Structural Steel

Structural steel is the backbone of commercial construction. Its extraordinary strength-to-weight ratio, ductility, predictability, and recyclability make it the material of choice for buildings, bridges, towers, and industrial structures worldwide. Steel is manufactured under strict quality controls, so its properties are highly consistent and predictable — an engineer can rely on steel behaving exactly as expected.

This lesson covers steel types, grades, shapes, properties, connections, and protection systems.

Steel is an alloy of iron and carbon (typically 0.2-2.0% carbon by weight). The manufacturing process:

1. Iron ore, coke (processed coal), and limestone are charged into a blast furnace and heated to approximately 2,800°F (1,540°C). The result is pig iron — iron with a high carbon content and impurities.
2. Steelmaking: Pig iron is refined in a basic oxygen furnace (BOF) or electric arc furnace (EAF) to reduce carbon content and remove impurities. The BOF process uses virgin iron ore; the EAF process melts recycled scrap steel. Today, approximately 70% of U.S. structural steel is produced from recycled scrap in EAFs — making steel one of the most recycled materials in the world.
3. Casting: Molten steel is cast into semi-finished shapes (slabs, billets, blooms) through continuous casting.
4. Rolling: Semi-finished shapes are reheated and passed through rolling mills to produce final shapes — wide-flange beams, channels, angles, plate, bar, and tube.

Steel grade designates the chemical composition and mechanical properties of the steel. The most common structural steel grades in the U.S. are designated by ASTM standards:

Grade	Fy (Yield Strength)	Fu (Tensile Strength)	Common Use
A36	36 ksi	58-80 ksi	Plates, bars, angles, channels. The historic "all-purpose" structural steel.
A992	50 ksi	65 ksi (min)	Wide-flange (W) shapes. The current standard for W shapes. Replaced A36 for most beams and columns.
A500 Grade B	42 ksi (round) / 46 ksi (rectangular)	58 ksi	Hollow Structural Sections (HSS) — round, square, and rectangular tube.
A500 Grade C	46 ksi (round) / 50 ksi (rectangular)	62 ksi	HSS — higher strength grade.
A572 Grade 50	50 ksi	65 ksi	Plates, bars, shapes. Similar to A992 but for non-W shapes.
A588	50 ksi	70 ksi	Weathering steel (Cor-Ten). Forms a protective rust patina and does not require painting in many applications.

Key concept: Higher yield strength (Fy) allows designers to use smaller, lighter members, saving material and cost. The shift from A36 (Fy = 36 ksi) to A992 (Fy = 50 ksi) for wide-flange shapes increased design efficiency by nearly 40%.

Steel is produced in standard cross-sectional shapes, each optimized for specific loading conditions:

Wide-Flange (W Shapes)

The most common structural shape. W shapes have a cross-section shaped like the letter "H" or "I" — two flanges connected by a web. They are efficient at resisting bending and are used as beams, columns, and girders.

Designation: W followed by nominal depth and weight per foot. Example: W14×30 means a wide-flange shape approximately 14 inches deep weighing 30 pounds per linear foot.

The AISC Steel Construction Manual contains tables listing the dimensions and properties of every standard W shape — hundreds of sizes ranging from W4×13 to W44×335.

HSS (Hollow Structural Sections)

Square, rectangular, and round tubes. HSS are efficient for columns (resist buckling equally in all directions for round/square) and for architectural applications where a clean appearance is desired.

Designation: HSS followed by dimensions and wall thickness. Example: HSS 8×8×1/2 means a square tube 8 inches wide with a 1/2-inch wall.

Channels (C and MC Shapes)

C-shaped sections, used for light framing, stair stringers, and lateral bracing. One flange projects from the web, making them useful where a flat surface is needed on one side.

Angles (L Shapes)

L-shaped sections with equal or unequal legs. Used for bracing, connections, lintels, and framing. Designation: L followed by leg sizes and thickness. Example: L4×3×3/8 means an angle with a 4-inch leg and a 3-inch leg, 3/8-inch thick.

Plates

Flat steel in various thicknesses (typically 3/16" to 4" or more). Used for base plates, gusset plates, stiffener plates, and built-up members.

Bars

Round, square, or flat steel bars. Used for rebar, anchor bolts, hangers, and miscellaneous applications.

Steel structures are assembled from individual members that must be connected. The strength of a steel structure often depends as much on its connections as on its members.

Bolted Connections

High-strength bolts are the most common connectors in structural steel:

• A325 bolts (now F3125 Grade A325): 120 ksi tensile strength. The standard structural bolt.
• A490 bolts (now F3125 Grade A490): 150 ksi tensile strength. Higher capacity for heavier loads.

Bolt types by load transfer:

• Bearing-type connections: Bolts transfer load through shear and bearing against the bolt hole wall. More economical.
• Slip-critical connections: Bolts are tensioned to a very high clamping force so that load is transferred by friction between the connected plates. Required where slip would be unacceptable (connections subject to fatigue, oversized holes, or slots).

Welded Connections

Welding fuses steel members together by melting the base metals and adding filler metal. Common structural weld types:

• Fillet weld: A triangular weld placed in the corner between two surfaces. The most common structural weld — approximately 80% of all structural welds are fillets.
• Complete Joint Penetration (CJP) groove weld: The weld extends through the full thickness of the member. Used for critical moment connections (beam-to-column moment frames). More expensive and requires more inspection than fillet welds.
• Partial Joint Penetration (PJP) groove weld: The weld extends partially through the member thickness. Less expensive than CJP but with reduced capacity.

Structural welding must comply with AWS D1.1 (Structural Welding Code — Steel) and is performed by certified welders.

Steel corrodes (rusts) when exposed to moisture and oxygen. Protection options include:

• Paint systems: Primer + intermediate coat + finish coat. The most common protection for structural steel in buildings. Shop-applied primer protects steel during transportation and erection; finish coats are applied after erection.
• Hot-dip galvanizing: Immersing steel in molten zinc. Creates a thick, durable coating. Used for exposed exterior steel, handrails, lintels, and miscellaneous metals.
• Weathering steel (A588/A709): Forms a protective oxidation layer (patina) that arrests further corrosion. Eliminates the need for painting in many applications. Commonly used for bridges and architectural features.
• Metalizing: Spray-applied zinc or aluminum coating for aggressive environments.

Steel does not burn, but it loses strength rapidly at elevated temperatures. At 1,000°F (538°C), steel retains only about 60% of its room-temperature strength. At 1,300°F (704°C), it retains only about 20%.

Building codes require structural steel to be protected by fire-resistant materials in most buildings. Common fireproofing methods:

• Spray-applied fire-resistive material (SFRM): A cementitious or fiber-based material sprayed directly onto steel members. The most common and economical method for concealed steel. Thicknesses vary from 3/8" to 2" depending on the required rating.
• Intumescent coatings: Paint-like coatings that expand (intumesce) when exposed to heat, forming an insulating char layer. Used where the steel will be exposed (architectural applications).
• Concrete encasement: Encasing steel columns or beams in concrete. Extremely effective but heavy and expensive.
• Board systems: Gypsum or mineral fiber boards wrapped around steel members. Used where SFRM is not suitable.