Finish floor elevation?? 5

[Schambach]

Not sure if this is the right venue to ask this question, but my exposure to other designs, methodologies, is quite limited and so I'm looking at this broad group to answer a simple question. If using datums to denote elevations of finish floor and framing do you call out finish floor as:

1.Dimension above sea level?
2. 0'-0"?
3. 100'-0"?
4. Other?

Thanks.

 

[JedClampett]

We always try to use the local datum elevation (usually above sea level) or a portion of it (4622 becomes 22). That way civil and structural are speaking the same language.

 

[Schambach]

Thanks JedClampett. I work in an industry where we don't know what the local datum elevation is and for ease of construction (single or double story wood structures) we have typically called finish floor 100'-0" so that when defining an elevation below finish floor we don't have to go to negative numbers. We've been approached about switching finish floor to 0'-0" and I've been asked to figure out what the rest of the industry does. Calling finish floor anything but 0'-0" or 100'-0" would be really confusing in our industry. What do others do who operate on the zero or 100 principle?

Thanks.

 

[jayrod12]

Generally we have main floor at 100'-0". Someone (Civil/Arch) then relates that to a geodetic elevation above sea level. My main drawback from that would be avoided if we'd do what JedClampett does, we could avoid coordination issues with grading elevations.

 

[SlideRuleEra]

Schambach - Using 0'-0" has an unintended consequense. If there is need to specify elevations below finish floor, the numbers go negative. Suddenly the plus or minus sign in front of a number becomes critical. All too easy for, say, - 1' to be copied or misread and + 1'. Not so much by using 99' to mean one foot below 100'.

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[cvg]

"civil engineer" piping in...

the term "sea level" is rarely ever used to define a datum and elevations are not measured to a nearby sea level. NGVD was established in 1929 and "loosely" based on a mean sea level.

https://en.wikipedia.org/wiki/Sea_Level_Datum_of_1929

It was superseded in 1988 by NAVD, which again only approximates sea level. Vertical datums in the US should refer to NAVD since "mean sea level" is not only undefined, but ever changing.

https://en.wikipedia.org/wiki/North_American_Vertical_Datum_of_1988

Also note that you can and should always establish a vertical bench mark on every project, even if you have to assume an elevation. Otherwise, you really have no control on the building elevation or site grading. It is really not that difficult since you can generally find an elevation point in the nearest paved street. And unless your work is on the coast, you rarely will have elevations below zero.

 

[JAE]

We also do what jarod12 stated - use 100'-0" on all Arch/Struct/Mech/Elec/Plumbing drawings.
Then typically on the Civil/site plan sheets there is a Fin. Floor Elevation xxxx.xx' = 100'-0" which equates the site elevation with the "working" elevation of 100.


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[Schambach]

Thanks all! Anybody see much 0'-0" out there or do you feel the majority of drawings (if not using NAVD) are 100'-0"? Appreciate the help!

 

[jayrod12]

I see the odd preliminary drawing at 0'-0". But after the first conversation with the architect, we get that changed to 100'-0" for exactly the reason SRE indicated.

 

[TehMightyEngineer]

A few years back we had a drawing where the architect insisted on using 0'-0" on their drawings (including some negative numbers for lower elevations). Despite our protest we were forced to match their elevations on our structural drawings. Thankfully no issues came of it but the contractor also noted the potential for confusion and would have preferred finished floor be set at 100'.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

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[Hokie93]

I have used datum elevation = [0'-0"] for 25 years and not had the first problem and, based on seeing competitor's drawings from time-to-time, the use of 0'-0" seems common. Perhaps the datum choice is somewhat tied to the area/region of practice. I have seen some architects use the actual (NAVD 88) elevations all the way up the building and I thought that was crazy and/or putting a lot of faith in the finished first floor elevation not needing to be revised part way through the project.

 

[wannabeSE]

We almost always use 100'-0" at the ground level for the reasons SRE mentioned. I don't like using civil elevations for a couple reasons. First, civil uses decimal feet dimensions (817.17') while we use ft-in dimensions (817'-2"). Call me a whiner, but I don't like looking at civil finish floor elevations: Basement @ 798.67', 1st @ 817.17', 2nd @ 835.17', 3rd @ 853.17', 4th @ 869.50', 5th @ 885.84' . . . Second, I have been on projects where the grading plans change at the last minute when everyone is rushing to finish. It is nice not having to go back and coordinate all of the elevations every time the grading changes.

I work on a lot of existing buildings. On the record drawings, it's probably an even split between 100' at the ground level and civil elevations. Once in a while I will see 0' at the ground. And there is another terrible option. Once or twice I have seen the structural drawings say "see architectural" (SAD) for all of the floor elevations. I was taught if a dimension has structural significance, it needs to be on the structural drawings.

 

[acwooten]

As you have probably realized by reading everyone else's responses, there is not much of an industry standard and you will likely see any of the ones you listed.

I currently work with municipalities and we almost always use the local datum values, though I'm sure there are companies in our industry that do it differently at times. The past companies I've worked at have used 0'-0", 100'-0", local datums, and more.

All that to say, it can change depending on the company you work for, the client you are serving, the EOR, etc. You should talk to all of these respective parties before making that decision.

 

[wikidcool]

I agree that the 100' elevation reference avoids the +/- confusion and is probably a safer option. That said, I agree with Hokie93. Most firms I have worked for use 0'-0", and I've never once experienced a problem because of it. Most of the negative number references I see are pretty common sense they are below, not above. Top of footing elevations, elevator pits, etc. There's never been much danger of someone trying to build the elevator pit 4ft above the slab.

 

[Okiryu]

I have typically seen 100’. Personally, I like that more since I do geotechnical work and do not want to have negative numbers when we have sites where good amount of fill is needed...

 

[dcarr82775]

100'-0" for datum
note somewhere tying datam to NAVD

 

[cvg]

First, civil uses decimal feet dimensions (817.17') while we use ft-in dimensions (817'-2"). Call me a whiner, but I don't like looking at civil finish floor elevations: Basement @ 798.67', 1st @ 817.17', 2nd @ 835.17', 3rd @ 853.17', 4th @ 869.50', 5th @ 885.84'

well, you had to go there...

they invented the decimal tape measure a long time ago and really, is 12' 6 3/16 inches easier than 12.52 feet?

 

[JAE]

...and really, is 12' 6 3/16 inches easier than 12.52 feet?

Yes.
Next question.



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