Approximate dimensions

[FSS]

How do other folks approximate dimensions on their drawings? For example, when I have a foundation plan based on a rather weak set of building plans (residential projects do not always have complete architectural drawings) I will usually put +/- after my dimensions and follow that with a note to contractor to verify dimensions in field. Same thing applies to remodel projects where dimensions are based on someone else's tape measure.

A colleague of mine told me +/- is not technically correct since 10' +/- could mean either 9' or 11'. All I am trying to say is that dimension should not be relied on as for sure number without checking first and slight modification is acceptable. Is there a better way?

 

[Maury]

I've used the +/- symbol to idicate approximate just as you have and haven't had problems with people understanding the meaning. I don't agree at all with your colleague's opinion that 10'+/- could mean either 9' or 11'. Another way would be to use an * or Note# next to the dimension with an explanation somewhere else on the sheet.

 

[Ussuri]

To my mind +/- indicates a tolerance so should be followed with a value of some description. What is wrong with using the word 'approx' in front of a dimension? I have seen that in the past.

 

[jike]

Here are some others ways that I have done or seen.

Use "Approx." before the dimension and "GC to determine" after.

Use "Field Determine" in lieu of the dimension.

The word "verify" seems to be overused and often mis-interpreted so I try to avoid using it.

One word of caution:

If your beam is designed for 10 feet and the GC "field determines" that it needs to be 15 feet, would that concern you? What control do you have preventing them from using the beam designed for 10 feet for a 15 foot span?

 

[whyun]

For dimensions that can vary, I use the worst case assumed in the design. For example, 10'-0" max or 10'-0" min.

For approximate dimensions that are existing, similar to jike, I may say "+/- 10'-0" field verify" or "10'-0" approx. field verify". Somewhere in the notes should say contact architect/engineer when discrepancies are found.

I also do not agree with your colleague. If the dimension says 10' +/- 1', it is true but +/-10' is read as "10 feet more or less"

 

[crossframe]

FSS,

I think "technically" your colleague is correct. When no tolerance is given, it becomes the value of the last digit.(10'+/- could mean anywhere from 9' to 11', and 10'-0"+/- could mean 9'-11" to 10'-1")

However in the "real" world, I think that everyone will understand you mean "about 10ft" and will take it in context. If you're talking about a beam length, it'll be 10', give or take. If you say the depth of a swimming pool is 10'+/-, I think everyone would understand 9' to 11' is an acceptable range.

 

[FSS]

Crossframe, I have been using same logic. It is indeed because he was "technically" correct that I posted the question. Did not want what I thought was common sense to get all twisted when someone else interprets my drawings.

 

[pba]

We put an Asterisk by the dimension - * and a note * = Contractor to confirm dimension. (or some other form of words). We use +/- to define tolerances where these differ from the general tolerances for the project which we defne in the Specification.

To my mind, the important thing is not the method, but to ensure that it cannot be misinterpreted (either willfully of unwittingly!). Clearly +/- 10 could be construed as 9 to 11 and this is clearly not your intention. A note defining what +/- means is required if you continue to use what seems to be to be quite a reasonable system.