BIM Clash Detection

Steps on how to streamline the BIM clash resolution process. 

This article is written with the intermediate AutoCAD MEP or Revit user in mind, assuming a general working knowledge of BIM coordination. Autodesk Navisworks (formerly known as JetStream) has become the most commonly used clash detection software in today’s U.S. construction and design build industry.

Navisworks is used to combine and navigate through many different types of 3D models. It supports and interacts with a wide range of file formats, including AutoCAD, Revit, Microstation, Inventor, Solidworks, 3D Studio Max, SketchUp and numerous others. The main appeal is its ability to append and combine files from multiple software platforms, but it can also render, animate, perform timelines, and quantify elements contained in the appended files. Navisworks is perhaps best known for performing clash detection tests between the 3D models of different AEC disciplines.

Contractors have primarily used this software in the past. However, as the AEC industry continues to evolve its design methods and processes, more owners are asking that architects and engineers use the software during the design phase  to reduce and even eliminate clashes prior to turnover of 3D models to the construction team.

In turn, this has put pressure on AEC design teams to educate their staff on the use of Autodesk Navisworks, Autodesk BIM 360 Glue, or other clash detection software. This article will focus on streamlining the process; not for clash detection itself, but for responding to clash detection reports and modifying the 3D model to address and remove clashes.

Clash detection workflows

A common workflow includes a BIM facilitator who performs clash tests at various intervals and notifies the AEC team of items that need to be addressed. There are different ways to go about this, and it often involves issuing a report showing a JPEG image of each clash along with other information, such as the clash name, status, grid location and building level, element I.D., etc. These reports can be customized to show various types of information to help identify the clashing items and to allow 3D model authors to locate and take appropriate action to remediate the issue. Clash tests are run between two specific disciplines (i.e. “Ductwork vs. Structure,” “Plumbing vs. Electrical,” etc.).

Clash reports are typically generated and distributed ahead of a multi-discipline coordination meeting, which can be attended either in-person or via a web meeting. The design team discusses the conflicts and decides which discipline will move its components in order to resolve the clash. A general hierarchy tends to guide the process. Clashes between MEP trades and structural steel typically result in MEP items being rerouted to avoid collisions with structural elements. Pressure pipe may be rerouted to avoid larger items such as sheet metal ducts. Ducts may need to be rerouted to accommodate sloped drainage piping that is restricted by the available vertical clearance within its area of travel.

It is important to note that a collaborative team approach is required for successful clash resolution. When team members work unilaterally to address clashes, the effort often results in confusion because the other team members are unaware of the change. In many cases both parties will address the same clash, which then only creates another set of clashes.

A successful clash resolution effort involves collaboration and agreement. Once the teams agree to a clash resolution, the BIM facilitator creates a saved viewpoint (screenshot), notates it with the agreed direction and then moves on to the next clash. Once all of the clashes have been reviewed in this way, the meeting is adjourned and each discipline makes its assigned changes before the next clash review is performed. This process is repeated at intervals as necessary to deliver a coordinated project.

The element of timing

Clash detection can become a somewhat pointless routine if started too early in the design process. During schematic design and design development, the design team works through the iterative process of coordination and routing, responding to changes in the architectural model and developing their own basic design and layout. At this stage, it can be reasonably expected that clashes will exist throughout the model. Detailed trade routing and coordination cannot be expected, and therefore going through the motions of clash detection too early in the process can become an exercise in futility. Depending on the complexity of the project, creating section views or 3D views to check for conflicts may be sufficient in the early stages of a project.

Clash detection becomes a more viable tool for the BIM designer later in the design process as layout issues are resolved and architectural changes subside. When the construction documents have reached 50 percent to 75 percent completion, clash detection becomes more practical for solving routing conflicts. It is generally better to start with a larger perspective and then focus in on a more granular level as the project moves toward completion.

Fixing clashes efficiently

Because of its historical contractor use, Navisworks is sometimes viewed as more of a “peripheral” software by architects and consulting engineers who use AutoCAD MEP or Revit to produce engineering drawings and bid documents. As a result, designers are often less familiar with Navisworks and its many useful features and tools. One such tool is an add-in called “Navisworks Switchback.” This is an often overlooked feature that can be beneficial to the designer to work out clashes in a much more efficient manner than the typical workflow.

The BIM facilitator presents the engineer or designer with a Navisworks “NWD” file, which contains a series of saved viewpoints that show clashes and are marked with instructions on who will move each item to resolve the clash.

The designer then attempts to figure out where the clash occurs so that s/he can then open AutoCAD MEP or Revit, navigate to the same location and make the necessary changes. Depending on how complete the model is at the time of detection, the area of the clash may be congested with duct, pipes, conduit, structural elements and the like, which can make it visually difficult to determine its location. The designer may need to temporarily hide elements, turn on gridlines, zoom to a wider view angle, or move about sufficiently to determine the clash location in the Navisworks model. S/he can also see the level and nearby gridlines displayed in the lower left hand corner of the screen to get a general idea of location; and then navigate to the same location in the AutoCAD MEP or Revit model to remediate the clash.

All of this takes time, and Navisworks Switchback can provide a better workflow. Here’s how it works:

The “Navisworks Switchback” Method (AutoCAD MEP and Revit)

1. Start by opening your discipline’s model in AutoCAD MEP or Revit as applicable. If using AutoCAD MEP, type “NWLOAD” at the command prompt and hit enter. The command line will then display “Navisworks Ready.” If using Revit, click on the “Add-Ins” tab, “External Tools” ribbon, and then pulldown and select “Navisworks Switchback.”

2. Open the Navisworks file. From the “View” tab, “Workspace” ribbon, “Windows” pull down, open the “Saved Viewpoints” and “Selection Tree” windows and make sure they are docked on either side of the screen. In the “Saved Viewpoints” window, click on a saved viewpoint that illustrates a clash. These viewpoints will typically be organized in folders and marked  or notated with comments. Once the viewpoint is displayed on the screen, notice that the grid location and building level of the clash are displayed in the lower left corner of the screen.

3. Now use the cursor to select the item that you will be modifying to resolve the clash. Notice that the selection tree has expanded to highlight the item that was selected. 

4. From the Selection Tree window, right click on the item in the selection tree and then click on “Switchback” from the shortcut menu. Navisworks now automatically opens the AutoCAD MEP or Revit file and creates a 3D view that is identical to the saved viewpoint in the Navisworks file. The item that you selected in the Navisworks saved viewpoint is also highlighted in this 3D view.

For editing purposes, 3D views generally do not work well. Having made note of the building level that was displayed in the lower left corner of the Navisworks screen, you can open a suitable view that depicts same building level in AutoCAD MEP or Revit. Be careful not to click in the 3D view, as it will deselect the item. Once the working view is open, the clash item (still selected) will be highlighted on the screen, and you can easily locate it and zoom in for editing. At this point, you can create section views and make modifications as necessary to fix the clash.

Using this method, you can easily switch back and forth between Navisworks and AutoCAD MEP or Revit to quickly and efficiently address all of the outstanding clashes.

The “Select by ID” Method (Revit only)

Another approach that can be used to locate clashing items in Revit is the “Select by ID” method. Depending on the situation, you may find this method quicker as it involves less steps.

1. Start by opening your discipline’s model in Revit.

2. Follow the step two instructions listed previously in the “Navisworks Switchback” method. In addition, open the “Properties” window and dock it on the right side of the screen above the “Saved Viewpoints” window.

3. Use the cursor to select the item that you will be modifying to resolve the clash. In the “Properties” window, click on the “Element ID” tab, which will indicate a numerical value. Right click on the value and select “Copy Value.”

4. Activate the Revit window. Taking note of the building level that was displayed on the Navisworks screen, open a suitable working view that depicts the same building level in Revit. Click on the “Manage” tab, “Inquiry” ribbon, and then click on “Select by ID.” Next, right click and paste the Element ID number at the prompt, and then click on “Show.” The element will now be highlighted on the screen and you can easily locate it and zoom in for editing.

The “Zoom Object” Method (AutoCAD MEP only)

While the “Select by ID” method is not available in AutoCAD MEP, this is a similar method that can be used. You may find it slightly less appealing because it involves some typing, whereas the Revit method is strictly copy, paste and click.

1. Start by opening your discipline’s model in AutoCAD MEP.

2. Follow the step two instructions listed previously in the “Navisworks Switchback” method. In addition, open the “Properties” window and dock it on the right side of the screen above the “Saved Viewpoints” window.

3. Use the cursor to select the item that you will be modifying to resolve the clash. In the “Properties” window, click on the “Entity Handle” tab, which will indicate a numerical value. Right click on the value and select “Copy Value.”

4. Activate the AutoCAD MEP window. Making note of the building level that was displayed on the Navisworks screen, open a suitable working view that depicts the same building level in AutoCAD MEP. At the command prompt type “Zoom” and click enter. Next, type “O” for object and enter, where you will then be prompted to “Select Objects.” Type in “handent” and paste the entity handle numerical value copied from the Properties window in Navisworks. Note that the text string must be entered using the following example syntax including punctuation: (handent “29F39”). The command prompt will display “1 found.” Hit the enter key again, and the screen will zoom directly to the object that you had selected in Navisworks. You can now make modifications as necessary to fix the clash.

Any one of the above methods will improve your workflow when responding to clash detection tests. Try out each one and determine which works best for you, depending on the situation and the BIM model platform. Often overlooked, Navisworks can be a powerful resource for the plumbing engineer. Take time to explore this software and learn some of its basic features to enhance your understanding of BIM coordination and clash detection. 

Aaron Coppersmith is a Senior Plumbing Design Engineer in the Southern California division of Southland Industries a national MEP building systems firm. He is a Certified Plumbing Designer (CPD), Green Plumbing Designer (GPD), accredited LEED BD+C, ARCSA AA, and dedicated BIM advocate. He can be reached at acoppersmith@southlandind.com

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