Navisworks Clash Detection 101 – Part #3

Part #3 – Understanding Clash Types and Tolerances

One of the most misunderstood parts of Navisworks clash detection is the settings for the clash type and the tolerances. Using the model from my hydraulic pipe sizing article I have put together an example of the different clash detection types at varying tolerances. I’ve added some columns to the model which are progressively offset at 25mm intervals. You can clearly see that there are columns that clash, and columns that do not clash with the drainage pipework.

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I’ve then created two selection sets as explained in the 2nd part of my clash detection series, one for columns and one for the drainage pipework. I have then created a series of clash detection rules that clash the columns selection set against the sanitary drainage selection set, the clash reports repeat at different intervals.

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For the hard and hard conservative clash reports, I have selected tolerances of 10, 20 and 50mm and for the clearance clash report, I have selected tolerances of 20, 50 and 100mm. When running the clash reports, I get the following results:

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As you can see, as you increase the hard clash tolerance, the clashes reduce and as you increase the clearance tolerance, the clashes increase.

Hard clashes are as the name implies – a hard clash but as you adjust the tolerance, the clash is ignored within that tolerance range. Allowing for a 50mm hard tolerance means that two items need to intersect or clash by 51mm or more before they are reported on. The clash shown below has been reported in our 10 and 20mm tolerance reports, but completely ignored in our 50mm report.

There is no denying that this is a clash, yet it’s not reported in the 50mm tolerance test at all.

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Clearance clashes are the exact opposite of a hard clash. Clearance clash reports check elements for minimum clearance requirements. In a clearance clash test with a 50mm tolerance, two elements will need to be 50mm apart or closer to be reported as a clash. This is particularly useful when you want to allow additional clearance for pipe, cabletray or duct hangers and brackets that have not been modelled.

As you can see in the reported clash below, the two elements do not intersect at all, yet they are still reported as a clash due to the minimum clearance not being met.

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But What if my Elements are smaller than my tolerance?

That’s actually an interesting question and can prove useful or troublesome in clash detection reports. I’ve added a 20mm gas pipe to my example model which in plan intersects with my 15mm cold water droppers that run to each basin. Again, you can clearly see that the newly added gas pipework clashes with the existing cold water pipework.

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I’ve in turn created a series of clash tests, this time I have only created hard clash tests with tolerances of 0, 10, 20 and 50mm.

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The reports give interesting results. Only the report with a 0mm tolerance has reported any clashes at all. Results like these have the chance to run you into trouble if the clashes can not be resolved on site – say for example a 65mm vent pipe rising inside a 90mm stud wall and a 32mm chilled water pipe running horizontally in the wall cavity; there is no way the two services will ever fit and yet they’re not reported in Navisworks.

These settings though can also be used to your advantage. For example if you have modelled all the hot and cold water pipework to the individual fixtures, you would never be expected to coordinate pipework of that size and how can you when the architect doesn’t model the studs within the walls? Using the tolerances to your advantage you can avoid having essentially false positive clashes reported.

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