Dimensions not adding up

[Anonymous]

I run into this from time to time. I've also run into it in other CADD/BIM software. Anyone know what can cause this?

[Anonymous]

Wrathchild wrote:
I don't really think my model could be the problem. I almost always (if not always) type in exact units when laying out my plans.

Even though accurate model building is the "always" the goal, one missed snap either in building or dimensioning and there it is. And sometimes if you're building with real material dimensions it can happen. As a final check I put it in 1/64" and adjust the model to get the right dimensions. That way the dimensional creep visits less often.

Dave

[Anonymous]

Dave wrote:

Wrathchild wrote:
I don't really think my model could be the problem. I almost always (if not always) type in exact units when laying out my plans.

Even though accurate model building is the "always" the goal, one missed snap either in building or dimensioning and there it is. And sometimes if you're building with real material dimensions it can happen. As a final check I put it in 1/64" and adjust the model to get the right dimensions. That way the dimensional creep visits less often.

Dave

I can't imagine not using real material dimensions. That seems like asking for trouble.

[Anonymous]

Wrathchild wrote:
I can't imagine not using real material dimensions. That seems like asking for trouble.

I agree, but the point is, let say you have 15/32" actual sheathing and inadvertantly snap to it rather than framing line (either in dimensioning or placing) then depending on your offset you can come up with dimensions that are off by some small amount which show up as a 1/64" when dimensioned at that setting or give the strings that do not add up at a higher setting.

[Anonymous]

Ahhhhhh the old "it just doesn't add up" thing comes up again - and again. Many moons ago in the Frenc days, this was a serious problem - once costing me a lot of money to have the foundation forms for a large residence to be re-set. All because of my own negligence in creating extremely accurate modeling. As a result of this error, I adopted the procedure of only creating in 1/64ths, including dimensioning, and making all of the graphics as correct as humanly possible - then at the end or at significant milestones re-setting the dims to 1/4. This was the best way at the time to assure that the strings would at least somewhat agree. Another method I used was to set a walls only layer set that had those elements that required best dimensioning practices and dimensioned that set. The slabs, ceilings and other elements then had to be the same as the walls. The whole thing was exacerbated when trying to use composite walls - my way around that one was to only draw studs -- not great but kept the plans simple and readable.

Now days with the structure display the process is much simpler, but I still draw with 64ths as the default and change to a more appropriate fraction at required times.

BTW the Frenc explanation at the time was to recognize the difference between 6 decimal places of the computer and the resolution of the screen and brain. Forcing the use of absolute input elements.

[Karl Ottenstein]

I would use accurate sizes for everything, including that 15/32 sheathing. One should not be snapping to wall skins for dimensioning if using complex walls IMHO, but should be using 'core' dimensioning, which is handled by clicking on a wall edge and letting ArchiCAD find the core. You end up having to delete some extra dimension points to clean up, but it saves problems.

Like Lew, I also recommend always working in 1/64 working units, and 1/64 working dimensions. Use some lower rounding dimension for your placeable views.

Cheers,
Karl

[Laura Yanoviak]

Karl wrote:
I would use accurate sizes for everything, including that 15/32 sheathing. One should not be snapping to wall skins for dimensioning if using complex walls IMHO, but should be using 'core' dimensioning, which is handled by clicking on a wall edge and letting ArchiCAD find the core. You end up having to delete some extra dimension points to clean up, but it saves problems.

Like Lew, I also recommend always working in 1/64 working units, and 1/64 working dimensions. Use some lower rounding dimension for your placeable views.

Cheers,
Karl

This is our practice. Also keep in mind construction tolerances -- accuracy is crucial, but allow for "creep".

[Djordje]

As discussed many times ... going metric would make this obsolete, but that is a different kettle of fish.

You should do the things as they are, and go for the best accuracy possible. And, don't expect things to add up if you have to round up. Or down.

[Erika Epstein]

An old-school technique was to put in a long string a +/- on one dimension to show where you the architect would allow these rounding errors to be resolved.

Keeping tolerances at 1/64" means you immediately notice iwhen your dimensions are off the accepted rounding of 1/8",1/2" or whatever is appropriate.
Model accurately. When you see a problem, stop, find and fix the error .

[Laura Yanoviak]

Erika wrote:
An old-school technique was to put in a long string a +/- on one dimension to show where you the architect would allow these rounding errors to be resolved.

Our practice is to omit the last (or least critical) dimension -- to never close a dimension string.

[Steve Jepson]

I also work to the 1/64th setting. I never have dimensions on a floor plan with fractions less than 1/2". Fractions are for details, not floor plans. Dimension everything to face of stud.

Dimensions on a floor plan are not so much for showing the size of things as they are for showing the location of things.

If you draw with precision you will never have to tweak your dimension text to get rid of the odd fractions.

The reason you can get bad dimension chains is because setting the dimension text to round up to some fraction does not change the level of precision with which you are able to place things.

Obviously if you are working sloppy and rounding off dimensions to the nearest 1/2" you are going to get some bad dimension chains.

The way to make sure you never have any difference between your overall dimension and the dimension chain is to use the snap grid.

If you have the snap grid set to the same level you want your fractions displayed at you will not have any trouble with bad dimension chains.