design accuracy
design accuracy
(OP)
I am planning on modeling an existing structure via staad|pro.
Knowing that it's a model and not the construction documents, is it acceptable to round your models dimension up to the next whole inch or even foot in some cases? I know that most FEA software's out there have the ability to go to that precision, and I've done so in the past.
Do any of you round the dimensions of your model to save time?
Knowing that it's a model and not the construction documents, is it acceptable to round your models dimension up to the next whole inch or even foot in some cases? I know that most FEA software's out there have the ability to go to that precision, and I've done so in the past.
Do any of you round the dimensions of your model to save time?






RE: design accuracy
RE: design accuracy
RE: design accuracy
Mike Halloran
Pembroke Pines, FL, USA
RE: design accuracy
>> This is compounded by your erroneous assumption that reducing precision speeds up the program. It does not. That's because the program cranks through the same 32-bit or 64-bit floating point calculations, regardless of what the actual value is.
TTFN
RE: design accuracy
Even customary rounding, e.g. using .63 instead of 5/8, adds up throughout the building process, and it doesn't average to zero. You end up with corners that look closed but aren't, lines that appear to touch end to end but really overlap, stuff like that. CAD programs get confused when presented with discontinuous geometry like that, and you'll waste so much time tracking down those little errors that any gains from using fewer keystrokes at input are more than consumed.
Mike Halloran
Pembroke Pines, FL, USA
RE: design accuracy
RE: design accuracy
One quick comment for the novice engineers out there though, do not mistake precision for accuracy. Precision is the number of digits you carry through your calculations. Accuracy is the degree of correctness of an answer or process. Many of our loadings are only accurate to two figures although most engineers I work with strive for three figures. When the loadings are only accurate to three figures (at best), don't waste resources preparing calculations with 5 significant figures. You are fooling yourself.
RE: design accuracy
RE: design accuracy
Structural engineering is not "rocket science". I agree with COEngineeEr.
RE: design accuracy
RE: design accuracy
I do a lot of 2 to 5 story buildings. I round everything off to the next greater even foot dimension in my calculations and computer analysis models.
We hardly know the loading withing an order of magnitude, so extreme precision in the length of meembers is complete overwehlmed by the inaccuracies inherent in our loading assumptions.
RE: design accuracy
It might make more of an impact on how it is rounded...meaning with some experience, one can round up or down depending on what dimension is being dealt with.
For example: for finding a moment in a beam, round the span up and round the loading up. For the structural depth of a slab design, round the dimension down so slightly more steel is required. Knowing which way to round for each situation is always helpful.
I also agree that for CAD, don't round. It will just cause problems down the road.
RE: design accuracy
I never draw or model anything, call it AutoCAD, SAP, Staadt etc to fractions of an inch. I just imagine the iron workers at the job site reading 13'-1-5/16" from a framing plan..... it has got to be hilarious.
I have found very sane to work with round numbers and agree to stay to the quarters of a foot numbers, when modeling. 12.25', 12.50,12.75' etc.
Also remember, most of us deal with buildings up to three stories high. 93% of construction is of that size.
Regards
RE: design accuracy
When I program, for example, I may have pi defined to 17 decimal places only because the programming language defaults to a 'double' variable and there is no overhead in doing so. The answer is not likely any more correct than if I had used pi defined to 5 decimal places.
And with CAD, I draw stuff to the 'exact' dimension just to prevent closure issues, and although I usually set the precision to 1/64, I realize that on site, the precision may not be any better than 1/8...
Dik
RE: design accuracy
Say that the column line locations are all established by dimensions from a common base line, such as this:
Because it is a large structure, you round off to the closest foot:
83'7" becomes 84'
84'5" becomes 84'
Your model has just merged two distinct column lines. In industrial buildings it is common to have unusually spaced column lines in different parts of the structure - what I just described can really happen.
You can do what you want, and I know that the above scenario is unlikely on your current project. However, I suggest that you use good practice now to avoid "unintended consequences" later.
www.SlideRuleEra.net
RE: design accuracy
If I start with the best precision available, I always end up more satisfied, and usually with less rework, often for the reason to which SlideRuleEra points.
RE: design accuracy
RE: design accuracy
Carpenter holding plans: Okay, turn that 43 degrees 18 mintues and 43 seconds
Carpenter holding speed square: Alright, 45 degrees. Shoot it.
It was hilarious for everyone but me.
RE: design accuracy
DaveAtkins