Dimensions and BIM
Dimensions and BIM
(OP)
I guess I'm a little old fashioned, but I am getting a little tired of some of the trends I am seeing with BIM. I will admit that I don't use it and more than likely (hopefully) never will. I work on mostly small industrial facilities and building less than 3 stories in height.
I have two consecutive projects working with architects who are using BIM. One project has the building grid dimensions down to 1/4 inch, the other has the dimensions down to 1/16 inch. Is this accuracy even needed, can't we keep things to the nearest 1/2 inch? There is no way a foundation guy is going to construct a building 100'-3 5/16" x 84'-2 1/4".
I imagine this is a sign of things to come, and I can only imagine the precision increase that happened from hand draw to CAD drawn.
I have two consecutive projects working with architects who are using BIM. One project has the building grid dimensions down to 1/4 inch, the other has the dimensions down to 1/16 inch. Is this accuracy even needed, can't we keep things to the nearest 1/2 inch? There is no way a foundation guy is going to construct a building 100'-3 5/16" x 84'-2 1/4".
I imagine this is a sign of things to come, and I can only imagine the precision increase that happened from hand draw to CAD drawn.





RE: Dimensions and BIM
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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RE: Dimensions and BIM
I had the opportunity to examine drawings of a new building here in Canada. The architects worked in metric. The drawings were converted to feet and fractional inches for the construction people.
Could that be your problem?
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JHG
RE: Dimensions and BIM
I typically only show dimensions to the 1/2", but understand if they get it within an inch, I am ok with it. They will just lose the other 1/2" on the other side.
RE: Dimensions and BIM
Does it really matter? I guess that's my point. Why are we being so precise when the guy pouring the foundations isn't going to be so precise.
drawoh,
No, both are US architects designing US projects. Although, I have run into that problem before on a project last year. The project was designed by a US architect in the US but the building was premanufactured using methods and computer program developed in Europe. In that instance if you added the string dimensions to the whole dimensions the length of the building was off by 1/2". I was skewered by the GC who thought it was my fault until I told him to talk to his client who bought the building.
I am bringing up this issue only in reference to something being lost. Similar in a way from when drawings were produced by hand to when they are now produced in CAD. When they were done by hand designers had to be conscious of these issues. This consciousness was lost with a move to CAD. I am guessing even more is going to be lost with a move to BIM.
RE: Dimensions and BIM
1) I agree that column lines should be spaced to the nearest inch, though I'd prefer the nearest 3". Unless you're retrofitting a structure, there's no need to have 20' - 5 5/8" column spacing (which is what I have on a recent project of mine). Nothing would have been lost had it been 20'-5 1/2" or 20'-6" between columns.
2) If the dimension calls out some fractional inches (or any value for that matter), then the CAD file or BIM model better be that exact dimension. Not 3/256" shorter/longer and then let the dimension tool round that difference into oblivion. The project mentioned above had that problem - after setting all the column lines I pulled an out-to-out dimension and the out-to-out was bigger than what the architect's file showed. Too many times I feel like I'm on clean up duty making the architect's poor modeling work.
RE: Dimensions and BIM
Cheers
Greg Locock
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RE: Dimensions and BIM
In the US at least tolerances on inch mechanical ("machine design" etc. - not construction/facilities related) drawings are commonly invoked by a scheme based on the number of decimal places behind the dimension, which then implies a certain 'default' tolerance value. So drawing to higher or lower 'resolution' can change the tolerances implied if you aren't careful.
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RE: Dimensions and BIM
I was asking more along the lines of why do you care that it is overly precise? At the end of the day, these things get spit out from a CAD program, and it would seem to require additional effort to make all the numbers imprecise for no real gain, as you stated, the accuracies are lower, so there's no gain for additional work.
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers
RE: Dimensions and BIM
If I'm going to show it, and it's not really going to be done in the field then why should the architect be so accurate to begin with.
RE: Dimensions and BIM
There is another issue which has nothing to do with BIM.
When you design something in CAD, it is easy to drag the shape and size you want. You go into your drawing, clicketty click, and the dimension comes out 11'-5-5/16", or perhaps 16771.9mm. You finalize the drawing! A lot of people make a point of forcing the model into a rounded off size. Some people don't. In the latter case, BIM inherits the mess.
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JHG
RE: Dimensions and BIM
Cheers
Greg Locock
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RE: Dimensions and BIM
That scares me. This is why I like doing my own drawings. In 3D CAD, before I finalize everything, I prepare all my fabrication drawings. With completely dimensioned drawings, I can see where it would be nice to round off numbers, and I can round them off, updating all the affected pieces. Then, I localize all the models and check everything in. Nothing I do gets anywhere near BIM.
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JHG
RE: Dimensions and BIM
.
Me wrong? I'm just fine-tuning my sarcasm!
RE: Dimensions and BIM
RE: Dimensions and BIM
One time, I made some little bridges to span between the trusses in my house's roof, so I could slide along them on my back, and use the space for light storage. They had to be short, so they could enter diagonally through the hatch from a closet.
I prefabbed about six of them, based on the nominal truss spacing of 2'-0'.
None of them fit; some were too long, some were too short.
The actual truss locations were off by up to 2 inches, and some trusses were warped out of plane.
So I got to trim the long bridges to fit the short bays,
and make the remainder one at a time, custom fitted.
</tangent>
I guess I got spoiled, working in automotive, military, aerospace ground equipment, and medical equipment, where the end product had better look like the design documents, and discovered deviations invite the participation of regulatory authorities.
Given the apparently accepted construction industry practice of somebody perhaps someday recording deviation from the design documents with 'as-builts', i.e. a second set of documents for which no one wants to pay, and for which no one takes responsibility, the whole BIM activity is 'as-built' on a very shaky foundation.
Mike Halloran
Pembroke Pines, FL, USA
RE: Dimensions and BIM
Robert Hale, PE
RE: Dimensions and BIM
BIM is done by the fresh out of college and the details of grid dimensions isn't looked at until part way through design if ever.
I am currently dealing with a few BIM grids that weren't drawn straight... for no reason skewed by 1degree in a rectangle building. That sucks! oh and the lovely 1/256" units with truncation down to 1/8"
RE: Dimensions and BIM
This has always been the case with CAD of all kinds, and AutoCAD is NO LESS susceptible to negligent or untrained errors. I've seen all of these exact same errors in AutoCAD type files. It is no better or worse in BIM, but like moving from PAPER to CAD in general, there are gobs of benefits.
I'm personally glad I no longer have to use Revit (or any Autodesk product), but when I did, I embraced it, understood its quirks, my shortcomings, and was got the work done easily. Then again the one architecture firm that required it's use (I was in structural design) was about a mile away from our office also pretty good about fixing models when discrepancies or errors were discovered that caused us grief.
ETA: Anecdote: I remember reviewing some shop drawings for a client who took our design drawings and had some foreign company make up the shop drawings real cheap. I'll never forget how hard I laughed when I saw a bar length dimensioned at X' 3-5/7" and other oddball fractions. It wasn't a typo, either. It was an unfamiliarity with American fractional systems, allegedly. Some people can't be saved, no matter the tools they use.
RE: Dimensions and BIM
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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