Mixing A706 and A615 Rebar

Author: Morgan

Oct. 07, 2024

Mixing A706 and A615 Rebar

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Mixing A706 and A615 Rebar

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Mixing A706 and A615 Rebar

tolchijb

(Structural)

(OP)

23 Dec 13 11:33

thread507-: A706 rebar required for seismic?

Is it standard practice in high seismic regions to specify A706 rebar only or do you only specify A706 at ductile elements and use A615 everywhere else. If you do mix rebar, how is it delineated as different in the field other than standard bar markings that are not going to scream that this rebar is different?

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RE: Mixing A706 and A615 Rebar

TXStructural

(Structural)

23 Dec 13 14:25

There is no need to mix. The price difference, if you can actually get A615, is inconsequential. Much of the bar in the western states is dual-spec (A615/A706), so you will likely get higher ductility without specifying it.

We do not encourage use of A706 elsewhere because of the limited availability of certain metals, such as vanadium, used in chemical grain refinement (the usual means of achieving greater ductility in rebar and structural steel.)

The other aspect of A706 which is important to seismic application is a cap on the upper bound of yield strength. Right now, A706 limits the strength to 18 ksi over the nominal value.

RE: Mixing A706 and A615 Rebar

tolchijb

(Structural)

(OP)

23 Dec 13 14:39

The project I am working on is in South Carolina so I am not sure if dual grade is the norm there. I guess I should make a call and ask the question.

I want to verify to make sure I understand your post. You would specify A615 with the specific requirements (actual yield no more than 18,000 psi above specified fy, and ratio of actual tensile strength to actual yield strength not less than 1.25) vs. specifying A706. You would do this for all bar, not just at ductile elements. Is this correct?

RE: Mixing A706 and A615 Rebar

TXStructural

(Structural)

23 Dec 13 16:24

No, specify A706 for the project if the material is available (I have a call in to find out for you.) If the contractor needs to sub A615, then you can agree if the material meets the requirements you mention. The hard part is that they would have to know every heat of steel provided for the ductile members meets the requirements.

RE: Mixing A706 and A615 Rebar

TXStructural

(Structural)

23 Dec 13 17:27

A706 is produced in limited quantities in South Carolina. The fabricators may also stock materials from other mills, but I would recommend specifying only those bars in elements that require A706 ductility be shown on drawings using a (D), with an appropriate plan note or general note indicating the (D) designates the use of A706 bar in the appropriate grade.

The supplier or contractor can then request a variance to use A615 with the appropriate properties, in lieu of A706.

RE: Mixing A706 and A615 Rebar

slickdeals

(Structural)

24 Dec 13 11:44

I would be wary of specifying the same bar size in A615 and A706, regardless of how bulletproof your drawings and notes are. Remember, if there a 50-50 chance that a contractor can screw it up, it's likely to happen 9 times out of 10. If the element is cast with the wrong grade of steel, you have no choice but to tear it down.

RE: Mixing A706 and A615 Rebar

TDIengineer

(Structural)

27 Dec 13 17:33

On a project I did, we did not specify A706 for jamb bars in a masonry wall as I usually only do this in concrete shear walls per the ACI code section. However, the state field engineer insisted. There was a change order cost by the contractor to go with A706 over A615 in California. it wasn't huge, but there was a cost difference.

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News

Do you know the difference between A706 and A615 bars?

Good morning! This is Back of the Envelope &#; the structural engineering newsletter that helps you become smarter each week.

Recently, a coworker shared a CRSI (Concrete Reinforcing Steel Institute) document called &#;Frequently asked questions about reinforcing bars.&#; (link)

Lots of really good info in there. My favorite one is about A706 bars vs. A615 bars.

We hear it all the time (and see it on the drawings) that for welded rebar and seismic applications, use A706, not A615&#;, but why?

The CRSI article explains it pretty well (it&#;s under the section &#;What is the availability and application of ASTM A706 reinforcing bars?&#;)

I&#;ll give you a CliffsNotes version below.

(Estimated reading time: 2 minutes and 18 seconds.)

Tightly Controlled Chemistry

First, the major difference between the two ASTM standards is the chemical make-up of the bars.

For A706 bars, chemicals are more tightly controlled to enhance weldability. There are maximum limits on carbon, manganese, phosphorus, sulfur, and silicon.

A615, on the other hand, only has a limit on phosphorous.

This is hugely important, which I&#;ll explain next.

Carbon Equivalent (C.E.) & Preheat Temperatures

AWS (American Welding Society) has an equation for calculating this thing called the &#;carbon equivalent&#; (aka C.E.). It is based on the chemical analysis of seven elements:

For A706, the requirement is that the C.E. must be 0.55% or less. (I.e., if you specify A706, you can be sure that the bars you get will have C.E. <= 0.55%)

And since C.E. is linked to the &#;minimum preheat temperature required for welding,&#; A706 bars can be welded at much cooler temperatures. (For example, based on the AWS Table below, #3 to #11 A706 bars can be welded at 50 degrees or less.)

So basically, no preheat for A706 in most common cases.

A615, on the other hand, has no C.E. requirements &#; meaning, the C.E. of any given bar is pretty much unknown.

Because of that, you have to assume the worst case, which could be above 0.75%. So if you want to weld it, the bar must be preheated to 300 to 500 degrees &#;.

That explains the weldability. How about seismic applications?

Strength Limit

For A706, there is a limit that the yield strength can be no more than 18,000 psi above the minimum specified yield strength. In other words, if you specify Grade 60 steel, the highest yield strength you might get is 78 ksi.

Also, regardless of the specific yield strength, you can be sure that the tensile strength will be at least 1.25 times the actual yield strength.

A615 doesn&#;t have such requirements.

This means that A706 is more suitable for blast-resistant and seismic-resistant applications where yielding needs to be tightly controlled.

Availability & Cost

You might be wondering, with all its benefits, is A706 more expensive and harder to find?

Apparently, it only costs a small premium and is widely available.

This means, you probably wouldn&#;t want to specify A706 everywhere in most cases, but you also don&#;t need to worry about breaking the bank for instances where you need a lot of them.

Markings

Oh one last thing.

When you are at a job site, how do you know if you are looking at an A706 or A615?

By looking at the markings. The letter followed by the bar size identifies the type of steel:

W = A706, and S = A615.

And there you have it. Hopefully this helps you know a little bit more about A706 and A615 than before!

P.S. This is a rehash of an article I wrote in my weekly newsletter, &#;Back of the Envelope" &#; where I teach you SE-related things in 5 minutes (or less), once a week.

If you enjoy reading stuff like this, consider following me on Linkedin or subscribing to the actual newsletter at BackOfTheEnvelope.co.

For more a706 rebarinformation, please contact us. We will provide professional answers.