ASTM A913 is the standard specification for high-strength (up to Grade 80) low-alloy steel shapes of structural quality, produced by thermo-mechanical rolling followed by a quenching and self-tempering process (QST). The ASTM specification covers Grades 50, 65, 70, and 80 for use in buildings, bridges, or other structures.
STRUCTURE Magazine talked with representatives from two steel suppliers to learn more about how A913 is used in building applications, considerations for its use, and overall capabilities.
STRUCTURE: What makes ASTM A913 different from conventional A992 steel? How is it made?
Dennis Pilarczyk: There are several differences between ASTM A913 and ASTM A992 steel, the most relevant of which to the structural engineer is the increased yield strength properties of ASTM A913 steel (up to 80ksi for A913, whereas A992 is typically 50ksi). This gives the designer the ability to support greater axial loads with less material by favoring A913 over A992.
What gives A913 higher strength, as well as improved weldability attributes, lies within the thermo-mechanical control and quenching and self-tempering (QST) processes. QST consists of in-line heat treatment with controlled cooling. During the QST process, intense water cooling is applied to the whole surface of the wide flange section directly after the last rolling pass. The cooling is interrupted before the core is affected by quenching and the outer layers are tempered by the flow of the heat from the core to the surface as the section’s temperature returns to equilibrium.
STRUCTURE: What is the history of A913 steel?
Joe Dardis: Many of the engineers I talk to don’t realize A913 has been around for over 30 years. It was first introduced to the global market by ArcelorMittal in 1990 and was adopted by ASTM in 1993 under the A913 specification.
The original A913 specification included Grades 65 and 70 but was expanded over the years. Grade 50 was added to the specification in 1995 and Grade 80 was added in 2019.
Code acceptance of A913 grew in the 1990s as it was introduced in the American Welding Society (AWS) D1.1 Structural Welding Code in 1996 and included in the AISC Specification for Structural Steel Buildings in 1999. Combined with growing familiarity of its economic benefits among designers, an increased demand for tall buildings, and greater availability, adoption of A913 grew. 30 plus years later, A913 has been used in hundreds of tall (and not so tall) steel buildings and is becoming the standard material for tall steel buildings worldwide.
STRUCTURE: What are the benefits associated with a higher yield strength?
Pilarczyk: The primary benefit associated with higher yield strength can be found in strength-controlled elements such as axially loaded columns, long-spanning trusses, and short-spanning girders. In these applications, the designer can often realize tonnage reductions from 15-30% when compared to ASTM A992 steel which has a 50ksi yield strength.
While tonnage reductions themselves can have a positive impact on a project from a design efficiency standpoint, there are several other benefits associated with A913. These would include a reduction in embodied carbon due to the elimination of member tonnage, lower crane requirements, smaller supporting and foundational elements, and reduced weld material when conjoining smaller members. These “trickle down” benefits of A913 steel can have a substantial impact on a project by way of reduced labor/material costs as well as schedule efficiencies.
STRUCTURE: How does the alloy content affect the weldability of A913? How does weldability affect economy?
Dardis: Since the thermo-mechanical control and QST processes provide additional grain refinement and strength, A913 is less reliant on conventional alloy-dependent strength enhancement. Subsequently, when compared to A992, A913 has lower carbon and alloy content, providing superior weldability characteristics.
It is important to note the meaning of weldability. Most steels are weldable, but the relative ease in which they can be welded is described as weldability. The better the weldability, the less precautions need to be taken, such as preheating the steel prior to welding.
Due to their superior weldability characteristics, A913 Grade 50 and 65 are prequalified in AWS D1.1 to be welded without preheat across all thickness levels, provided the base metal temperature is above 32 degrees F and the filler metal has a diffusible hydrogen content of 8 ml/100g (H8) or less.
The absence of preheat requirements can significantly reduce fabrication costs, particularly for thicker sections. For instance, A992 steel over 2 ½ inches thick must be preheated to 225 degrees F. This can add hundreds, if not thousands of man-hours on a large project in addition to increased material and energy consumption. Further, preheating can become more complex and costly if it needs to be done in the field.
A913 Grade 70 is also a prequalified base metal in AWS D1.1, albeit with slightly higher preheat requirements than A992 steel. A913 Grade 80 is currently not a prequalified base metal, which means the fabricator must establish their own weld procedure and qualification testing prior to fabricating A913 Grade 80.
STRUCTURE: How does A913 differ from A992 in terms of toughness requirements?
Pilarczyk: The QST process helps to refine the grain structure of A913 steel when compared to A992 steels. Through sophisticated controlled cooling and rolling sequences, grain size is restrained against overgrowth, creating what is called a refined grain structure that improves toughness.
The ASTM A913 specification requires a Charpy V-notch (CVN) impact test meeting a toughness of 40 foot-pounds at 70 degrees F. This CVN requirement is included in the specification and is not a supplementary requirement. On the other hand, CVN testing is always a supplementary requirement for A992.
Additionally, the Supplementary Specification S30 in ASTM A6 can be utilized to get Charpy impact testing at the alternate core location when the flange thickness equals or exceeds 1.5 inches. Lower temperature CVN testing can be achieved, upon request, for applications in extreme environments.
STRUCTURE: In what applications is A913 most beneficial? In what applications is A913 not as beneficial?
Dardis: The most obvious application of A913 is for use in tall buildings. Gravity columns in tall steel buildings often utilize some of the heaviest rolled shapes available.
The use of A913 in tall buildings allows designers to maximize weight savings given the typical large axial column loads and potentially reduce or eliminate the use of built-up sections or reinforced rolled shapes, saving not only on material costs but also fabrication costs. Additionally, the smaller column size inherent to steel buildings is further realized through the use of an even smaller section, allowing the owner to maximize leasable space and potentially increase revenue.
A913 is also regularly used in truss applications. In tall buildings, A913 can be used in outrigger systems or transfer trusses. Similar to gravity columns, members in these applications can have high axial loads and designers can take full advantage of the higher yield strength. Long-span trusses in stadiums, arenas, convention centers etc. can also benefit from a higher yield strength and reduced member size. In these scenarios, a reduction of the dead load of the truss is an additional benefit.
Another common application is in seismic design. The higher yield strength can optimize columns in the “strong column-weak beam” design as well as satisfy the sometimes large column strength demands in seismic braced frames.
Given all these benefits, I am often asked why not just use A913 everywhere? While A913 is a great solution in many applications, it won’t help with stiffness requirements. For example, steel sections used for floor framing and lateral systems are often governed by deflection, vibration, or drift. Increasing yield strength and reducing member size will reduce the stiffness of these systems. It is worth noting, however, that in cases where extensive welding is required, A913 Grade 50 can be used in lieu of A992 for weldability benefits.
STRUCTURE: What are the not so obvious applications that the engineering community should be aware of?
Dardis: A rule of thumb that I always tell designers is if your member is strength-controlled and weighs roughly 100 pounds/foot or more, you can save money by utilizing A913. While you will run into this scenario in almost every tall steel building or long-span truss, there are many other projects where this applies.
Multi-story hospitals, labs and higher education facilities will often have higher dead and live loads, long spans, skyways, and assembly spaces with non reducible live loads, which could all contribute to larger loads on members. Similarly, many industrial facilities will have equipment or storage loads. While the weight savings may not be as pronounced as a tall building, A913 can still be used effectively to save material and reduce fabrication costs in these types of applications when strength is governing the design.
STRUCTURE: What is its acceptance in codes and standards?
Dardis: A913 Grade 50, 65, 70, and 80 are approved materials in the AISC 360 –22 Specification. A913 Grade 50, 65 and 70 are approved materials in the AISC 341 –22 Seismic Specification with some limitations.
AISC 341 permits the use of A913 Grade 50 in any part of the seismic force resisting system provided that a max yield to tensile ratio is specified (supplementary requirement S75). A913 Grade 65 and 70 are permitted for use in column sections where the steel is not expected to yield, such as special moment frame and braced frame systems.
STRUCTURE: Is it available? Who makes it and in what shapes and grades? Is there a longer lead time than A992?
Pilarczyk: A913 steel is produced domestically by Nucor at the Nucor-Yamato Steel Company facility in Blytheville, Arkansas. This mill is capable of supplying A913 in Grades 50, 65, 70, and 80 for wide flange products, including jumbo sections.
Since the process to create A913 steel with the quenching and self-tempering (QST) is in-line with traditional A992 steel, it is produced in the same roll cycles and frequency as any other wide flange products. It should be noted that, due to the QST process, a minimum amount of thickness must be present in the steel section’s web and/or flange. If there is not enough thickness in the web or flange, the rapid cooling will quench the entire cross-section of the steel and not allow for adequate self-tempering to occur. As such, not all wide flange sections are available to meet the A913 specification. For example, a W14x90 is the smallest section available in the W14 family. Please consult your Nucor or ArcelorMittal representative to gather all available sections.
Dardis: ArcelorMittal produces A913 Grade 50, 65, 70 and 80 for the North American market at its Differdange mill in Luxembourg. ArcelorMittal rolls A913 in shape sizes up to W14x1000 and W36x925.
STRUCTURE: What are the sustainability benefits? Is there a difference in embodied carbon for A913 versus A992?
Pilarczyk: Fundamentally, due to the in-line nature of A913 production, the embodied carbon required to produce it does not differ from A992 steel. All A913 steel is produced in steel mills that utilize electric arc furnace (EAF) technology. This is the cleanest process available to make steel and is reliant on a feedstock of scrap material to make new steel out of old steel in a circular manner.
However, substantial sustainability benefits can be realized when using higher strength A913 steel versus 50ksi A992 steel. When higher strength steel is used to reduce the member size required for column, truss, or short span girder elements, the embodied carbon is similarly reduced. Remember the global warming potential of steel is typically expressed in terms of the carbon dioxide needed to produce a ton of steel.
For example, if a W14x605 A992 Grade 50 member is required to support the anticipated loading conditions for a column element, then switching to a W14x500 A913 Grade 65 will reduce embodied carbon by over 17%. Multiply this across an entire project and it becomes apparent how substantial the savings may be. Greater reductions can be expected when utilizing A913 Grade 80 wide-flange sections.
STRUCTURE: What changes should we look out for in the near future?
Dardis: ArcelorMittal has done extensive research and testing on the weldability of A913 Grade 70 and 80 and has successfully demonstrated that the prequalification status of these grades should be reviewed. As a result, there will be significant changes effective with the release of AWS D1.1 2025.
The minimum prequalified preheat temperatures of A913 Grade 70 will be reduced and match A913 Grade 50 and 65 up to a material thickness of 2 ½ inches. Above 2 ½ inches, the minimum preheat will be reduced from 300 degrees F to 150 degrees F (electrode classification H8 or lower). In addition, A913 Grade 80 will be introduced as a prequalified base metal with the same preheat requirements as A992 (electrode classification H4 or lower).
This will have a dramatic effect on the overall economy of using these grades. Designers will be able to better utilize the improved weight savings of the stronger grades, while worrying less about the trade-offs in fabrication cost due to the more stringent preheat requirements.
Pilarczyk: Many changes are on the horizon when it comes to A913 steel, but also for the steel industry as a whole. A913 Grade 80 steel is becoming more available in the market, in sizes that have not existed in the past.
It was mentioned earlier that electric arc furnaces (circular steelmaking) is the cleanest way to produce steel, however substantial efforts are underway to continue pushing the boundaries for how we can decarbonize the industry. Using fusion, fission, and hydrogen technologies (among others) to displace fossil fuel energy sources are exciting developments to monitor.
Market demands are a major driving force for steel manufacturers to understand what solutions are needed to meet the challenges the design and construction communities are facing. It is important for the design, construction, and fabrication communities to communicate early and often with steel manufacturers to ensure the latest innovations are being employed in projects for more efficient, sustainable outcomes.
STRUCTURE: What is the cost premium, if any, for A913? Can that premium be offset in another area?
Dardis: The efficiency of the QST process means that steel manufacturers can produce A913 without a significant increase in cost. As a result, the premium on A913 is minimal and can easily be overcome by the weight savings.
Steel is bought on a basis of weight, so weight savings directly translate to dollar savings. If A913 reduces tonnage by 15-30%, there is a huge net savings even by the most conservative estimates. If you add in the other benefits, such as lower preheat requirements and reduced fabrication costs, the net savings gap widens.
STRUCTURE: What resources are available to assist design teams?
Pilarczyk: Both Nucor and ArcelorMittal—the two sources of A913 steel—have technical information available to provide guidance when designing, procuring, and working with A913 steel. However, possibly more relevant is the call to action for the design and construction communities to establish relationships with their steel suppliers. Early collaboration can net considerable benefits for the project team. ■