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Do you know the difference between A706 and A615 bars?

Dec. 16, 2024

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.

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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.

ASTM A706 Grade 80 Reinforcement

SUBJECT: ASTM A706 Grade 80 Reinforcement

This design memorandum defines WSDOT policy for the use of ASTM A706 Grade 80 reinforcement. ASTM A706 Grade 60 reinforcement remains the preferred reinforcing steel type for WSDOT bridges and structures.

Reinforcement conforming to ASTM A706 Grade 80 may be used in Seismic Design Category (SDC) A for all components. For SDCs B, C and D, ASTM A706 Grade 80 reinforcing steel shall not be used for elements and connections that are proportioned and detailed to ensure the development of significant inelastic deformations for which moment curvature analysis is required to determine the plastic moment capacity of ductile concrete members and expected nominal moment capacity of capacity protected members.

ASTM A706 Grade 80 reinforcing steel may be used for capacity-protected members such as footings, bent caps, oversized shafts, joints, and integral superstructure elements that are adjacent to the plastic hinge locations if the expected nominal moment capacity is determined by strength design based on the expected concrete compressive strength with a maximum usable strain of 0.003 and a reinforcing steel yield strength of 80 ksi with a maximum usable strain of 0.090 for #10 bars and smaller, 0.060 for #11 bars and larger. The resistance factors for seismic related calculations shall be taken as 0.90 for shear and 1.0 for bending.

ASTM A706 Grade 80 reinforcing steel shall not be used for oversized shafts where in-ground plastic hinging is considered as a part of the Earthquake-Resisting System (ERS).

ASTM A706 Grade 80 reinforcing steel shall not be used for transverse reinforcement in members resisting torsion.

For seismic hooks, fy shall not be taken greater than 75 ksi.

Modifications to Resistance Factors for Conventional Construction (AASHTO LRFD Bridge Design Specifications 5.5.4.2.1):

For sections in which the net tensile strain in the extreme tension steel at nominal resistance is between the limits for compression-controlled and tension-controlled sections, f may be linearly increased from 0.75 to that for tension-controlled sections as the net tensile strain in the extreme tension steel increases from the compression controlled strain limit, ecl, to the tension-controlled strain limit, etl.

This variation f may be computed for prestressed members such that:

Contact us to discuss your requirements of a706 rebar. Our experienced sales team can help you identify the options that best suit your needs.

and for nonprestressed members such that:

where:

et = net tensile strain in the extreme tension steel at nominal resistance

ecl = compression-controlled strain limit in the extreme tension steel (in./in.)

etl = tension-controlled strain limit in the extreme tension steel (in./in.)

For sections subjected to axial load with flexure, factored resistances are determined by multiplying both Pn and Mn by the appropriate single value of f. Compression-controlled and tension-controlled sections are defined as those that have net tensile strain in the extreme tension steel at nominal strength less than or equal to the compression-controlled strain limit, and equal to or greater than the tension-controlled strain limit, respectively. For sections with net tensile strain εt in the extreme tension steel at nominal strength between the above limits, the value of f may be determined by linear interpolation, as shown in Figure 1.

Figure 1&#;Variation of f with Net Tensile Strain εt

Modifications to General Assumptions for Strength and Extreme Event Limit States (AASHTO LRFD Bridge Design Specifications 5.7.2.1):

Sections are compression-controlled when the net tensile strain in the extreme tension steel is equal to or less than the compression-controlled strain limit, ecl, at the time the concrete in compression reaches its assumed strain limit of 0.003. The compression-controlled strain limit is the net tensile strain in the reinforcement at balanced strain conditions. For Grade 60 reinforcement, and for all prestressed reinforcement, the compression-controlled strain limit may be set equal to ecl = 0.002. For nonprestressed reinforcing steel with a specified minimum yield strength of 80.0 ksi, the compression-controlled strain limit may be taken as ecl = 0.003. For nonprestressed reinforcing steel with a specified minimum yield strength between 60.0 and 80.0 ksi, the compression controlled strain limit may be determined by linear interpolation based on specified minimum yield strength.

Sections are tension-controlled when the net tensile strain in the extreme tension steel is equal to or greater than the tension-controlled strain limit, etl, just as the concrete in compression reaches its assumed strain limit of 0.003. Sections with net tensile strain in the extreme tension steel between the compression-controlled strain limit and the tension-controlled strain limit constitute a transition region between compression-controlled and tension-controlled sections. The tension-controlled strain limit, etl, shall be taken as 0. for nonprestressed reinforcing steel with a specified minimum yield strength, fy = 80.0 ksi.

In the approximate flexural resistance equations fy and f &#;y may replace fs and f &#;s, respectively, subject to the following conditions:

o fy may replace fs when, using fy in the calculation, the resulting ratio c/ds does not exceed:

where:

c = distance from the extreme compression fiber to the neutral axis (in.)

ds = distance from extreme compression fiber to the centroid of the nonprestressed tensile reinforcement (in.)

ecl = compression-controlled strain limit as defined above.

If c/d exceeds this limit, strain compatibility shall be used to determine the stress in the mild steel tension reinforcement.

o f &#;y may replace f &#;s when, using f &#;y in the calculation, if c &#; 3d&#;s, and fy &#; 60.0 ksi. If c < 3d&#;s, or fy > 60.0 ksi, strain compatibility shall be used to determine the stress in the mild steel compression reinforcement. Alternatively, the compression reinforcement may be conservatively ignored, i.e., A&#;s = 0.

When using strain compatibility, the calculated stress in the nonprestressed reinforcing steel may not be taken as greater than the specified minimum yield strength.

When using the approximate flexural resistance equations it is important to assure that both the tension and compression mild steel reinforcement are yielding to obtain accurate results. The current limit on c/ds assures that the mild tension steel will be at or near yield. The ratio c &#; 3d&#;s assures that mild compression steel with fy&#; 60.0 ksi will yield. For yield strengths above 60.0 ksi, the yield strain is close to or exceeds 0.003, so the compression steel may not yield. It is conservative to ignore the compression steel when calculating flexural resistance. In cases where either the tension or compression steel does not yield, it is more accurate to use a method based on the conditions of equilibrium and strain compatibility to determine the flexural resistance.

Values of the compression- and tension-controlled strain limits are given in Table 1 for common values of specified minimum yield strengths.

Specified Minimum Yield Strength, ksi

Compression Control, ecl

Tension Control, etl

0.002

0.005

Modifications to Development of Reinforcement (AASHTO LRFD Bridge Design Specifications 5.11.2):

Development lengths shall be calculated using the specified minimum yield strength of the reinforcing steel. Reinforcing steel with a specified minimum yield strength up to 80 ksi is permitted.

For straight bars having a specified minimum yield strength greater than 75 ksi, transverse reinforcement satisfying the requirements of AASHTO LRFD Bridge Design Specifications 5.8.2.5 for beams and 5.10.6.3 for columns shall be provided over the required development length. Confining reinforcement is not required for slabs or decks.

For hooks in reinforcing bars having a specified minimum yield strength greater than 60 ksi, ties satisfying the requirements of AASHTO LRFD Bridge Design Specifications 5.11.2.4.3 shall be provided. For hooks not located at the discontinuous end of a member, the modification factors of AASHTO LRFD Bridge Design Specifications 5.11.2.4.2 may be applied.

Modifications to Splices of Bar Reinforcement (AASHTO LRFD Bridge Design Specifications 5.11.5):

For lap spliced bars having a specified minimum yield strength greater than 75 ksi, transverse reinforcement satisfying the requirements of AASHTO LRFD Bridge Design Specifications 5.8.2.5 for beams and 5.10.6.3 for columns shall be provided over the required splice length. Confining reinforcement is not required for slabs or decks.

Background:

ASTM A706 Grade 80 reinforcing steel is commercially available in the United States. The use of steel with this higher capacity could provide benefits to concrete bridge construction by reducing member cross sections and reinforcement quantities, leading to savings in material, shipping, and placement costs. Reducing reinforcement quantities will also reduce congestion problems leading to better quality of construction. The use of high-strength steel reinforcement with high-strength concrete should result in more efficient use of both materials.

ASTM A706 Grade 60 reinforcement remains the preferred reinforcement type for all WSDOT bridges and structures.

Further research is required to establish the shape and model of the stress-strain curve, expected reinforcing strengths and strain limits for concrete components made with ASTM A706 Grade 80 reinforcing steel.

If you have any questions regarding these issues, please contact Bijan Khaleghi at 360-705-.

cc: Mark Gaines, Bridge Construction -

F. Posner, Bridge and Structures &#;

Note: Click here for a PDF of this design memo.

For more information, please visit astm a615 rebar.