures such as viaducts and tunnels that are controlled by an alignment and profile. Put that in an environment where the alignment changes often, and the structural engineer, using a tool like Revit software, finds it hard to accommodate that change. AutoCAD Civil 3D software is great for designing the alignment and profile—with complex curve elements and subassemblies for the mass modeling of complex structures—but we need Revit software for the detailed concrete...

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