Any convention for specifying spacing of metric bars in slabs?

[ajk1]

Is 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?

 

[jayrod12]

Every 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"

 

[Tommy385]

Rounding 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..

 

[Manitou90]

Most 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.

 

[ajk1]

to Jayrod - are you in Canada? I am, although I did not say so in my post. Canada officially went metric in construction in 1977 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?

 

[kingnero]

the following is commonly used in Europe:

 

[jayrod12]

I'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.

 

[Canuck65]

I 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.

 

[ajk1]

Thanks for the info. Very much appreciated.

 

[ukbridge]

Increments 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.

 

[NorthCivil]

From 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

 

[Agent666]

I'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.

 

[TLHS]

I'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.

 

[hokie66]

In 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.

 

[Trenno]

Specifying 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.

 

[hokie66]

In 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.

 

[ajk1]

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.

 

[hokie66]

We probably don't get it right in text, because on drawings, calculations, etc. we don't include the mm. It is just 50, 300, 18000, etc.

 

[rapt]

Don'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!

 

[Ingenuity]

From US NIST on SI Unit rules and style conventions: Link