What is the difference between yield strength (Fy) and tensile strength (Fu)?

Yield strength (Fy) is the stress where permanent (plastic) deformation begins — typically 0.2% offset for steels without a distinct yield plateau. Tensile strength (Fu), also called ultimate tensile strength (UTS), is the maximum stress the steel can withstand before necking and fracture occur. On the engineering stress-strain curve: (1) Elastic region (0 to Fy) — linear, reversible deformation governed by Hooke's Law (σ = E·ε). (2) Yield plateau — constant stress with increasing strain, characteristic of mild carbon steels like A36. (3) Strain hardening — stress rises above Fy as dislocations accumulate, reaching the peak at Fu. (4) Necking — localized cross-section reduction begins at Fu, engineering stress decreases until fracture. Fu is always greater than Fy. The Fy/Fu ratio indicates ductility: A36 (0.45-0.62) has a large gap between yield and fracture = high ductility; A992 (0.77) has less gap = adequate ductility for seismic; A514 (0.77-0.91) has a narrow gap = limited ductility, requires special detailing. In AISC design, Fy controls yielding limit states (φ = 0.90), while Fu controls fracture limit states (φ = 0.75) because fracture is sudden and less predictable than yielding, warranting a higher safety margin.

What structural steel has the highest tensile strength?

Among structural steels approved for building construction, ASTM A514 (quenched and tempered) has the highest tensile strength at 110-130 ksi (760-895 MPa), with yield strength of 100 ksi. A514 is used for heavily loaded columns in high-rise buildings, mining equipment, crane runway girders, and heavy excavation equipment where the 100 ksi yield strength enables significant material savings. In bridge construction, A709 Gr 100 (essentially A514 chemistry with bridge-specific requirements) reaches Fu = 110-130 ksi. Moving beyond structural grades: maraging steels (18Ni grades) achieve Fu = 250-350 ksi in aerospace and tooling applications. Tool steels reach extreme values: D2 = 250-300 ksi, M2 high-speed steel = 280-350 ksi. Cold-drawn wire for prestressing strand (ASTM A416 Gr 270) has Fu = 270 ksi minimum. However, high tensile strength comes with tradeoffs: (1) Reduced ductility — A514 elongation is 14-18% vs 21% for A992. (2) Weldability — A514 requires careful preheat (minimum 50°F for thin sections, 225°F for sections over 1-1/2 inch), low-hydrogen electrodes (H4 or H8), and controlled heat input (max 2.5-3.0 kJ/mm). (3) Cost — A514 is 2-3x the cost of A572 Gr 50 per pound, though the weight savings may offset this for critical applications. (4) Fracture toughness — higher-strength steels have lower Charpy V-notch values at cold temperatures; this must be checked for service temperature per AISC 360 Appendix 3.

Why does AISC use Fu (tensile strength) for fracture and block shear checks?

AISC 360 uses Fu for fracture-related limit states because fracture is a sudden, brittle failure mode that occurs when the stress reaches the material's ultimate tensile strength on the reduced (net) cross-section. The key design philosophy: (1) Yielding (Fy-based) is ductile — large deformations provide visible warning before failure, so higher resistance factor φ = 0.90 is used. (2) Fracture (Fu-based) is brittle — occurs with little or no warning at stress concentrations (bolt holes, cope locations, weld toes), so a lower resistance factor φ = 0.75 is applied. The five principal fracture checks using Fu are: (a) Tension member fracture on net section: φPn = φFuAe, where Ae = An·U (effective net area with shear lag factor U per AISC Table D3.1). (b) Bolt bearing at bolt holes: φRn = φ2.4d·t·Fu per AISC J3.10(a), checking the connected part material. (c) Bolt tearout: φRn = φ1.2Lc·t·Fu per AISC J3.10(b), where Lc = clear distance from hole edge to edge or end. (d) Block shear rupture: φRn = φ[0.6FuAnv + UbsFuAnt] ≤ φ[0.6FyAgv + UbsFuAnt] per AISC J4.3 — note that both Fy and Fu appear in block shear, with Fu controlling the net area contributions. (e) Weld metal strength: φRn = φ0.6FEXX·Awe, where FEXX is the electrode's tensile strength classification (E70XX means Fu = 70 ksi minimum). The common thread: all involve stress concentrations where localized yielding cannot redistribute load before fracture initiates.

How does tensile strength change with steel thickness?

Tensile strength generally decreases as plate thickness increases, due to the effect of cooling rate during hot rolling. Thicker sections cool more slowly after rolling, resulting in coarser grain structure and lower strength. ASTM specifications account for this by specifying minimum Fu values that decrease with thickness group. For A36: plates up to 8 inches must meet Fu = 58-80 ksi, but the upper end of the range is typically achieved only in thicknesses under 1 inch. For plates over 8 inches, the minimum Fu drops to 58 ksi without an upper bound. For A572 Gr 50: Fu = 65 ksi minimum for all thicknesses up to 4 inches, but the actual mill values are often 65-70 ksi for thin plates (under 1/2 inch) and 65-67 ksi for thick plates (2-4 inches). For quenched and tempered A514: thickness has a dramatic effect — plates under 3/4 inch achieve Fu = 110-130 ksi, while plates over 2-1/2 inches are limited to Fu = 100-120 ksi due to reduced hardenability at the core. The mechanism: cooling rate (dT/dt) at the mid-thickness of a plate is inversely proportional to the square of thickness (t²), so doubling the thickness reduces the cooling rate by approximately 4x, producing a coarser, lower-strength microstructure. For design, always use the certified mill test report (CMTR) Fu values for the specific heat and thickness, not generic tables. AISC Specification requires that the specified minimum Fu be used for design, not the reported (typically higher) mill values, unless specifically permitted by the Engineer of Record.

What is the tensile strength of Grade 8.8 bolts?

Grade 8.8 bolts have a minimum tensile strength (fub) of 800 MPa and yield strength (fyb) of 640 MPa per EN 1993-1-8 Table 3.1. The tensile stress area for an M20 bolt is 245 mm².

What is the Fu value for S355 steel?

S355JR steel has a minimum tensile strength Fu of 470 N/mm² for thicknesses between 3 mm and 100 mm per EN 10025-2 Table 2. This value is used in bearing and net section calculations per EN 1993-1-8.

Steel Tensile Strength — Fu Values by Grade & Application

Tensile strength (Fu), also called ultimate tensile strength (UTS), is the maximum stress a steel specimen can withstand during a tensile test before it fractures. While yield strength (Fy) governs most structural design checks, tensile strength is critical for tension member design, bolt bearing, and fracture-critical applications.

What Is Tensile Strength?

During a tensile test, the steel specimen is pulled until it breaks. The stress-strain curve shows:

Elastic region (0 to Fy): Linear, reversible deformation
Yield plateau: Constant stress, increasing strain (mild steels only)
Strain hardening: Stress increases to the maximum (Fu)
Necking: Localized thinning before fracture

The tensile strength is the peak of the curve. For structural design:

Yielding (Fy) controls ductile failure modes
Fracture (Fu) controls brittle failure modes
AISC checks both: φFyAg for yielding and φFuAe for fracture on net section

Tensile Strength by ASTM Specification

Carbon and Low-Alloy Structural Steels

ASTM Spec	Grade	Fu (ksi)	Fu (MPa)	Fy (ksi)	Fy/Fu	Elongation (%)
A36	—	58-80	400-550	36	0.45-0.62	20-23
A992	50	65	450	50	0.77	21
A572	42	60	415	42	0.70	20
A572	50	65	450	50	0.77	18
A572	55	70	485	55	0.79	17
A572	60	75	520	60	0.80	16
A572	65	80	550	65	0.81	15
A588	—	70	485	50	0.71	18
A514	—	110-130	760-895	100	0.77-0.91	14-18
A709	36	58	400	36	0.62	20
A709	50	65	450	50	0.77	18
A709	50W	70	485	50	0.71	18
A1043	36	58	400	36	0.62	21
A1043	50	65	450	50	0.77	19

Hollow Structural Sections

ASTM Spec	Grade	Fu (ksi)	Fy (ksi)	Notes
A500 Gr B (round)	—	58	42	Most common round HSS
A500 Gr B (rect)	—	58	46	Most common rect HSS
A500 Gr C (round)	—	62	46	Higher strength round
A500 Gr C (rect)	—	62	50	Higher strength rect
A501	—	58	36	Hot-formed HSS
A1085	—	65	50	Tight tolerances, Charpy required

Fasteners

ASTM Spec	Grade	Fu (ksi)	Application
A307	Gr A	60	Common (unfinished) bolts
A325	1/2-1 in	120	High-strength structural bolts
A325	>1-1.5 in	105	Oversized high-strength bolts
A490	—	150	Alloy steel structural bolts
A490M	—	150	Metric equivalent
F3125 Gr A325	—	120	Consolidated spec
F3125 Gr A490	—	150	Consolidated spec

Stainless Steel

UNS Number	Type	Fu (ksi)	Fy (ksi)	Elongation (%)
S30400	304	75-90	30-40	40-50
S31600	316	75-90	30-40	40-50
S32100	321	75-90	30-40	40-50
S34700	347	75-90	30-40	40-50
S41000	410	65-90	30-65	15-25
S43000	430	65-75	30-40	20-25
S17400	17-4PH	135-170	110-145	8-14

Specialty and Tool Steels

Steel Type	Fu Range (ksi)	Typical Application
Spring steel (1070)	100-140	Springs, clips
Spring steel (1095)	120-180	High-stress springs
4140 alloy	95-180	Shaft, gears, forgings
4340 alloy	110-220	Aircraft, heavy forgings
8620 alloy	80-130	Case-hardened parts
Tool steel (O1)	200-280	Cold-work tools
Tool steel (D2)	250-300	Blanking dies
Tool steel (A2)	230-280	Forming dies
Tool steel (M2)	280-350	Cutting tools
Tool steel (H13)	210-280	Die casting dies

Tensile Strength vs Yield Strength in Design

AISC Design Checks Using Fu

Design Check	AISC Section	Formula	When It Controls
Tension fracture	D2	φFuAe (φ=0.75)	Net section of tension members
Bolt bearing	J3.10	Based on Fu of connected part	Thin connected elements
Bolt tearout	J3.10	1.2φFuLCt (φ=0.75)	Short edge distances
Block shear	J4.3	Combination of Fy and Fu	Gusset plates, cope blocks
Weld metal	J2.4	0.6FEXX (FEXX = electrode Fu)	Fillet weld capacity

Block Shear Strength (Uses Both Fy and Fu)

Block shear rupture combines tension fracture on one plane with shear yielding or fracture on the perpendicular plane:

φRn = φ × [0.6FuAnv + UbsFuAnt] ≤ φ × [0.6FyAgv + UbsFuAnt]

where φ = 0.75, Anv = net shear area, Ant = net tension area, Agv = gross shear area, Ubs = uniformity factor.

Tensile Testing Methods

Method	Standard	Specimen	Speed
Standard tensile	ASTM E8/E8M	Round or rectangular	Per specification
Elevated temperature	ASTM E21	Subsize round	Per specification
Fastener testing	ASTM F606	Full-size bolt	Controlled rate

Typical specimen dimensions:

Round: 0.500 in diameter, 2.0 in gauge length
Rectangular (plate): 1.5 in wide, gauge length = 2.0 in
Subsize: 0.250 in diameter, 1.0 in gauge length

Frequently Asked Questions

What is the tensile strength of A36 steel? A36 has a minimum tensile strength of 58 ksi (400 MPa). The range is 58-80 ksi depending on thickness. For plates over 8 inches thick, Fu = 58 ksi minimum.

What is the difference between yield and tensile strength? Yield strength (Fy) is the stress where permanent deformation begins. Tensile strength (Fu) is the maximum stress before fracture. Fu is always greater than Fy. The gap between them determines how much the steel can deform before breaking (ductility).

Why does AISC use Fu for fracture checks? Fracture is a sudden, brittle failure mode that occurs at the tensile strength. It is less predictable than yielding and has no warning signs. AISC uses Fu with a lower resistance factor (φ = 0.75 vs 0.90 for yielding) to provide additional safety margin against brittle failure.

What steel has the highest tensile strength? Among structural steels, ASTM A514 (quenched and tempered) has the highest at 110-130 ksi. Tool steels can exceed 300 ksi but are not used for structural applications. Maraging steels reach 350+ ksi in aerospace.

Does cold working increase tensile strength? Yes. Cold working (cold rolling, drawing) strain-hardens the steel, increasing both Fy and Fu while reducing ductility. Cold-formed steel members (AISI S100) rely on this strength increase.

Steel Yield Strength — Fy values by grade
Steel Stress-Strain Curve — Complete stress-strain behavior
Steel Grades — ASTM specifications
Tension Member Design — AISC Chapter D checks
Steel Material Properties — Comprehensive material data

Disclaimer

This is a calculation tool, not a substitute for professional engineering certification. All results must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in construction, fabrication, or permit documents. The user is responsible for the accuracy of all inputs and the verification of all outputs.