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Navisworks clash deep dive 2: false-positives

False-positive clashes can be a headache on BIM projects. This article explains how to remove them prior to generating your Navisworks reports.

In a previous post, we discussed the correct configuration for Navisworks clash detective. After setting it up, the next crucial step is to know how to remove and minimise false-positive clashes. Although Navisworks clash detective can be a very useful resource, it can also create trouble if we do not know how to properly interpret or filter the results. It is therefore paramount to rationalise the clash report tests in order to exclude (remove) ‘false’ clashes.

Clash rules to remove false-positives in Navisworks

Clash rules can allow you to ignore (discard) clashes that either you are not currently interested in, are not real clashes, or are issues with ‘unfinished’ work, which you will review in a later stage. Let us explore in more detail each of the pre-defined rules that can help us minimise false-positive clashes.

Ignore items in same layer

This will ignore any clashes between objects on the same layer as the current selection. For example, it can be useful if you want to ignore clashes on the same level within a Revit file.

Ignore items in same group/block/cell

This will remove false-positive clashes in Navisworks between objects in the same group, block or cell as the selection. A group can, for example, be a block definition from AutoCAD files or cell definition from MicroStation files. This can be useful if you want to ignore clashes on the same block e.g. a lighting column, which may be shown as clashes in the Navisworks detective because they are touching each other, but they are actually part of the same building element.

Ignore items in same file

This will ignore items within the same file, so if your model contains multiple CAD files and you simply 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 is currently incomplete, 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 acts as a single object in the Selection Tree in Navisworks. 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 parts of the model may 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 and remove false-positive clashes in Navisworks. Whereas ‘ignore Items in the same composite object’ will ignore clashes between parts of a single object, this rule 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 vice versa.

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 centre lines. Navisworks will draw a cylinder as a series of triangles to improve performance. 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 they were drawn accurately and completely round.

By knowing how the pre-defined clash test rules work, we can select the ones that will help us eliminate or minimise the false-positive clashes. In a well-designed, coordinated BIM model, by ticking the right rules, we can reduce the reported clashes from a few thousand to just a couple of hundred. Talking from experience, we tested the effectiveness of these clash rules and went from over 3,000 clashes to just 300 clashes in one of our new build construction projects. So around 90% of the initial clashes reported by Navisworks were ‘false-positive’. Now that we know how to use the pre-defined rules, we can save significant time in our BIM projects and focus on solving the real clashes.

Have you got any questions about removing false-positive clashes? Ask us in the comments below and we will be happy to help!

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