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Any convention for specifying spacing of metric bars in slabs?
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(Structural)
(OP)
14 Feb 19 12:18Is there any convention for specifying the spacing of metric bars in slabs?
For example, should they be specified to the nearest calculated 1 mm, or to 5 mm or to 10 mm?
i.e., if the calculated spacing is 153 mm, should it be specified as 10M@150 or 10M@153 or 10M@155?
(Structural)
14 Feb 19 12:30Every 25mm for me. Generally the guys tying the bars have an imperial tape, so if you say 15m@150 you're actually getting 15m@6"
(Structural)
14 Feb 19 12:33Rounding to 5 mm is acceptable (for example 75mm) but rounding to 10 mm is more usual. in your case I would go with 150 mm.p.s.Generally the guys tying the bars don't have a tape..
(Structural)
14 Feb 19 13:31Most common for me (Scandinavia) is rounded to 50 (100, 150, 200 etc). Every now and then I'll round to 25mm (100, 125, 150, 175 etc), but not often.
(Structural)
(OP)
14 Feb 19 14:00to Jayrod - are you in Canada? I am, although I did not say so in my post. Canada officially went metric in construction in so I am very surprised that imperial tapes are still being used on construction sites in Canada, but it has been a long time since I was on site. Are you sure that they use imperial tapes in Canada, or are you perhaps not in Canada?
(Mechanical)
14 Feb 19 14:12the following is commonly used in Europe:
(Structural)
14 Feb 19 14:13I'm in Winnipeg. Trust me when I say they're typically using imperial tapes. I actually find it rare when I see a true metric tape on site.
When I specify a 250x600 gradebeam the forms are typically 10"x24" because the plywood is in 48" wide sheets, and the pre-manufactured snap ties are for 10" forms.
(Structural)
14 Feb 19 14:55I work in Ontario and can confirm that most contractors still operate in imperial, regardless of whether or not the drawings are metric or imperial. It is common for them to have dual measure tapes. Slowly some of them are starting to use metric. In your example 10M @ 150 would be typical. Generally 50mm increments make sense, however, 25mm increments are common as well.
(Structural)
(OP)
14 Feb 19 15:20Thanks for the info. Very much appreciated.
(Structural)
14 Feb 19 20:01Increments of 25mm, 98% of the time I always use a 150mm spacing unless its for beams when I'd specify the number of bars in a face instead.
(Civil/Environmental)
14 Feb 19 20:24From my experience practicing in Canada -
For private sector jobs, I've spec'd metric bar sizes - 10M, 15M etc, at imperial spacings (12", 16")
For government funded projects, all specs had to be in metric. So all work was in metric.
Tape measures are widely available in imperial, and dual imperial/metric. purely metric tape measures are special order items.
specifying rebar spacing should never be to the nearest millimeter, but to the nearest 10mm. I would never spec a 305mm spacing.
and if i specified a 300mm spacing, I would expect a competent contractor to to acheive spacing of 280-320mm
(Structural)
14 Feb 19 20:56I'll put in a vote for rounding down to the nearest 25mm. If its an odd dimension the expectation is that the actual spacing is less than this. For example design might be 162.5mm spacing, gets specified as 150mm, on site you get 143mm on average based on distance over which the bars are distributed. Most being at 150mm and an extra makeup spacing at the edges perhaps.
Only in specific cases would you goto nearest 5mm, in particular I think the only time I've done this is where the particular detailing warrented it, for example in beam column joints for the close spacing of stirrups to fit them in.
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(Structural)
14 Feb 19 21:37I'd usually go to 50mm. 25mm if I've got a good reason. Anything less than that and I'd look at the drawings weirdly. I would look really closely if someone specified bars at, like, 285mm.
(Structural)
14 Feb 19 22:30In Australia, the spacing is usually given in increments of 10 mm. 160, 170, etc. As to box tapes, the curious thing I have found is that while the tape is metric only, the length of the box is often given in inches. I have a Stanley one, made in Thailand, metric only, and the length of the box is 3 1/8". My Dewalt tape, made in the USA, also metric only, has a box length of 3 3/8". Makes it useless for interior measurements. They must use the same box whether metric, imperial, or dual.
(Structural)
14 Feb 19 22:40Specifying bars in a slab at 5mm increments would give the impression you've just copied out your first single span one way slab worked example from the textbook.I would go with 50mm increments, and only drop down to 25mm in very rare cases.Make it easy for the guys on site and that way you should get less mistakes.
(Structural)
14 Feb 19 23:41In a flat slab or flat plate, another common practice, which I agree with, is to specify the number of bars in each column strip or middle strip. That way, they are just spread over the strip equally, and the inspector just has to count, not measure. In a one way slab, giving the spacing is better.
(Structural)
(OP)
17 Feb 19 03:02
I meant my question to apply to slabs and walls, though I unfortunately failed to say so.
There seems to be no general agreement, so perhaps it does not matter if we specify 125 mm as a spacing or 120 mm as a spacing, if the calculated spacing is say 123 mm.
On another point, I am puzzled why people write the metric number with no space between the digit and the metric unit. For example, why do people write 50mm rather than the correct format of 50 mm with a space? No one would omit the space if it were imperial units, such as 2inches or 2in. , they would write 2 inches or 2 in. In the published Standards, there is always a space, as far as I know. Perhaps they are trying to avoid having the digit being separated from the unit if it occurs at the end of a line, but the way to do that is to put in a "hard" space, which can be done easily in Microsoft Word although I don't know if a hard space can be used in CAD or Bluebeam.
(Structural)
17 Feb 19 03:35We probably don't get it right in text, because on drawings, calculations, etc. we don't include the mm. It is just 50, 300, , etc.
(Structural)
17 Feb 19 04:16Don't know why but I have always placed the unit immediately after the number without a space, for everything, stress, forces, distances etc.
If the calculated result was 123, I would round 120. Rounding down to the nearest 5 is ok (about 1/16in) but 10mm is probably more common. Rounding to nearest 25 would be a throwback from imperial inch days.
never understood the logic of rounding up!
(Structural)
17 Feb 19 04:23From US NIST on SI Unit rules and style conventions: Link
(Civil/Environmental)
17 Feb 19 21:22For me it's more important how many bars exist in 1 meter as the required reinforcement is calculated per meter length.
(Structural)
(OP)
17 Feb 19 22:33Thanks Ingenuity - your attachment is but one more verification that proper format is to inlcude a space between the digit and the unit of measurement. i.e. 100 mm, not 100mm.
(Structural)
18 Feb 19 00:13The space between number and unit depends on which style guide you're following but I think using the space is far more common. I tend not to use a space if non-breaking space unavailable such as this firm.
Also more common IME not to use a dot after an abbreviation so 2 in. is replaced by 2 in in modern use. That particular case has the issue that the abbreviation is a common word though. 2" might be the go. Inches are old so using the dot matches - in for a penny, in for a pound.
As to the initial question, I'd round reinf spacing similar to what you do in inches in the same situation. If you specify to half-inch, go wit 10mm. If 2 inches, go with 50mm increments.
(Structural)
(OP)
6 Mar 19 20:36To me leaving out the space with metric units is wrong and irrational, since I have never seen that done with imperial units. In all the CSA Standards that I have looked at, the space is included. That is also what CSA told me when I put this question to them. I suspect that it is written with the space in American Standards publications (such as ACI 318) as well, but I don't have any at my fingertips here at home to check. But I realize it is what it is despite my feelings about it.
Regarding metric tape measure availability/use in our area (southern Ontario) our chief field engineer says that metric tapes and dual metric/imperial tapes are readily available and routinely used on construction sites. I would be astonished if that were not the case, since we have been officially metric in construction jobs since , and the vast vast majority of out projects have been hard metric drawings for a very long time.
I am not sure though if bar chairs are in metric sizes...I sure hope they are.
(Structural)
6 Mar 19 20:52"I am not sure though if bar chairs are in metric sizes...I sure hope they are."
If you think the placement of reinforcing steel is accurate enough that it matters, you're dreaming.
(Structural)
6 Mar 19 21:36Regarding rebar chairs, and granted this was 15 years ago, there was a conversation had with the local rebar suppliers around here asking how they determined the chair heights based on the parameters we provide on our drawings for top slab reinforcing only. For bottom reinforcing they generally have the proper chairs available to give the heights specified.
1) The rebar guys consider the clear cover specifications on our drawings as a minimum, not a set number.
2) When determining required chair height, they use nominal bar diameter including deformations, even though theoretically you could have the valleys line up. For example a 15m bar is actually 18mm nominally with deformations, therefore in their calculations they assume the double mat of top steel thickness to be 36mm (18x2) however if the valleys lined up it would be at most 30mm, therefore moving your reinforcing further from the top surface by 6mm.
3) When the detailer is calculating the chair height they round down to the nearest 1/2", moving the steel further down from the top surface again. I do not know if metric chairs are manufactured or if they just use imperial ones.
So what does this all mean: Let's assume a 150 thick slab with 20mm cover and 2 layers of 15m bars.
If you design to the absolute tightest tolerances your effective depth to the primary top reinforcement is 122.5mm (150-20-15/2).
Using the guidance for the detailer's calculation the worst case underside of reinforcing dimension is 94mm (150-20-18*2)and since they do not make a 94mm chair they would round down to the nearest 1/2" increment or 89mm.
Now let's say that you have the valleys line up, and this chair discrepancy, then your actual effective depth to the primary top reinforcement is 111.5mm (89+15+15/2) or a reduction in effective depth of 9%.
This is a worst case scenario type thing, but none the less something to keep in mind. Also as I indicated this conversation with the rebar detailer's happened 15 years ago (but there's a quite clear memo that floats around our office regarding this).
I generally then tend to take off 5-6mm from my effective depth when designing slabs (or alternatively do not take them past 90% capacity).
Take from this what you will.
(Structural)
6 Mar 19 21:47Thanks for the additional info, jayrod. I didn't know the specifics, but I knew from various design guides, etc. and from being in the field watching rebar placement, that it's not wise to count on rebar ending up exactly where you designed it to be - in either direction.
(Structural)
6 Mar 19 21:51It does matter what size bar chair is used. In Australia, bar chairs are typically available for 20, 25, and 30 cover bottom, then in 10 mm increments up to 300 for top bars. Something special after that.
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