Using Clash Rules in Navisworks to reduce False Positives

To many the single most valuable tool in Navisworks is Clash Detection. I've seen many different figures from $5,000 upwards as the average cost of fixing an issue on site as opposed to resolving the issue first in your virtual model.

However managing the process of tens of thousands of potential clashes can be daunting and time consuming. It doesn't have to be; this is the start of a series of blog posts about getting the most out of clash detection in Navisworks, and this first post is a comprehensive guide to setting up clash rules to reduce false positives.

First up, a video with an introduction to Clash Detective Rules: -

So Clash Rules can allow you to ignore clashes that you aren't currently interested in, aren't real clashes, or are issues with 'unfinished' work which you will review later. Here are some more details about each pre-defined rule.

• Ignore Items in same layer

  This will ignore any clashes between objects on the same layer as the selection, if you are not sure what level the layer is, look for this icon () in the Selection Tree. For example, if you wanted to ignore clashes on the same level from a Revit 2013 file.

• Ignore Items in same group/block/cell
  This will ignore any clashes between objects in the same group, block or cell as the selection. A group can be a block definition from AutoCAD files or cell definition from MicroStation files - look for this icon () in the Selection Tree.
• Ignore Items in same file

  This will ignore items within the same file, so if your model contains multiple CAD files and you just want to check for clashes against other files then select this option. It can be used when you are assuming that a file will be self-clashed in the future. For example, you want to check your Level 2 Mechanical File that isn't yet complete against the structural work. This will ignore any clashes within that Mechanical File.

• Ignore Items in same composite object

  A composite object () is a group of geometry that is considered a single object in the Selection Tree. For example, a window object might be composed of a frame and a pane, or a cavity wall may be made of several layers. This rule would ignore any clashes between the two parts of the same object, For example, the window pane and the frame may be modeled so that the parts touch or overlap. This rule would ignore clashes between them.

• Ignore Items in previously found pair of composite objects

  This is one of the most useful rules to reduce the number of false positives. Whereas 'Ignore Items in the same composite object' will ignore clashes between parts of a single object, this will ignore multiple clashes between different composite objects. For example, if you have a window and pane composite object and a cavity wall composite object, this will report a single clash between the window and the wall rather than multiple clashes between the window frame and the leaves of the wall and the window pane and the leaves of the wall.

• Ignore Items in coincident snap points

  This covers any items found clashing that have snap points that coincide, for example pipe runs and fittings that have snap points at the end of center lines. Navisworks will draw a cylinder as a series of triangles to improve performance (try Hidden Line mode to see how they are drawn). Depending on the faceting factor of the cylinder, some of the triangles may clash with each other, even if the pipe or fittings would fit snuggly if this was drawn as completely round.

Here's a few examples of which rules you may want to use, and when.

• If you are using a cavity wall in your model which is made of several layers; outer leaf, insulation, inner leaf, plasterboard, you can use the rules "same composite object" to reduce clashes from touching leaves, and "previously found pair of composite objects" to count any clashes with the wall as a single clash rather than four.
• If you are running a check of a pipe run with fittings you can use the rules "coincident snap points" and "previously found pair of composite objects" to reduce clashes reported between fixtures and the pipes and within the fixtures.
• If you are looking to check for major clashes between pipes and structure on an early pass, you probably don't want to look for clashes within each model. Use "same file" and "previously found pair of composite objects" to avoid looking within a file of the same discipline and only report a single clash result wherever one discipline intersects a composite object made of several parts.

In a future blog we'll go into more detail about creating your own rules, why you would want to, and how to do it, if there's anything you want to know about rules, leave a comment or use the links on the left to email us.