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.

 

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.

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:

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.

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.

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.

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.

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.

Part #2 – Sets

Sets are a big part of clash detection in Navisworks, they can make the difference between a great clash report and one that is a waste of time. Simply clashing one model against another is not going to give you great results in most instances. Say for example you clash an architectural model against an electrical model, that’s all you need to do right? Wrong. Every electrical item that is recessed into either the wall or ceiling will be reported as a clash which this actually isn’t a clash, you end up with a time consuming administration overhead to manually approve clashes that shouldn’t have been reported on in the first place. This post will help you get your head around sets and how to make them work for you.

Selection sets are a group of objects selected either from the view window or from the selection tree window. You can manage sets from the sets window, if your sets window isn’t visible on screen, head to the View tab, and from the Windows button, select Set from the list.

When you run a search in the find items window, the search results will be highlighted in blue in the view window, as well as in the selection tree window.

When you select an object or a group of objects, they will be displayed in the Selection Inspector window along with their properties.

 

Now that I have explained the functions of each of the window elements, I’ll show you how to put them to use.

Using the Find Items window, you can search for elements based on specific information. In this instance, I have searched for all air terminals located on level 1. The find items window will allow you to search for elements based on the element properties which includes user parameters.

Following the same method, I have created a selection set of all the lights on level 3. In the next installment, I’ll explain tolerances and how they affect your clash results.

Part #1 – The Clash Detective Window

To the average engineer or modeller, Navisworks and clash detection go hand in hand, some people think that they are one and the same. If you’re not familiar with the terminology, clash detection is a process that identifies interference between elements in your model where as Navisworks is an Autodesk product that facilitates automated detection of the interferences between modelled elements. These interferences, or clashes may be between different models or within the same model, they could be between disciplines or within the same discipline.

The concept behind clash detection during the design process is that you can identify and fix problems before construction begins, therefore saving time and money down the track. Provided that it is configured correctly, Navisworks can help to speed up the process and reduce human error during model inspection by running automated clash reports.

The problem with Navisworks however is not everyone knows how to use it effectively. Maybe you’ve been to training with a reseller in the past and you walked out the door still scratching your head. After spending a whole day working on 4D timelining, 3 hours showing you how to animate an automatic door in Simulate and less than an hour spent learning the very basics of clash detection in Manage or maybe you’ve had no training at all? Either way you’re only just barely making your way through your clash detection sessions.

Over my next few articles I will outline a few of the clash detection basics to get you more confident in your adventures into clash detection, I’m not going to claim to solve all your clash detection woes, but whatever your experience, it’s always good to have something to refer back to.

In this first installment, I’ll explain each of the parts of the Clash Detective interface.

Beneath the selection windows are a group of buttons which allow you to change what geometry is clashed in the test.

 

The Output Settings section allows you to select the output format of your clash reports. It allows you to select either the reports from the current clash test or from all clash tests either as a combined or separated format. You can also choose from HTML, XML or recorded viewpoints as your clash report format.

In the next installment, I’ll build on what I’ve shown today and explain how to create selection sets to get better results from your clash tests.