Module 5: Introduction to Structural Steel - Theory & Concepts

Module 5: Introduction to Structural Steel

Overview

Structural steel is an alloy of iron and carbon (typically less than

0.3\%

carbon for structural applications), along with other alloying elements like manganese, silicon, copper, and vanadium to enhance strength, ductility, weldability, and corrosion resistance. It is the premier material for high-rise buildings, long-span bridges, and industrial facilities due to its extremely high strength-to-weight ratio.

Properties of Structural Steel

The behavior of steel under load is characterized by its stress-strain curve, which exhibits a distinct linear elastic region followed by a massive plastic yielding plateau before strain hardening and eventual rupture.

The Stress-Strain Curve

Understanding the standard tensile test for structural steel is fundamental to grasping both ASD and LRFD design philosophies. The idealized stress-strain curve consists of four distinct regions:

Elastic Region: Stress is directly proportional to strain (Hooke's Law). The slope of this line is the Modulus of Elasticity ( $E = 200,000 \text{ MPa}$ ). If the load is removed, the specimen returns to its original length.
Yielding (Plastic) Plateau: Once the stress reaches the Yield Stress ( $F_y$ ), the steel begins to deform plastically (permanently) without any significant increase in applied stress. This immense ductility allows steel structures to absorb massive amounts of energy.
Strain Hardening: After substantial yielding, the internal crystalline structure of the steel reorganizes, and it begins to resist additional stress again until it reaches its Ultimate Tensile Strength ( $F_u$ ).
Necking and Rupture: Beyond $F_u$ , the cross-sectional area of the specimen rapidly decreases (necks down), leading to a drop in engineering stress until final fracture occurs.

Checklist

Yield Stress ( $F_y$ ): The boundary of the elastic region. This is the most critical property for determining the nominal strength of a member against yielding failure modes.
Ultimate Tensile Strength ( $F_u$ ): The absolute maximum stress the steel can withstand before rupturing. This governs fracture failure modes (e.g., net section rupture).
Modulus of Elasticity ( $E$ ): $200,000 \text{ MPa}$ ( $29,000 \text{ ksi}$ ). Constant for all structural grades. Governs stiffness and buckling calculations.
Ductility: The ability of steel to undergo massive plastic deformation before failure. This property is vital for redistributing local stress concentrations safely.

Toughness and Brittle Fracture

Charpy V-Notch (CVN) Toughness

While yielding (

F_y

) is ductile, steel can fail via sudden, brittle fracture under certain conditions (low temperatures, high strain rates like impact loads, and severe stress concentrations like notches).

Toughness: The ability of steel to absorb energy and deform plastically before fracturing. It is measured using the Charpy V-Notch (CVN) impact test.
Transition Temperature: Steel transitions from ductile behavior at room temperature to brittle behavior at lower temperatures. Structural designs in cold environments or those subjected to dynamic loading require specific minimum CVN energy absorption values at specified testing temperatures.

Key Takeaways

Yield Stress ( $F_y$ ) is the primary parameter for design, while Ultimate Tensile Strength ( $F_u$ ) dictates failure.
The Modulus of Elasticity ( $E$ ) is constant for all structural steel at approximately $200,000 \text{ MPa}$ .
High ductility allows steel structures to absorb immense energy before failure.

Common ASTM Steel Grades

Structural steel is classified by the American Society for Testing and Materials (ASTM). The NSCP adopts these specifications for structural design in the Philippines.

ASTM Grades in Structural Design

A36: The traditional carbon structural steel. $F_y = 36 \text{ ksi}$ ( $250 \text{ MPa}$ ), $F_u = 58-80 \text{ ksi}$ ( $400-550 \text{ MPa}$ ). Commonly used for plates, angles, and channels.
A992: The modern standard for wide-flange (W) shapes. It is a high-strength, low-alloy steel. $F_y = 50 \text{ ksi}$ ( $345 \text{ MPa}$ ), $F_u = 65 \text{ ksi}$ ( $450 \text{ MPa}$ ). It offers superior strength and guaranteed weldability compared to A36.
A500: The standard specification for cold-formed welded and seamless carbon steel structural tubing (HSS - Hollow Structural Sections) in round, square, and rectangular shapes. Typically $F_y = 42 \text{ ksi}$ to $50 \text{ ksi}$ .
A572 (Grade 50): High-strength low-alloy columbium-vanadium structural steel. Often used for heavy plates and shapes where A992 is not applicable.

Key Takeaways

ASTM A992 ( $F_y = 50 \text{ ksi}$ ) is the standard grade for Wide-Flange (W) shapes in modern construction.
ASTM A36 ( $F_y = 36 \text{ ksi}$ ) remains standard for plates and miscellaneous sections.
ASTM A500 governs cold-formed Hollow Structural Sections (HSS).

Built-Up Sections and Plate Girders

When standard rolled shapes are insufficient for the required loads or span, designers resort to built-up sections. These are massive beams or columns fabricated by welding together separate steel plates.

Built-Up Members

Built-up members, particularly Plate Girders, allow the designer to tailor the cross-section perfectly to the bending moment envelope. The flanges can be made extremely thick at the mid-span (where bending is highest) and thinner near the supports. The web can be made incredibly deep to resist massive shear forces. However, these extremely deep, thin webs become highly susceptible to web buckling and require the use of intermediate transverse stiffeners (vertical plates welded to the web).

Standard Steel Shapes

Steel is hot-rolled into specific cross-sectional shapes optimized for various structural purposes. The nomenclature is standardized (e.g., W14x90).

Checklist

W-Shapes (Wide-Flange): The workhorse of steel construction. They have parallel inner and outer flange surfaces. The nomenclature W $14\times90$ means a wide-flange shape approximately 14 inches deep, weighing 90 pounds per linear foot. Used for both beams and columns.
S-Shapes (Standard I-Beams): Older shapes with sloped inner flange surfaces. Less efficient in bending than W-shapes and harder to connect to due to the sloped flanges.
C-Shapes (Channels): A web with two flanges on one side. Prone to twisting if not loaded through their shear center. Often used as purlins, girts, or built-up members.
L-Shapes (Angles): Two legs intersecting at 90 degrees (equal or unequal legs). Primarily used for tension members, light bracing, and bolted connections.
HSS (Hollow Structural Sections): Square, rectangular, or round tubes. Excellent torsional resistance and architectural appeal, making them ideal for exposed columns and truss members.

Key Takeaways

Standard steel shapes are optimized for specific structural functions (e.g., W-shapes for major beams and columns, HSS for columns with multi-directional loading).
Nomenclature (e.g., W $14\times90$ ) defines the shape type, approximate depth, and weight per foot.

Design Philosophies (ASD vs. LRFD)

The NSCP/AISC manual provides two parallel, equally acceptable methodologies for structural steel design: Allowable Strength Design (ASD) and Load and Resistance Factor Design (LRFD).

ASD vs. LRFD

Allowable Strength Design (ASD): A deterministic approach. The required strength ( $R_a$ ) is calculated using unfactored (service) load combinations. The available strength ( $R_n/\Omega$ ) is the nominal strength of the member divided by a single safety factor ( $\Omega$ , typically 1.67 for yielding).
$R_a \le R_n / \Omega$
Load and Resistance Factor Design (LRFD): A probabilistic approach. The required strength ( $R_u$ ) is calculated using statistically factored load combinations (e.g., $1.2D + 1.6L$ ) to account for the variability of loads. The available strength ( $\phi R_n$ ) is the nominal strength multiplied by a resistance factor ( $\phi$ , typically 0.90 for yielding) to account for variations in material strength and dimensions.
$R_u \le \phi R_n$

Degradation and Failure Mechanisms

While steel is non-combustible and incredibly strong, it must be protected from extreme heat and environmental degradation to maintain structural integrity.

Steel Degradation Mechanisms

Fire Exposure: Structural steel begins to lose stiffness ( $E$ ) and yield strength ( $F_y$ ) around $400^\circ\text{C}$ ( $750^\circ\text{F}$ ) and loses roughly 50% of its strength by $600^\circ\text{C}$ . To prevent premature collapse in a fire, steel members are protected using intumescent paint (which swells and insulates when heated), spray-applied fire-resistive materials (SFRM), or encasement in concrete or gypsum board.
Corrosion: Unprotected steel oxidizes (rusts) when exposed to oxygen and moisture, gradually losing cross-sectional area and strength. Corrosion is mitigated through protective coatings (primers, epoxy paints), galvanizing (zinc coating), or specifying weathering steel (like ASTM A588) that forms a stable, protective patina.

Lamellar Tearing

Through-Thickness Weakness

Lamellar tearing is a unique structural failure mode that occurs in highly restrained welded joints (like thick plate T-joints or corner joints).

Because steel is hot-rolled, microscopic non-metallic inclusions are flattened into planes parallel to the surface.
While strong longitudinally and transversely, the steel is significantly weaker in its "through-thickness" (Z-axis) direction.
When large, heavily restrained welds shrink as they cool, they pull violently on the base metal in the Z-axis, causing the base metal to tear underneath the weld in a step-like pattern.
Mitigation: Designing joints to avoid through-thickness shrinkage stresses, using lower-strength weld metal, or specifying steel with improved Z-axis properties.

Key Takeaways

Structural steel features a high strength-to-weight ratio, massive ductility, and a constant elastic modulus ( $E \approx 200,000 \text{ MPa}$ ).
ASTM A992 ( $F_y = 50 \text{ ksi}$ ) is the modern standard for W-shapes.
W-shapes are the most efficient and common sections for primary framing (beams and columns).
ASD relies on unfactored loads and a single safety factor ( $\Omega$ ).
LRFD utilizes statistically factored loads and resistance factors ( $\phi$ ) to provide a more uniform reliability index across different load combinations.

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