Introduction to Structural Steel - Theory & Concepts

Introduction to Structural Steel Design

$Learning Objectives$

Explain the material behavior that makes structural steel efficient, ductile, and susceptible to instability.
Recognize common rolled and hollow structural shapes and relate geometry to structural behavior.
Distinguish specified material properties from measured coupon behavior.
Apply the LRFD and ASD strength formats without treating either method as a universal set of load combinations.
Identify the exact code/standard edition behind a load factor, resistance factor, material property, bolt value, or detailing rule.
Interpret carbon equivalent, toughness, residual stress, and fire behavior without turning screening concepts into unsupported acceptance decisions.

$Course code-basis rule$

The Philippine course context names NSCP 2015. Current-practice material in this course may also reference later AISC/ASCE documents. These must not be silently combined. Every worked design should identify the adopted load standard, steel specification, connection standard, and seismic provisions that apply to that example.

$Why structural steel performs well$

Structural steel combines high strength-to-weight ratio, ductility, predictable elastic stiffness, shop fabrication, rapid erection, and recyclability. These advantages do not remove the need to check instability, fracture, fatigue, fire, corrosion, connection behavior, and serviceability.

$F_y$

The specified or method-defined stress associated with the onset of significant permanent deformation. The exact determination may be a yield point, offset yield, or proof stress depending on the product specification and material response.

$F_u$

The maximum engineering tensile stress reached during a standardized tension test, calculated using the original specimen area.

$E$

The slope of the linear elastic stress-strain response. Structural steel design commonly uses $E=29,000\text{ ksi}$ (about $200,000\text{ MPa}$ ).

$Specified properties are not a synthetic stress-strain curve$

$F_y$ , $F_u$ , and $E$ are design/material anchors. They do not uniquely determine the full yield plateau, strain-hardening curve, elongation, necking, or fracture strain of an actual heat of steel. Those characteristics require product-specific or measured data.

$Interactive schematic$

The following tool distinguishes the quantitative elastic branch and specified strength anchors from the intentionally schematic post-yield shape.

Specified Fy

ksi

Specified Fu

ksi

Illustrative engineering strain

in/in

Elastic modulus used29,000 ksi

Idealized yield strain Fy/E0.00172

Current regionidealized yield plateau

Illustrative engineering stress50.0 ksi

Only the elastic branch and specified Fy/Fu anchors are quantitative inputs. The plateau, strain-hardening and necking shapes are schematic. Actual coupon response, elongation, toughness, residual stress and fracture strain must come from material-specific test data.

Toughness, Residual Stress, and Weldability

$Toughness$

The ability of a material to absorb energy before fracture. Notch toughness is particularly important where low temperature, dynamic loading, thick material, stress concentrations, or seismic/fatigue demands make brittle fracture a concern.

$Charpy V-Notch testing$

A Charpy V-Notch (CVN) test reports absorbed impact energy for a specified specimen orientation and test temperature. A CVN value is not interchangeable with yield strength or tensile strength, and the test temperature is part of the requirement.

$Residual stress$

Uneven cooling after rolling and thermal effects from fabrication/welding can leave self-equilibrating residual stresses in steel. Residual stress affects the onset of local yielding and member stability and is one reason real compression-member strength differs from ideal Euler behavior.

$Carbon equivalent is a screening index, not a welding approval$

Carbon-equivalent expressions combine carbon and selected alloying elements to help assess hardenability and susceptibility to hydrogen-assisted cracking. There is no universal CE formula or universal CE threshold that by itself decides whether preheat is required.

The governing product/welding specification and welding procedure must control the formula and required actions. Preheat and welding controls can also depend on material group, thickness, restraint, hydrogen level, heat input, process, joint configuration, and service requirements.

$Do not make this inference$

A calculation such as “CE is below 0.40%, therefore no preheat is required” is not a complete structural-welding decision. CE can inform the assessment, but the qualified/prequalified welding procedure and governing code requirements control the actual preheat and fabrication requirements.

Common Structural Shapes

$Shape families$

W-shapes: efficient strong-axis beams and common columns; parallel flange surfaces.
S-shapes: older American Standard beams with tapered inner flange surfaces.
HP-shapes: bearing-pile shapes proportioned for pile service.
C and MC channels: open singly symmetric sections used in framing, bracing, and built-up members.
L angles: equal- or unequal-leg members widely used in bracing, trusses, and connections.
WT/ST/MT tees: tee sections, often produced from rolled I-shape families.
HSS: round, square, and rectangular closed sections; efficient for torsion and biaxial compression behavior.
Plate/built-up sections: fabricated when rolled sections cannot efficiently meet the required strength, stiffness, or geometry.

$Interactive shape explorer$

Use the shape visualizer to connect section geometry with typical structural behavior. Shape selection is only the first step; strength, stability, serviceability, connections, availability, fabrication, and fire/corrosion requirements still govern design.

Standard Steel Shapes Viewer

Drag to rotate • Scroll to zoom

W-Shapes (Wide Flange) are the most common shape for beams and columns due to their high moment of inertia relative to their weight.

Structural Steel Grades

$Material specification must match product form$

A grade name is not enough. Verify the exact ASTM product specification, shape/product form, thickness range, specified $F_y$ / $F_u$ , toughness or supplementary requirements, weldability requirements, and material traceability.

Common examples in U.S.-based steel design include ASTM A992 for many W-shapes, ASTM A36 for many plates/angles and miscellaneous products, ASTM A572 grades for structural plates/shapes, and ASTM A500/A1085 families for HSS. The actual project specification governs.

LRFD and ASD Strength Formats

LRFD strength requirement

Factored required strength must not exceed design strength.

R_u \le \phi R_n

Variables

Symbol	Description	Unit
$R_u$	Required strength from the applicable strength load combinations	-
$R_n$	Nominal resistance for the governing limit state	-
$\phi$	Resistance factor prescribed for that limit state	-

ASD strength requirement

Required strength from ASD load combinations must not exceed allowable strength.

R_a \le \frac{R_n}{\Omega}

Variables

Symbol	Description	Unit
$R_a$	Required strength from the applicable ASD load combinations	-
$R_n$	Nominal resistance for the governing limit state	-
$\Omega$	Safety factor prescribed for that limit state	-

$Load combinations are not defined by AISC strength format alone$

AISC defines member and connection resistance, but the required load combinations come from the adopted loading/building standard. Do not assume that “LRFD” automatically means one fixed list regardless of code edition or jurisdiction.

Load Combination Traceability

$ASCE 7-16 D/L/W/S teaching subset$

The interactive tool below intentionally evaluates a limited ASCE 7-16 subset using only dead load $D$ , floor live load $L$ , wind effect $W$ , and snow effect $S$ . It includes the companion live-load exception as an explicit user choice rather than silently applying $0.5L$ .

It does not model earthquake $E$ , roof live $L_r$ , rain $R$ , fluid $F$ , lateral earth pressure $H$ , or sign-dependent combinations. A real design must evaluate every applicable effect under the adopted standard.

Dead load D

kips

Live load L

kips

Wind effect W

kips

Snow effect S

kips

Use 0.5L companion factor
Only when the ASCE 7-16 occupancy exception is satisfied; garages and public-assembly areas are excluded.

Strength design

1.4D28.0 kips

1.2D + 1.6L + 0.5S77.0 kips

1.2D + 1.6S + L70.0 kips

1.2D + 1.6S + 0.5W47.5 kips

1.2D + W + L + 0.5S74.0 kips

0.9D + W33.0 kips

Allowable strength design

D20.0 kips

D + L50.0 kips

D + S30.0 kips

D + 0.75L + 0.75S50.0 kips

D + 0.6W29.0 kips

D + 0.75L + 0.45W + 0.75S56.8 kips

0.6D + 0.6W21.0 kips

Scope: this is an educational ASCE 7-16 D/L/W/S evaluator. Earthquake, roof-live, rain, fluid, lateral-earth-pressure and special load effects are not included. Use the code edition adopted by the project or jurisdiction for design.

Limit States

$Strength and serviceability$

Strength limit states include yielding, rupture, buckling, fracture, connection failure, fatigue, and other conditions that limit safe resistance. Serviceability limit states include excessive deflection, vibration, drift, or other performance conditions that impair intended use without necessarily representing imminent collapse.

Fire and Corrosion

$Fire resistance is an assembly/system property$

Steel does not burn as a combustible framing material, but its stiffness and strength deteriorate as temperature rises. Fire resistance depends on the member, applied load, restraint, connection behavior, thermal exposure, section factor, and protection system—not one universal “failure temperature.”

$Corrosion protection is a system$

Coatings, galvanizing, weathering steel, drainage, detailing, inspection, maintenance, and environmental exposure work together. A corrosion-resistant material or coating cannot compensate for persistent water traps, incompatible metals, inaccessible crevices, or poor surface preparation.

$Before using any steel-design equation$

Identify the adopted building/loading standard and edition.
Identify the AISC steel specification edition used for member/connection resistance.
Identify RCSC/AWS/seismic companion standards where applicable.
Confirm units and material/product specification.
Confirm the equation's scope: member type, cross-section class, load case, connection type, and limit state.
Evaluate all applicable competing limit states rather than selecting one favorable equation.
Preserve the code basis in the calculation package and drawings.

Key Takeaways

Structural steel combines high strength, stiffness, ductility, and constructability, but stability, fracture, fatigue, fire, corrosion, and connection behavior remain essential design concerns.
$F_y$ , $F_u$ , and $E$ are not enough to predict an entire coupon stress-strain curve.
Carbon equivalent is a weldability screening concept; it does not by itself approve a welding procedure or determine preheat.
LRFD and ASD are resistance/checking formats; required load combinations come from the adopted loading standard.
Code-edition traceability is part of engineering correctness. NSCP 2015 course examples and later AISC/ASCE provisions must be labeled rather than blended silently.

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Introduction to Structural Steel Design

$Learning Objectives$

$Course code-basis rule$

$Why structural steel performs well$

$F_y$

$F_u$

$E$

$Specified properties are not a synthetic stress-strain curve$

$Interactive schematic$

Schematic Steel Stress-Strain Behavior

Toughness, Residual Stress, and Weldability

$Toughness$

$Charpy V-Notch testing$

$Residual stress$

$Carbon equivalent is a screening index, not a welding approval$

$Do not make this inference$

Common Structural Shapes

$Shape families$

$Interactive shape explorer$

Structural Steel Grades

$Material specification must match product form$

LRFD and ASD Strength Formats

LRFD strength requirement

ASD strength requirement

$Load combinations are not defined by AISC strength format alone$

Load Combination Traceability

$ASCE 7-16 D/L/W/S teaching subset$

Load Combination Explorer

Strength design

Allowable strength design

Limit States

$Strength and serviceability$

Fire and Corrosion

$Fire resistance is an assembly/system property$

$Corrosion protection is a system$

$Before using any steel-design equation$

Introduction to Structural Steel Design

Learning Objectives

Course code-basis rule

Why structural steel performs well

Yield Strength (FyF_yFy​)

Tensile Strength (FuF_uFu​)

Modulus of Elasticity (EEE)

Specified properties are not a synthetic stress-strain curve

Interactive schematic

Toughness, Residual Stress, and Weldability

Toughness

Charpy V-Notch testing

Residual stress

Carbon equivalent is a screening index, not a welding approval

Do not make this inference

Common Structural Shapes

Shape families

Interactive shape explorer

Structural Steel Grades

Material specification must match product form

LRFD and ASD Strength Formats

LRFD strength requirement

ASD strength requirement

Load combinations are not defined by AISC strength format alone

Load Combination Traceability

ASCE 7-16 D/L/W/S teaching subset

Strength design

Allowable strength design

Limit States

Strength and serviceability

Fire and Corrosion

Fire resistance is an assembly/system property

Corrosion protection is a system

Before using any steel-design equation

$Learning Objectives$

$Course code-basis rule$

$Why structural steel performs well$

$F_y$

$F_u$

$E$

$Specified properties are not a synthetic stress-strain curve$

$Interactive schematic$

$Toughness$

$Charpy V-Notch testing$

$Residual stress$

$Carbon equivalent is a screening index, not a welding approval$

$Do not make this inference$

$Shape families$

$Interactive shape explorer$

$Material specification must match product form$

$Load combinations are not defined by AISC strength format alone$

$ASCE 7-16 D/L/W/S teaching subset$

$Strength and serviceability$

$Fire resistance is an assembly/system property$

$Corrosion protection is a system$

$Before using any steel-design equation$