A572 Grade 50 Mechanical Properties

PRELIMINARY — NOT FOR CONSTRUCTION. Values are minimum specified per ASTM A572/A572M. Actual mill values typically exceed minimums. Must be independently verified by a licensed Professional Engineer before use in design.

Property Value (Imperial) Value (Metric) Notes
Yield Strength Fy 50 ksi min 345 MPa min For t ≤ 2 in typical plates
Tensile Strength Fu 65 ksi min 450 MPa min
Fy/Fu Ratio (typical) ~0.77 ~0.77 Not specified, typical value
Elongation (8 in / 200 mm) 18% min 18% min
Elongation (2 in / 50 mm) 21% min 21% min
Modulus of Elasticity E 29,000 ksi 200,000 MPa All structural steels
Shear Modulus G 11,200 ksi 77,000 MPa
Density ρ 490 lb/ft³ 7,850 kg/m³
Poisson's Ratio ν 0.30 0.30
Thermal Expansion α 6.5 × 10⁻⁶ /°F 11.7 × 10⁻⁶ /°C

Available A572 Grades

A572 includes multiple grades covering different strength levels:

Grade Fy (ksi) Fu (ksi) Typical Use
42 42 60 Rarely used; where 36 ksi is insufficient and 50 ksi is overkill
50 50 65 Most common grade — structural plates, gusset plates, base plates
55 55 70 Bridge and heavy structural applications
60 60 75 High-strength plates for heavy girders
65 65 80 Specialized heavy plates; availability may be limited

Grade 42 has largely been replaced by Grade 50 in practice because the incremental cost difference is negligible relative to the strength gain. For typical structural platework, Grade 50 is the default specification unless thickness or ductility considerations dictate otherwise.

A572 Grade 50 Type 1 vs. Type 2

A572 Grade 50 comes in two types based on notch toughness requirements:

Feature Type 1 Type 2
CVN testing Not required Required (typically 15–20 ft-lb at specified temp)
Application General structural use Primary tension members; fracture-critical
Cost Lower 5–10% premium
Availability Widely stocked May require mill order
Typical temp rating Not rated 40°F, 10°F, or -10°F depending on order

Specify Type 2 when:

For most building applications (gusset plates, base plates, stiffeners loaded in shear or compression), Type 1 is adequate. For bridge primary members and seismic frame components, Type 2 is standard.

Chemical Composition

A572 Grade 50 is a microalloyed steel — small additions of columbium (niobium), vanadium, or both provide strength without the high carbon content of plain carbon steels:

Element Content (Heat) Content (Product) Role
Carbon (C) 0.23% max 0.26% max Lower than A36 (0.25–0.29%) for better weldability
Manganese (Mn) 1.35% max — Strength; counters sulfur
Phosphorus (P) 0.04% max — Embrittlement control
Sulfur (S) 0.05% max — Hot shortness control
Silicon (Si) 0.40% max — Deoxidation
Columbium (Cb/Nb) 0.005–0.05% 0.005–0.05% Grain refinement; precipitation strengthening
Vanadium (V) 0.01–0.15% — Precipitation strengthening

The carbon equivalent for A572 Grade 50 typically falls in the range 0.40–0.45, slightly higher than A992 (0.40–0.43 typical) but still readily weldable with standard E7018 electrodes for thicknesses up to 1-1/2 inches.

Carbon Equivalent

For typical A572 Gr 50 chemistry: CE ≈ 0.23 + 1.20/6 + 0.07/5 = 0.23 + 0.20 + 0.014 = 0.444

At CE ≈ 0.44, A572 Gr 50 is near the 0.45 threshold where preheat becomes recommended for thicker sections. This is slightly higher than A992 and attributable to the higher manganese content permitted by the specification.

Thickness-Dependent Strength

Unlike A992 (W-shapes only, standardized section dimensions), A572 plates can range from thin sheet to heavy plate, and strength can vary with thickness:

Plate Thickness Fy min (ksi) Fu min (ksi) Notes
t ≤ 2 in (50 mm) 50 65 Standard range
2 in < t ≤ 4 in (100 mm) 50 65 Verify with specification edition
4 in < t ≤ 6 in (150 mm) 46 65 Reduced yield for heavy plates
t > 6 in Consult mill — Beyond standard scope

Always verify the applicable thickness bracket from the governing ASTM edition before entering Fy into a design equation. The edition year matters — earlier editions of A572 had more aggressive thickness-dependent reductions.

Product Forms and Typical Applications

Product Form Typical Thickness/Gauge Common Applications
Structural plates 1/4 in – 6 in (6–150 mm) Gusset plates, base plates, stiffeners, connection plates
Structural shapes Per AISC Manual W, S, HP shapes (often dual-certified A992)
Bars 1/4 in – 6 in dia Anchor rods, pins, tie rods with higher strength than A36
Sheet piling Per manufacturer catalog Retaining walls, cofferdams, flood protection
Plates for welded girders 3/8 in – 3 in Plate girders, crane girders, transfer girders

A572 Grade 50 vs. A36 — When to Upgrade

Design Parameter A36 A572 Gr 50 Impact of Upgrade
Plate thickness (tension yielding) t = P / (φ × 36 × b) t = P / (φ × 50 × b) ~28% thickness reduction
Weld volume t² × length (0.72t)² × length ~52% fillet weld volume reduction
Plate weight t × area × 490 0.72t × area × 490 ~28% weight savings
Material cost per ton Baseline ~1.05× Negligible
Total installed cost Baseline ~0.85× (lighter, less welding) Savings through lighter fabrication

For a typical gusset plate 3/8 in × 12 in × 18 in:

Worked Example — A572 Gr 50 Base Plate Design

Given: W14×132 column, Pu = 800 kips, f'c = 4 ksi, A572 Gr 50 base plate.

Step 1 — Required plate area (bearing on concrete): A1_req = Pu / (φc × 0.85 × f'c × √(A2/A1)) — assume A2/A1 = 1.0 conservatively φc × 0.85 × f'c = 0.65 × 0.85 × 4 = 2.21 ksi A1 = 800 / 2.21 = 362 in²

Step 2 — Plate dimensions: Try B = N = √(362) ≈ 19.0 in → use 20 in × 20 in = 400 in² Bearing stress: fp = 800 / 400 = 2.00 ksi

Step 3 — Cantilever bending of base plate: Flange projection (critical cantilever): m = (N − 0.95d)/2 = (20 − 0.95 × 14.5)/2 = (20 − 13.78)/2 = 3.11 in Web-side projection: n = (B − 0.80bf)/2 = (20 − 0.80 × 14.7)/2 = (20 − 11.76)/2 = 4.12 in Critical cantilever: max(m, n, λn') = 4.12 in

Step 4 — Required plate thickness: Mu = fp × ℓ_crit² / 2 = 2.00 × 4.12² / 2 = 17.0 kip-in/in strip t_req = √(4 × Mu / (φ × Fy)) = √(4 × 17.0 / (0.90 × 50)) = √(68.0 / 45.0) = √(1.511) = 1.23 in → use 1-1/4 in plate

Check against A36 alternative: t_req_A36 = √(4 × 17.0 / (0.90 × 36)) = √(68.0 / 32.4) = √(2.099) = 1.45 in → 1-1/2 in plate required

A572 Gr 50 saves 0.25 in thickness (a 20% reduction) and approximately 98 lb of steel for this base plate.

Quick Reference Card

Parameter Value
Yield Fy 50 ksi / 345 MPa
Tensile Fu 65 ksi / 450 MPa
Fy/Fu (typical) ~0.77
CE (typical) 0.40–0.45
E 29,000 ksi / 200 GPa
Ry (seismic) 1.10
Product forms Plates, shapes, bars, sheet piling
Type 1 CVN Not required
Type 2 CVN Required per order

References