Clash Detection 

I had an RFI come in the other day. It was a request to change a couple dozen duct sizes on a building. The Owner, Mechanical Engineer and I were surprised. During the course of the design we ran clash detection. We felt good about the clearances for the ducts. The challenge with changing to many ducts is the impact on the system as a whole. But one step we always take is in our specifications is for the contractor to provide an above ceiling coordination drawing.

I should note that this is a design build project. The A/E has taken the BIM model to an LOD of 300 and we have been sharing the model with the contractor. The subcontractor(s) then have been using the model(s) to develop their shop drawings. This normally works fine, but now begins the change from paper construction documents to paper shop drawings. As the designer, we show design intent. Over the past 10 years, I’ve observed that as the skilled tradesmen have retired, we lost an import skill set. The drawings are no longer just design intent but intended to be instructions on how to build a building. But let’s save that thought for another time and look at what we can do to bring solutions to the table.

Going back to the team’s surprise on the ducts and beams colliding. There were a couple of factors. We did our last clash detection at 95% CD. Lessoned learned, do it at 100% and then once more at a 105%. The structural engineer needed to beef up a couple of beams. Plus, at the same time lowered the roof by about 8”. OK, not a real big deal, at the thickness beam we still have over 24” clearance. The original design targeted 36”. 

We reran our clash detection and we are still OK. Yet we are scratching our heads and asking what was different? The contractor had lowered the roof another 4”. This happened because the columns were shorter than designed and in order to maintain the proper roof sloped. The roof as a whole had to be lowered.

So now we have a blending of real-world construction and design intent. Working closely with the Mechanical Engineer, we developed a solution that did not require flattening of the ducts.  There are a couple of elements in the BIM model that don’t read well when doing a Clash Detection. One of those is flex duct. This cleared up 25% of the clashes. Now to resolve the other 75%. 

The solution simply was to have the individual supply duct come off the main duct from the bottom instead of the side. This addressed 95% of the ducts. But now we have clearance issues with cable trays and sprinkler pipes. It seems that last 5% is always the tough part. But we flipped the location of a couple of elements and... Drum roll please… It worked. We posted the response to the RFI, clouded the old collisions and made the models available.

There is a lot of power in using clash detection software. But it still requires some thought in pulling all of the pieces together. Ten years ago, we would have just worked it out in the field. I know without a doubt that there is a lot that can be learned in the field. That’s why I have enjoyed my time spent in Construction Administration. The good news, but working it out in the digital world we now have a ready to go solution when the ducts show up on the site.

…and now for something completely different.
It is physically impossible for pigs to look up into the sky.


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Year One


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Friday, 23 February 2024