development and lap length of rebar - is db nominal or actual diameter

[ajk1]

In CSA A23.3 - 2014, Design of concrete structures, the formula for compression deveopment and lap length of rebar is given in terms of d_b. In the definition of symbols at the front of the Standard, it says d_b = diameter of bar. It does not say whether it is the nominal diameter or the actual diameter. For some bars, such as a 10M, there is a significant difference (10 mm vs. 11.3 mm, etc.). The table for compression development length in the latest CAC Handbook seems to be based on nominal diameters. In the same table in an older edition of the Handbook it seems to be based on actual diameter
Does anyone know where d_b is stated to be the nominal diameter?

 

[Chickenhawk6451]

Regardless of what the book states, there's merit in taking the nominal diameter anyway as it is a smaller value and therefore more conservative for development lengths. AS3600 indicates nominal diameter, ignoring bar ribs

 

[ajk1]

The nominal dia of a 10M bar is 10 mm. The actual diameter is 11.3 mm. The development length is a multiple of the diameter, so it is more conservative to use the actual dia. In other bar sizes, it is more conservative for 25M, 30M, 45M, but less conservative for 20M, 35M and 55M, although the differences are quite small, except for the 10M where it is significant.

 

[ajk1]

The tables in CAC Hndbook for tension and compression development lengths seem to use the nominal diameter. So I suppose that is what should be used.

 

[KootK]

I'm pretty sure that's right but I couldn't find definitive proof in under 5 min. I was hoping that the definition of db would say nominal diameter but it doesn't. There are plenty of other cases of nominal diameter being used for db though where they say it explicitly. I could have sworn that, someplace, the give you the green light to base Ab on nominal db. And tension development length is based on Ab rather than db so, implicitly tension development length could be based on nominal db. Regardless, as you say, the handbook tabulation is pretty good circumstantial evidence, if not quite CSI level proof.

 

[skeletron]

I remember coming across this too while updating some spreadsheets. It seems in most cases (for example Annex D: Anchorage) they are referring to the nominal diameter, so I assume that the nominal is implied. But it begs the question: is the difference really significant? I mean, there's also the tolerance for placement of bars in A23.1 which is >>> the difference between nominal/actual bar diameter. IMHO, I just like to see a bit of gravy on a design one that exceeds the level of significance of nominal vs. actual.

 

[rapt]

Just trying to look at it from a common sense approach, development lengths are based on an average bond stress over that length.

To then convert that to a number * db, it would be logical that db be the actual diameter as it is dependent on the surface area of the bar!

 

[Chickenhawk6451]

Just trying to look at it from a common sense approach, development lengths are based on an average bond stress over that length.

To then convert that to a number * db, it would be logical that db be the actual diameter as it is dependent on the surface area of the bar!

Strictly speaking and in reality, you're right however I guess this boils down to what the governing code requires. Just looking through AS3600, it specifies nominal diameter, excluding ribs etc. This would lead to conservative results with the way the code is written bu American or Canadian codes may differ.

At the end of the day, difference should be minimal so why not just take the conservative number? should result in a minimal amount of extra reo

 

[rapt]

Chickenhawk6451

You are missing the point.
In Canada, they use nominal metric dimensions on imperial bars as akji showed above. In Australia we use metric bar diameters (mostly). They are not talking about outside diameter of the deformations.
For one bar size, the nominal diameter might be higher than the actual while for another it might be lower.

So the logical approach is to use actual bar diameter.

 

[Settingsun]

Deleted a bunch of questions that wikipedia answered. I assumed this was a metric/imperial issue but it's fully metric.

I'd say use 11.3 mm. It's a function of the force in the bar (area) vs a perimeter. The ratio would be underestimated by using 10 mm.

 

[canwesteng]

I'll give you the definitive proof in under 5 mins. Clause 2.4.3, at least in the 2004 code at my desk

 

[BridgeSmith]

I would say it depends on what was used in calibrating the equations for the development length and lap length. The AASHTO bridge design spec defines db as nominal diameter.

Honestly, the laps and development lengths are fairly approximate anyway. From strength and shear capacity, to bond strength, etc. concrete properties are highly variable, as are field conditions.

The only time I would consider the actual vs. nominal diameter is where you have 'soft conversions', where you're using imperial bars in a metric design, or vice versa. In that case, I might convert the nominal diameter of actual bars for calculation (use 12.7mm for a #13 metric bar if #4 imperial bars will be used in construction), but given the aforementioned variability, I probably wouldn't bother even in that case.

Edit: One additional note: Also, per ASTM A615, the actual and nominal diameters are almost identical, based on the perimeter and weight listed, so the actual surface area will be very close to that calculated for the nominal diameter. Although, the height of the deformations is what really matters to the necessary development and lap lengths. Bond strength is a minor component of the overall strength of the connection.

 

[JAE]

Not specific to the codes being referenced in this question, but ACI 318 uses db - defined as nominal diameter - in all their development length sections. No mention of actual diameters.

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[ajk1]

Thanks everyone.
Just one point: Canada has been using hard metric bars since at least 1977. There are no imperial bars in general use in Canada. This is very different than in the U.S. where I believe that they do NOT use hard metric bars today; they use soft metric--in other words, thy use imperial bars, but measure them in metric and call it metric.

The background is that there was an understanding between Canada and the U.S. engineering organizations back in the early 1970's that the U.S. would go hard metric with the same bar sizes as Canada, and Canada ran the proposed Canadian bar sizes past various American authorities and they were ok with it. But when it came time for the U.S. to convert to metric (many years after Canada had done so), the Americans changes their minds, possibly in response to American producers who did not want the expense of changing their production facilities.

It seems illogical to base the bar area on actual dimensions (and therefore the bar force on actual area), but then use nominal bar dimensions in the calculation for development length.

Example: Canadian 10M bar has an area of 100 mm², which is based on an actual diameter of 11.3 mm -- i.e. pi x diameter² / 4 = 3.14 x 11.3² / 4 = 100 mm².

10M bar = 100 mm²
15M bar = 200
20M bar = 300
25M bar = 500
30M bar = 700
35M bar = 1000 mm²

Anyway, you all are right that the differences in bar development lengths are not much. Still, it would have been tidier if the CSA Code had defined that db is the nominal bar diameter.

 

[canwesteng]

The code does define db as nominal diameter. See my post above

 

[skeletron]

This is the clause @canwesteng is referring to:

"2.4.3 Bar diameter for calculations
Except for calculations involving bar areas, the diameter, db, of metric reinforcing bars may be taken as the
bar designation number."

Note, that this appears in A23.3-04 but has been removed (or moved) in A23.3-14

 

[BridgeSmith]

"It seems illogical to base the bar area on actual dimensions (and therefore the bar force on actual area), but then use nominal bar dimensions in the calculation for development length."

Not necessarily. If the equations for development length, based on research and theory, were calibrated for nominal sizes, then nominal sizes are correct to use.

 

[Settingsun]

So, does this calculation 'involve' the bar area? At a fundamental level it does even though it may not appear in the equation. Should they simply have said "except for calculation of bar area..."?

 

[rapt]

ajk1

The nominal dia of a 10M bar is 10 mm. The actual diameter is 11.3 mm.

This looks very much like a soft conversion to me!

steveh49,

It is dependent on surface area which is related to actual diameter.

 

[ChadV]

Note, that this appears in A23.3-04 but has been removed (or moved) in A23.3-14

It's now Clause 3.3.3, no change to the text.