1. Clash Detection Terminology
Hard, Clearance and Duplicate Clashes
This PPT explains what is Clash Detection, the types of
Clash Detection and why it is required |

2. Clashes arise as the models of various disciplines – Architectural, Structural, Mechanical, Electrical, Plumbing, Fire-fighting, etc. are combined to make a composite model. Elements of various disciplines collide with each other as they are modelled in the same space by different consultants. Finding these clashes before starting the project is important as it has been proven too costly to detect the clashes at site and fix them.
Clashes: |

3. Hard Clash Clearance Clash Duplicate Clash
A clash in which the geometry of selection A intersects that of selection B by a distance of more than the set tolerance.
Hard Clash
A clash in which the geometry of selection A may or may not intersect that of selection B, but comes within a distance of less than the set tolerance.
Clearance Clash
A clash in which the geometry of selection A is the same as that of selection B, located within a distance of between zero and the set tolerance. A tolerance of zero would therefore only detect duplicate geometry in exactly the same location.
Duplicate Clash |

4. Clash Status Approved Reviewed New Active Resolved
Each clash has a current status associated with it and each status has a colored icon to identify it. This status is updated automatically by Clash Detective or can be set manually if desired.
The statuses are as follows:
Clash Status
Approved
A clash previously found and approved by someone
A clash previously found and marked by somebody As reviewed
Reviewed
A clash found for the first time in the current run of the test
New
A clash found in a previous run of the test and not resolved
Active
A clash found in a previous run of the test and not found in the
Current run of the test. It is therefore assumed to be resolved
Resolved |

5. A standard Hard clash test type applies a Normal Intersection Method, which sets the clash test to check for intersections between any of the triangles defining the two items being tested (all Autodesk Navisworks geometries are composed of triangles)
 Intersection Method
This may miss clashes between items where none of the triangles intersect. For example, two pipes that are exactly parallel and overlap each other slightly at their ends. The pipes intersect, yet none of the triangles that define their geometry do and so this clash would be missed using the standard Hard clash test type. However, choosing Hard (Conservative) reports all pairs of items, which might clash. This may give false positives in the results, but it is a more thorough and safer clash detection method. |

6. Hard clash severity depends on whether the Conservative or Normal Intersection Method has been applied. If Normal, the greatest penetration between a pair of triangles is measured. If Conservative, the greatest penetration of space around one item into the space around another is measured.
For hard clashes, the severity of a clash depends on the intersection of the two items intersecting. Hard clashes are recorded as a negative distance. The more negative the distance, the more severe the clash.
Severity |

7. Tolerance controls the severity of the clashes reported and the ability to filter out negligible clashes, which can be assumed to be worked around on site. Tolerance is used for hard clash, clearance clash and duplicate clash types of clash test.
Tolerance
Any clash found that is within this tolerance will be reported, whereas clashes outside of this tolerance will be ignored. So, for Hard clashes, a clash with a severity of between zero and the tolerance value will be ignored, whereas for Clearance clashes, a clash with a severity of more than the tolerance value will be ignored as it is further away than the distance required. Similarly, a Duplicate clash with a severity of more than the tolerance value will be ignored as it is likely to be a separate, yet identical piece of geometry. |

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