On Center Spacing Definition

[Civilling]

I have elementary questions about rebar on center spacing specified on drawings. If I have a new 10' x 2.5' x 1' height slab and have determined I need a 13#4 longitudinal bars and 4#4 transverse bars per temperature and shrinkage, how do I specify this on drawings? Would I specify #4 at 9" on center, each way, 3" clear typical? Does the contractor read that as 9" maximum on center spacing or exact spacing? i.e. they measure the 2.5' direction, they would lay 4#4's because the max spacing is 9", vs could they potentially lay 3#4s exactly at 9" on center with more clear cover?

 

[jayrod12]

You could just put, 4 #4 transverse evenly spaced or something along those lines. That's what I do in my footings.

 

[P205]

I would either state the total number (with no spacing, leave it to them to space evenly) or I would state spacing as maximum --> #4 @ 9" o.c. MAX.

 

[dhengr]

Civilling:
You might also try to straighten out your longitudinal and transverse bars terminology, you can hardly get 13 - #4 longitudinal bars in a 2' width. But, 13 - #4's x 1'-11", t&b (or bot. only, etc.) and 4 - #4's x 9'-5" bot., evenly spaced can hardly be misplaced, or mis-fabed. You can tabulate these for various odd shaped ftgs. on the given job, and you should have a couple typ. details showing various clear (clr.) dimensions, general ftg. notes, etc. Which layer of rebar do you want as the layer with the most favorable “d” value? Ftg. dimensions are sometimes described as length, width and thickness, along with top of ftg. elev.

 

[Civilling]

Thanks.

I typically will specify the number or bars (i.e. 4#4) since I am unsure how the OC spacing will be interpreted, and our pads are typically small. However, with something larger, say 10', I was advised by a structural engineer that contractors like OC spacing specified so they can measure out the distance and lay a bar. Since the spacing almost never works out evenly, I was wondering how the contractor usually interprets it.

 

[BSVBD]

4 #4 Long Bar ("Long"itudinal are typically your "Long" bar)
13 #4 Short Bar (Transverse are typically your short bar)

Within recent years, when I determine it necessary, I've been specifying "Long Bar" and "Short Bar" instead of the transverse. I've had more than one phone call asking what "transverse" is, what's the difference, etc.

4 #4 Longitudinal Bar
13 #4 Transverse Bar

4 #4 Longitudinal Bar at 8" o.c. (If you lay 4 rebar in a 30" width at 9" o.c. you will end up with only 1.5" clear each side.)
13 #4 Transverse Bar at 9.5" o.c.

You should have General Notes or a "Typical Rebar Layout" diagram or other which specifies required spacing, even spacing, specific side and bottom clear covers, etc...

For Example:
For Isolated Square Footings, I typically specify 5 #5 Bar E.W. (Each Way)
For Rectangular Footings, I specify 5 #6 Long Bar and 12 #5 Short Bar
For Continuous Wall Footings, I specify 4 #5 Longitudinal Bar and #4 Trans Bar at 12" o.c.

 

[Settingsun]

I was wondering how the contractor usually interprets it.

That will vary from contractor to contractor, from person to person, and from circumstance to circumstance. But generally in the way that makes the most profit for the contractor. Assume the worst.

So, you definitely need to specify the 4 bars since the possible outcome of only three bars is a significant loss of strength. 12 vs 13 is a bit less critical.

This document (link below) is from an Australian state road agency which really loves its drafting standards. Look at figure 6f on page 11 (17th page of the PDF file) for the 'proper' way to handle this situation. (Although I've never seen the first 'preferred' option actually used, only ever the 'approx' option.) We specify bar centres in 5mm increments = 1/5 inch.

http://www.rms.nsw.gov.au/business-...iers/design-documents/825701x/sdadmsect23.pdf

(Alternative link: section 23)

http://www.rms.nsw.gov.au/business-...es-manuals/structural-drafting-detailing.html

 

[P205]

I agree with the general sentiment that specifying the total number is the safest bet. It also helps for reviewing shop drawings that you can compare number for number, instead of having to do the math yourself during the review process.

 

[Civilling]

Thanks everyone. That was very helpful. The link was helpful too.

 

[Civilling]

I have another related question regarding callouts. If a mat footing leader says 24 #4 bars EW, T&B, does that typically mean there are 24 total bars (6 bars bottom, 6 bars top, each way), or does that typically mean 24 bars bottom, 24 bars top, each way totaling 96 bars?

I understand this may be determined by looking at the mat footing dimensions, but I want to standardize this with my drawings and want to be consistent with what others do/expect.

 

[jayrod12]

In that situation I would read it as 24 bars each way top and bottom totalling 96 bars.

 

[Nor Cal SE]

I agree with Jayrod12's thought on the most recent question from 6/21/17. But to avoid confusion, if it's my drawing, I simply write it as "24 #4 bars EW, T&B (96 total)". The parenthetical addition removes all doubt.