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 in this email.

(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.

(Source: CRSI)

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

Andy Lin
Back of the Envelope/Structural Engineer HQ