huangyhg

dimension tolerance
i am reviewing a drawing which has dimensions on plan which are to the 1/8". my question is how reasonable is this to assume? especially due to the fact it is a foundation plan. i would like to round it to the nearest inch and call it good. any thoughts are welcome
aci 117 covers tolerances. typically, a variation of plus or minus 1/4" is considered acceptable. here is a link for more info:
construction usually follows the adage that we tend to measure with a micrometer, mark it with a crayon, and cut it with an axe.
agree with sra....dimensioning and tolerances are different. i see no point in dimensioning to 1/8-inch for anything (except floor flatness) in cast-in-place concrete, since you can't reasonably be expected to repeatedly achieve it.
i disagree: as a steel erector and pipe fabricator field engineer, the concrete/steel/foundation/floor plan/structural plans better d*mn well not be "rounded off" to any "closest inch".
you put down what is required. +/- 1/16 if needed, +/- 1/4 if needed - or whatever is required for your design.
the erector will layout the concrete forms and rebar per dwg notes and general guidelines and your explicit tolerances.
the anchor bolts will get laid out around the column lines as close as possible: i use a scratch awl, lasers, and tape measures - or carpenter squares and plumb bobs - to mark centerlines to avoid possible errors about what "side" of a pencil mark or magic marker is correct. we use layout blueing and scratch marks on tanks and steel and piping when needed to mark locations. again - to be as accurate to the dwg as possible, don't ever deliberately introduce errors and mis-matched parts just for the "rounding up" on drawings.
when very close tolerances are not required for pipes or eqpt connections or drilled points, we use magic markers and chalk marks and string lines.
no.
do not ever "round off" on your srawings. start as accurately as needed, then let the accumulating errors (drill bits sliding, errors, marking errors, alsight angle or perpendicularity erros "force" changes from the true points. (allow margins for those accumulating errors (laarger bolts holes in column baseplates for example, at least one field cut pipe weld in the assembly, 1/8 gaps in the erected steel beams and girders)
i understand that 1/8" or 1/16" dimensions look goofy on plans and contractors or other engineers might see them as incorrect or unreasonable. but the 1/8" mark on a tape measure is a random line in space just like the 1" mark. so, i agree with racookpe1978 in that let the errors come as they come. and they will come in the form of tolerances.
i personally try to avoid 1/8" or 1/16" dimensions by making them not need to be that way, but i wouldn't round them for the sake of an easy number. if the architect wants a column in a certain place, sometimes that number comes out to the sixteenth of an inch, so, it is what it is.
right - and review your drawings (er, submissions to the forum) for typo's and goofs to avoid the first level (or primary) errors as well. like my errors (those typo's that i failed to check and correct) on the previous entries, it is up to you to begin with an accurate drawing.
then allow prudent tolerance margins be assigned to facilitate economical and on-schedule construction.
to avoid having 1/16" and 1/8" all over your drawings, why not dimension 6'-8 3/16" as 2037?
as a concrete contractor you are often faced with such tolerances. anchor bolts, embedments for precast structures are usually the dimensions that need to be strictly adhered to. 1/8" is possible with concrete, 1/16" is not really possible, with regards to horizontal layout.
what i have found is that 3/8" is allowable for most anchor bolts (3/4" bolts that are generally for erection purposes only).
to reiterate racookpe, it all depends what follows...if you are building a wall that has to fit a precast panel, than it is important to maintain 1/8 of an inch. asthetically you would joints to be the same with/thickness.
most recently i have been involved in the construction of a printing press, tolerances were to be within 1/16 due to equipment requirements.

quote (hokie66):
to avoid having 1/16" and 1/8" all over your drawings, why not dimension 6'-8 3/16" as 2037?
sure, and introduce almost a quarter of a millimeter error into the system??!!

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