Dimensions and BIM

[SteelPE]

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.

 

[IRstuff]

Does it really matter, other than perhaps an esthetic or pedantic question? Is 100' 3 1/2" really any looser, given that the as-built accuracies are what they are?

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529

 

[drawoh]

SteelPE,

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?

--
JHG

 

[PEDARRIN2]

In Revit, you can set the units to be as precise as you want and since the dimensions are "actual" you can show dimensions that will make a contractor laugh.

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.

 

[SteelPE]

IRstuff

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.

 

[WinelandV]

Two thoughts:

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.

 

[GregLocock]

From an actual engineering point of view there is no difference in the accuracy required of a dimension of 100 ft, or 100ft3 in and 3/8inch. You may have some silly arbitrary tolerance scheme in place, but without knowing how that works we can just guess. I'm pretty sure that when I order 1800 mm window units that the odd 3/8 inch in column spacing will cause issues.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[KENAT]

I started a response along the lines mentioned by Greg than ran to a meeting instead. At least in my mechanical world fundamentally the issues aren't what dimensional accuracy the parts are modeled or drawn to but what the tolerances applied are.

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.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[IRstuff]

"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."

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 forum1529

 

[SteelPE]

Why do I care, because I am required to use the dimensions shown on the architectural drawings. It would reflect really poorly if my drawings didn't match. Sometimes when the grids are in fractions of an inch it makes the dimensions shown on the drawings a complete mess. Then there is the fact that I like to model everything as accurately as I can. So having bays in fractions of an inch cause me to spend more time with my computer models than what I really should be doing.

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.

 

[drawoh]

SteelPE,

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.

--
JHG

 

[GregLocock]

That is a point i was carefully avoiding. In a previous career i would carefully work out the dimensions of a part. Then I'd flick it over to a drafter who would prepare the machining drawings, and carefully round off all my exact dimensions to nice round numbers.since this all got typed into a cam program it made no odds whether it was a round number or three decimal places. I'll be the first to admit that the initial analysis was +/-20% at best, but it was my best guess.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[drawoh]

GregLocock,

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.

--
JHG

 

[HCBFlash]

In answering the question "...is this necessary?...", it depends. If the BIM is used correctly, such dimensioning can have advantages, but few projects have active management of the drawings and verification of materials and assemblies so involved even in producing "as-builts". On a project for a very large "electronics" manufacturer in the USA has been running, BIM has been used to good effect, however the building owner/end user demanded it, and had full-time rep's on-site, and they were pretty good at keeping on top of stuff. Also, good tolerances were used and as-builts were constant and used continuously. However, as stated, this was not simply office space or other common commercial occupancies, and I speak as one involved in the very extensive electrical systems in these facilities. We do build assemblies shown in BIMs to dimensions and tolerances much finer than those used by other trades and systems. BIM certainly is the way things are headed, but must really be used to produce a higher quality product with approximately the same amounts of labor, as opposed to a new electronic, paperless labor reduction technique or cost cutting method for the construction phase of buildings.

.


Me wrong? I'm just fine-tuning my sarcasm!

 

[BUGGAR]

I'm the guy stretching the tape in the field. I can read the tape to sixteenths. I can even read the tape to "thirty seconds". But I am not paid to do math in the field. It angers the boss. Do whatever accuracy you have a jones for but make all the numbers add up correctly!

 

[MikeHalloran]

<tangent>
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

 

[RobertHale]

SteelPE, I completely sympathize with your plight. I think one of the worst things with this issue are the busts in dimension strings due to rounding. I have a project where the architect initially had the dimension tolerance set to the nearest 6" but when you started to measure the dimensions between even grid lines were in the odd multiples of 256ths of an inch. It finally got fixed but before it did you would get things like 10'-0" + 10'-0" + 10'-0" = 31'-0"

Robert Hale, PE

 

[EngineeringEric]

To me the problem isn't the program, it is oversight. If the PM or more senior staff don't look at a drawing or train a junior arch/engr then they will blindly draft something that is 100'-3.125". A program is only as good as the user, a hand draft set of documents is only as good as the draftsman (and they were good and usually experienced by the time they were doing plans, at least i think... before my time!)

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"

 

[JNieman]

I will never understand the complaints about BIM being the ability to have oddball dimensions or dubiously-placed objects.

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.

 

[IRstuff]

At a previous company, we had a "Drawing Manual" that was essentially our internal style guide covering things like numerical precision, etc.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm