A913 High-Strength Steel — 65-70 ksi W-Shapes for High-Rise Construction

ASTM A913 is the specification for high-strength low-alloy structural steel shapes — specifically wide-flange (W) sections — produced by the quenched and self-tempered (QST) process. Available in four grades (50, 60, 65, and 70) with minimum yield strengths from 50 to 70 ksi (345 to 483 MPa), A913 provides strength 20–40% higher than standard A992 W-shapes while maintaining excellent weldability and toughness.

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A913 Mechanical Properties by Grade

The Fy/Fu ratio limit of 0.85 is maintained across all grades — the same requirement that was introduced for A992 after the 1994 Northridge earthquake. This ensures ductile behavior and reliable plastic hinge formation in seismic applications, making A913 acceptable for use in Special Moment Frames (SMF) and Intermediate Moment Frames (IMF) under AISC 341.

The Quenched and Self-Tempered (QST) Process

The QST process is what makes A913 fundamentally different from traditional quenched and tempered (Q&T) steels like A514. Instead of reheating after quenching — which requires a second furnace and adds cost — QST uses the residual heat in the core of the section to temper the surface.

How QST Works (Step by Step)

Step 1: Hot rolling. The W-shape exits the rolling mill at approximately 1,500–1,650 degrees F (815–900 degrees C), fully austenitic.

Step 2: Intense water spray quenching. The entire surface of the hot shape is sprayed with water at controlled pressure and flow rate. The surface cools rapidly (50–100 degrees F per second), transforming to martensite in a surface layer 0.5 to 1.5 inches deep. The core remains hot — above 1,100 degrees F — because the heat must conduct through the flange and web thickness to reach the surface.

Step 3: Self-tempering. When the water spray stops, heat from the uncooled core conducts outward and reheats the martensitic surface layer to approximately 1,000–1,200 degrees F (540–650 degrees C). This is the "self-tempering" step — no external furnace required. The core provides the heat, and the surface receives the tempering treatment.

Step 4: Air cooling. The entire section cools to ambient temperature. The final microstructure is tempered martensite at the surface (high strength + ductility) transitioning to fine-grained ferrite-pearlite or bainite in the core (good toughness). The gradient in microstructure is intentional — the surface carries the highest bending stress in a beam or column, so maximum strength at the surface is efficient.

Advantages of QST Over Traditional Q&T

The QST process provides a graded strength profile that is well-suited to bending members: the highest strength is at the flange tips and web surface where bending stress is maximum. In compression members, the slightly lower core strength has minimal impact because buckling governs long before material yielding.

Available Sizes and Sections

A913 W-shapes are produced by a limited number of mills worldwide. Availability is concentrated in the most efficient high-rise column sections:

The heaviest A913 section, W14x730, has flange thicknesses of 4.91 inches and is used primarily for super-tall building columns in the lower 20–30 stories where axial loads are enormous. Section weight availability depends on the producing mill — ArcelorMittal (Luxembourg sections, "Jumbo" shapes) and Nucor-Yamato (Blytheville, Arkansas) are the primary North American suppliers.

For column design with A913, the practical benefit is clear: a W14x500 in A913-65 replaces a W14x730 in A992 (27% weight savings) for the same axial capacity in the inelastic buckling range. For a 100-story building, this can save 2,000–4,000 tons of steel.

Weldability of A913

One of the most important advantages of A913 over traditional high-strength steels is its excellent weldability. The low carbon equivalent — typically 0.35 to 0.40 — is achieved because strength comes primarily from the QST process rather than from high carbon and alloy content.

Carbon Equivalent Comparison

Welding Recommendations

• Filler metal for Grade 50: Standard E70XX electrodes. The weld metal (70 ksi tensile) over-matches the base metal (50 ksi yield, 65 ksi tensile).
• Filler metal for Grade 60/65: Use E80XX electrodes for partial-joint-penetration and fillet welds where weld strength must match base metal strength. E70XX can be used for full-penetration welds in tension because the larger section area of the base metal governs.
• Filler metal for Grade 70: E90XX or E100XX electrodes recommended for CJP welds in tension. For fillet welds, the weld metal strength should equal or exceed the expected stress in the connected element.
• Preheat: For A913-65 and A913-70 with thicknesses over 1.5 in., apply a minimum preheat of 100–150 degrees F per AWS D1.1 Table 3.2 for low-hydrogen electrodes. This is considerably less than the 300–400 degrees F required for A514 of similar thickness.
• Heat input: No maximum interpass temperature restriction for A913 below 600 degrees F, unlike A514 (400 degrees F max).

Seismic Welding (AISC 358)

For moment connections in Special Moment Frames (SMF) using A913 columns, the weld metal must satisfy AISC 358 toughness requirements: 40 ft-lb at 70 degrees F for the weld deposit and 20 ft-lb at 70 degrees F for the heat-affected zone. E71T-8 and E70T-7 electrodes typically meet these requirements for the column-bracket and continuity-plate welds. Consult the specific prequalified connection procedure for electrode selection.

A913 in High-Rise Construction

A913 has become the standard specification for high-rise building columns in North America, largely because of the QST process's favorable combination of strength, cost, and weldability. Key landmark projects include:

• One World Trade Center (New York, 2014): A913-65 and A913-70 W14x500 and W14x730 columns in the lower 40 stories, saving approximately 3,500 tons compared to A992.
• 432 Park Avenue (New York, 2015): A913-65 W14x370–W14x500 columns in the slender residential tower, where weight reduction was critical for foundation load management.
• Wilshire Grand Center (Los Angeles, 2017): A913-65 composite mega-columns (W14x730 concrete-filled) in the seismic-resistant core, using A913's ductile yield behavior under cyclic loading.

The economic threshold for A913 upgrade from A992 typically occurs at approximately 20 stories, where column sizes in A992 reach W14x300–W14x400 and switching to A913-65 allows reducing to W14x211–W14x283, providing weight savings that offset the material cost premium.

AISC 360 Design Values

AISC 360-22 does not tabulate separate design values for A913 grades. Use the general provisions with the actual specified Fy and Fu:

Column design (Chapter E): The critical stress Fcr in the inelastic buckling range depends on Fy. Higher Fy shifts the boundary between inelastic and elastic buckling (the transition slenderness lambda_c = 1.5). For KL/r values between 60 and 100, A913-65 columns develop proportionally higher strength than A992 columns — but not in direct proportion to Fy because the inelastic buckling curve flattens at higher yield strengths.

Connection design: When connecting A913-65 or A913-70 columns to A992 beams, the connection (shear tab, end plate, continuity plates) must be designed for the forces transferred — not for the full capacity of the column. A connection that develops 60% of the connected beam's plastic moment is adequate regardless of the column grade.

Cost Analysis: A913-65 vs A992

The case for A913 is strongest when column weight savings cascade through the building: lighter columns mean smaller foundations, reduced seismic mass, fewer truckloads to the site, and faster erection (fewer crane picks per column). For buildings over 40 stories, the total structural cost savings from A913-65 typically range from 5–12% compared to an all-A992 design.

A913 vs A514: Product Form Determines Choice

The decision is simple: if you need a rolled W-shape with higher strength than A992, specify A913. If you need plate products with strength above 65 ksi (up to 100 ksi), specify A514.

A913 Chemical Composition (Typical, Grade 65)

The controlled vanadium and niobium additions provide precipitation strengthening and grain refinement during the hot rolling and self-tempering stages. These microalloying elements are far more cost-effective than chromium, nickel, or molybdenum (used in A514) for achieving the 65–70 ksi strength range.

Frequently Asked Questions

What is the difference between A913 and A992?

A992 is the standard W-shape specification with Fy=50 ksi and Fu=65 ksi. A913 is the high-strength W-shape specification with grades up to 70 ksi yield. A913 uses the QST (quenched and self-tempered) manufacturing process, while A992 uses controlled rolling and air cooling. A913 is more expensive (10–25% premium) but provides 20–40% higher strength. Both have the same maximum Fy/Fu ratio of 0.85.

Can I use A913 for seismic design?

Yes. A913 grades 50, 60, 65, and 70 are all permitted for use in Special Moment Frames (SMF) and Intermediate Moment Frames (IMF) under AISC 341-22, provided the Fy/Fu ratio does not exceed 0.85 and the material meets the Charpy V-notch toughness requirements for the applicable temperature zone.

What filler metal should I use for welding A913-65?

For A913 Grade 65, use E80XX electrodes for fillet welds and partial-joint-penetration (PJP) welds where the weld metal must develop the base metal strength. For complete-joint-penetration (CJP) welds in tension, E70XX electrodes are acceptable because the base metal cross-section at the joint is larger than the weld area, and failure will be governed by the base metal. For A913 Grade 70, upgrade to E90XX or E100XX for CJP tension welds.

Does A913 require preheat before welding?

For thicknesses up to 1.5 inches, A913 typically does not require preheat when using low-hydrogen electrodes in ambient temperatures above 32 degrees F. For thicknesses over 1.5 inches, apply a minimum preheat of 100–150 degrees F per AWS D1.1. This is significantly less demanding than A514 (quenched and tempered plate), which requires 125–400 degrees F preheat depending on thickness.

How much weight can I save by using A913-65 instead of A992?

For columns in the 20–60 story range, switching from A992 to A913-65 typically reduces section weight by 20–30% for the same required axial capacity. For a W14x730 in A992 (730 lb/ft), the A913-65 equivalent is approximately W14x500 (500 lb/ft) — a savings of 230 lb/ft per column. In a building with 24 perimeter columns over 60 stories, this saves approximately 2,000 tons of steel.