Material Properties of Structural Steel

Structural Steel

Steel designed to support loads in buildings, bridges, and large structures; comes in various shapes/sizes and is classified by type, use, and design

Rebar (Steel Reinforcement Bar)

Deformed steel bar embedded in concrete to improve its overall strength and performance

Four Mechanical Tests for Rebar

Tensile, bend, compression, and fatigue testing

ASTM E8

Standard test method for determining tensile properties (yield strength, UTS, elongation, reduction of area) of metallic materials under controlled uniaxial loading

ASTM A615

Standard ASTM Specification for Deformed and Plain Carbon Steel Bars for Concrete Reinforcement

Stress Formula

σ = F/A, where F is applied force and A is cross-sectional area

Strain Formula

ε = ΔL/L₀, where ΔL is change in length and L₀ is original gauge length

Elastic Region

Region of the stress-strain curve where the material fully returns to its original shape upon unloading

Proportional Limit

The highest stress level at which stress remains directly proportional to strain (Hooke's Law: σ ∝ ε)

Hooke's Law

σ ∝ ε; stress is directly proportional to strain within the proportional limit

Modulus of Elasticity (Young's Modulus)

A material property that measures stiffness; represented by the slope of the stress-strain curve in the linear elastic region

Elastic Limit

The highest stress a material can experience while still remaining elastic (returning to original shape); slightly above the proportional limit

Modulus of Resilience

Energy per unit volume a material can absorb without undergoing permanent deformation; area under the elastic region of the stress-strain curve

Yield Point

The point where large strain occurs with little or no increase in stress; marks the onset of plastic (permanent) deformation

Offset Yield Method

Method to determine yield strength by drawing a line parallel to the elastic modulus offset by 0.2%–2% strain; yield strength is the intersection of this line and the stress-strain curve

Plastic Region

Region beyond the yield point where deformation is permanent and the material does not return to its original shape

Strain Hardening

Post-yield phenomenon where stress increases with increasing strain due to dislocation interactions resisting further deformation

Ultimate Tensile Strength (UTS)

The maximum stress a material can withstand; beyond this point, the cross-sectional area begins to reduce significantly (necking begins)

Necking Region

Localized reduction in cross-sectional area after UTS is reached; engineering stress decreases even as strain increases

Fracture Point

The final point on the stress-strain curve where the material completely breaks

Modulus of Toughness

Total energy per unit volume a material can absorb before fracture; represented by the total area under the stress-strain curve from zero stress to fracture

Deformed Bar

A steel bar with surface protrusions (ribs) intended for use as reinforcement in reinforced concrete construction

Rebar Deformation Spacing Requirement (ASTM A615 §7.1)

Deformations shall be spaced at substantially uniform distances; deformations on opposite sides shall be similar in size, shape, and pattern

Rib Angle Requirement

Deformations shall be placed at an included angle of not less than 45° to the bar axis (ASTM A615 7.2)

Maximum Average Spacing of Deformations

Shall not exceed 0.7 times the nominal diameter of the bar (ASTM A615)

Maximum Gap of Deformations (ASTM A615)

Gap (chord) between ends of deformations shall not exceed 12.5% of nominal perimeter; sum of all gaps shall not exceed 25% of nominal perimeter

Measuring Average Spacing of Deformations

Determined by measuring the length of a minimum of ten spaces and dividing by the number of spaces included (ASTM A615)

Measuring Average Height of Deformations

Based on three measurements per deformation: one at center and two at quarter points of overall length (ASTM A615)

Modulus of Resilience

Area under curve up to elastic limit, Energy absorbed without permanent deformation

Strain Hardening Region

Stress increases with strain, Dislocation interactions resist further deformation

Grade 80 Tensile Requirements

Min. tensile strength: 100,000 psi [690 MPa]; Min. yield strength: 80,000 psi [550 MPa]; UTS/yield ratio ≥ 1.10

True

The proportional limit and elastic limit are not the same point. – between them, behavior is non-linear but still elastic.

Fracture Point

Material completely breaks, Final point on curve

Higher Grade Rebar

Higher strength = often lower ductility (less elongation)

Steel Type Marking – "S"

Indicates Carbon Steel per ASTM A615

Steel Type Marking – "W"

Indicates Low-Alloy Steel per ASTM A706

Steel Type Marking – "SS"

Indicates Stainless Steel per ASTM A955

Rebar Grade Marking System

Grade 40 = no grade markings; Grade 60 = marked "60"; Grade 75 = "75"; Grade 80 = "80"; Grade 100 = "100"; Grade 120 = "120"

Modulus of Elasticity

Slope of linear elastic region, Stiffness – higher E = stiffer material

Modulus of Toughness

Total area under entire stress-strain curve, Energy absorbed up to fracture (toughness)

CS

Steel type marking for Low Carbon Chromium

Yield Point

Large strain with little/no increase in stress, Where plastic deformation begins

True Stress vs. Engineering Stress

Engineering stress uses original cross-sectional area; true stress accounts for actual reduced area — the true stress-strain curve continues rising after UTS due to necking

Ductile vs. Brittle

Ductile materials (like rebar) show significant plastic deformation and necking before fracture; brittle materials fracture with little to no plastic deformation

Factor of Safety

Yield strength is the design threshold; knowing it ensures the material meets the required safety factor before permanent deformation occurs

Nominal Perimeter Formula

Nominal perimeter = 3.1416 × nominal diameter (ASTM A615)

Rebar Deformations

Surface ribs increase mechanical bond (interlocking) between rebar and concrete, transferring tensile forces effective