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?

 

[hoshang]

For me it's more important how many bars exist in 1 meter as the required reinforcement is calculated per meter length.

 

[ajk1]

Thanks 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.

 

[Settingsun]

The 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.

 

[ajk1]

To 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 1977, 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.

 

[BridgeSmith]

"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.

 

[jayrod12]

Regarding 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.

 

[BridgeSmith]

Thanks 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.

 

[hokie66]

It 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.