Steel and Metal Selection for Engineering Design: Aluminum Association Standard Structural Shapes

Aluminum Association Standard Structural Shapes

Lightweight, Corrosion-Resistant Alternatives

When corrosion resistance, weight savings, or conductivity requirements rule out carbon steel, aluminum structural sections fill the gap. Available in 6061-T6 alloy — which offers a good balance of strength, weldability, and machinability — these shapes mirror the steel section families.

Aluminum I-Beams

Aluminum Channels

Wire and Sheet-Metal Gages — Where the practitioner's Nightmare Began

The Story Resumes

Remember the practitioner? His purchasing manager confused two different gage systems. The truth is, that mistake is shockingly easy to make. There are at least six major wire gage systems and four sheet-metal gage systems in active use around the world. None of them agree with each other. And gage numbers are not self-explanatory — a "16 gage" wire can be anywhere from 0.0508 to 0.0650 inches depending on which system you use.

The only safe practice: specify dimensions in decimal fractions of an inch (or millimeters), not gage numbers. Gage numbers are gradually being discarded, and the decimal system is now being used quite generally. But you will encounter gage numbers on legacy drawings, supplier catalogs, and older specifications — so you need to know how to decode them.

Wire Gage Systems — A Quick Decoder

United States Steel Wire Gage: Used by practically all steel producers in the United States. Also known as the the practitioner and a legacy wire-gage reference, the source wire-gage reference, and a legacy wire-gage reference Wire Gages.

American Wire Gage (AWG) / a legacy wire-gage reference: Formerly universal for copper and aluminum wire in the United States. Now specified in decimal fractions.

British Standard Wire Gage (S.W.G.) / Imperial Wire Gage: Legalized in Great Britain. Used for most sheet-metal products there.

Music (Piano) Wire Gage: Used specifically for music spring steel wire — one of the highest quality types used for mechanical springs.

Birmingham / Stub's Iron Wire Gage: Nearly obsolete. The only gage recognized by Acts of Congress, but rarely used in practice.

Stub's Steel Wire Gage: Used for drill rod and specialty steel wire applications.

Wire Gage Comparison Table (Decimal Inches)

Critical Observation: Look at gage number 16 across all systems. The AWG gives 0.0508", the US Steel Wire Gage gives 0.0625", the British S.W.G. gives 0.0640", and Birmingham gives 0.0650". That is a spread of over 27% — more than enough to cause a structural failure or a procurement disaster.

Gages for Tubing Wall Thickness

At one time, the Birmingham or Stub's Iron Wire Gage was used to specify wall thickness for seamless brass, seamless copper, seamless steel, and aluminum tubing. The a legacy wire-gage reference Wire Gage was used for brazed brass and brazed copper tubing.

Current best practice: Wall thicknesses are now specified by decimal parts of an inch, though steel pressure tubes and mechanical tubing may still reference the Birmingham gage. In Great Britain, the Standard Wire Gage (S.W.G.) is used for some steel tube specifications.

Rule of thumb: If a tubing spec gives you a gage number, convert it to decimal immediately and verify against the correct gage system before ordering.

Sheet-Metal Gages

The System That Almost Nobody Fully Understands

Sheet-metal gaging is where the practitioner's story started — and where confusion remains most persistent. Here is the landscape:

Manufacturers' Standard Gage for Sheet Steel: Based on a weight of 41.82 pounds per square foot per inch of thickness. This is the standard for steel sheets in the United States.

United States Standard Gage (1893): Established by Congress. Based on 40 pounds per square foot per inch of thickness — the weight of wrought iron, not steel. Because the thickness equivalents were derived from wrought-iron weights, they are not correct for steel. This older gage is a frequent source of confusion.

Birmingham Gage (B.G.): Used in Great Britain for black iron/steel sheet, hooping, and galvanized flat/corrugated steel sheet. Legalized in 1914. Not the same as the Birmingham Wire Gage.

Sheet-Metal Gage Comparison (Decimal Inches)

Notice the Galvanized Sheet column. Galvanized sheets are thicker at the same gage number than bare steel sheets because the gage accounts for the zinc coating weight. A 16-gage galvanized sheet is 0.0635" while a 16-gage bare steel sheet is 0.0598" — if you design for one and receive the other, your bend allowances, blank sizes, and weights will all be wrong.

Preferred Thicknesses for Uncoated Flat Metals

The ANSI Standard That Brings Order to Chaos

American National Standard ANSI B32.1-1952 (R1994) establishes preferred thicknesses for uncoated thin flat metals and alloys under 0.250 inches. These are based on the 20-series and 40-series of preferred numbers (ANSI Z17.1), which create a systematic geometric progression.

• 20-series: Each thickness is approximately 12% greater than the next smaller one
• 40-series: Each thickness is approximately 6% greater than the next smaller one
• 80-series: For intermediate thicknesses when the 40-series is insufficient

Preferred Thicknesses — Under 0.250 Inch (ANSI B32.1)

Material-specific notes:

• Copper and copper-base alloys: Below 1⁄4 inch, specify by the 20-series preferred numbers
• Zinc sheets: Usually ordered by decimal thickness, though a zinc gage exists
• Aluminum: Formerly designated by the a legacy wire-gage reference Wire Gage, now specified in decimals or fractions

Preferred Metric Thicknesses for All Flat Metal Products

The Global Standard for Metric Sizing

American National Standard ANSI/ASME B32.3M-1984 establishes preferred metric thicknesses, widths, and lengths for flat metal products of rectangular cross section. Use the Preferred Thickness column first; choose Second or Third Preference only when no suitable Preferred size is available.

Important caveat: Not all metals and grades are produced in each size listed. Always consult producers or distributors to determine whether a particular product and size combination is available.

Improvement method and result

Six months after the Harborview disaster, the practitioner's shop had the lowest error rate of any fabricator in the region. Here is what changed:

1. Decimal-Only Procurement. Every material order now specifies thickness, width, and length in decimal inches or millimeters — never gage numbers alone. Gage numbers appear only as reference in parentheses.

2. The Gage Verification Protocol. When any gage number appears on an incoming spec, the estimating team cross-references it against the correct gage system for that specific material before it reaches purchasing. A laminated copy of the wire and sheet-metal gage tables lives at every workstation.

3. Section Property Verification. Before finalizing any structural design, the engineering team pulls section properties from the AISC Manual (or this reference guide) and verifies that the actual geometry — not just the nominal designation — meets the design requirements. A W 14 × 730 and a W 14 × 22 share a nominal depth designation but could not be more different in every other dimension.

4. The Three-Signature Rule. Material orders above a threshold value require sign-off from the designer, the project engineer, and the purchasing agent — with each confirming the material specification independently.

the practitioner never lost money to a gage confusion error again.

Your Next Step: Build Your Own Verification System

You now hold a comprehensive reference covering every major rolled steel section, wire gage, sheet-metal gage, and preferred thickness standard used in structural engineering and manufacturing. But having the data is only half the battle. Using it systematically is what separates the shops that thrive from the ones that survive.

Here is what you should do right now:

• Print the gage comparison tables and post them where material specifications are written and orders are placed
• Establish a decimal-first policy for all material ordering — gage numbers for reference only
• Verify section properties against actual geometry, not nominal designations
• Cross-reference unfamiliar gage systems before accepting any specification at face value

The difference between a gage number in one system and the same number in another system can be 15%, 20%, even 30%. In structural applications, that margin can be the difference between a building that stands and one that does not.

What verification system does your shop or firm use to prevent gage and specification errors? What is the most expensive material ordering mistake you have witnessed — and what process change prevented it from happening again?

Data in this guide is taken from the Manual of Steel Construction, 8th Edition, 1980, with permission of the American Institute of Steel Construction; the Aluminum Association; and relevant ANSI/ASME standards. Always verify against the latest edition of applicable standards for active design work.

Why Material Properties Are the Foundation of Every Manufacturing Decision

Before you select a process, before you program a toolpath, before you write a purchase order—you need to understand what the material will do when you subject it to heat, pressure, electrical discharge, or chemical attack.

Material properties govern:

• Whether a casting cracks or holds under impact loading
• Whether an EDM electrode lasts one pass or fifty before corner wear destroys detail
• Whether a weld joint matches the base metal in strength, or becomes the weakest link
• Whether a hard-facing deposit survives the abrasion, corrosion, or temperature it was designed to resist
• Whether a laser cut is clean or leaves a heat-affected zone that compromises the part

Every section that follows is a deep dive into the material properties that control outcomes in each of these manufacturing domains. Bookmark it. Print it. Tape it to the wall above your machine.

Cast Iron — The Oldest Engineering Material You Still Can't Ignore

The Four Pillars of Cast Iron

Cast irons and cast steels form one of the largest families of ferrous alloys in manufacturing. In general, cast irons contain more than 2% carbon and 1 to 3% silicon. The mechanical and physical properties depend strongly on two things: the shape and distribution of free graphite and the type of matrix surrounding the graphite particles.

The four basic types are white iron, gray iron, malleable iron, and ductile iron. Understanding what separates them is the difference between a casting that performs for decades and one that shatters on its first day.

Gray Cast Iron: The Workhorse

Composition: 1.7–4.5% carbon, 1–3% silicon

Gray iron gets its name from the dark-colored fracture surface produced by excess carbon in the form of graphite flakes. Those flakes are both its superpower and its limitation—they provide outstanding vibration damping and machinability, but they also act as stress concentrators that limit tensile strength.

The ANSI/ASTM A48-76 Classification System:

The prefix number directly indicates minimum tensile strength in thousands of psi. Class 20 = 20,000 psi minimum. Class 60 = 60,000 psi minimum.

High-strength gray irons produced by the Meehanite-controlled process offer specialized variants: general engineering types, heat-resisting types, wear-resisting types, and corrosion-resisting types.

White Cast Iron: Pure Hardness, Zero Forgiveness

When nearly all the carbon exists in the combined or cementite form (iron carbide), you get white cast iron—named for its silvery-white fracture.

Critical properties to remember:

• Ductility: practically zero
• Compressive strength: usually exceeds 200,000 psi (compare to gray iron's 65,000–160,000 psi)
• Impact resistance: lower than gray iron
• Design constraint: sharp corners and thin sections cause foundry failures

Most white iron castings serve either as maximum wear-resistance applications or as the starting material for producing malleable iron through heat treatment.

Chilled Cast Iron: The Best of Both Worlds

Here is a concept that solves a problem many engineers face: you need a gray iron casting for its machinability and damping, but you need wear-resistant surfaces in specific areas. Chilled cast iron delivers exactly this by placing metal chills in the mold at locations where rapid cooling will produce a white iron surface layer, while the bulk of the casting remains gray iron.

Alloy Cast Iron: Engineered Properties

Add nickel, chromium, molybdenum, copper, or manganese in sufficient quantities to gray iron and you fundamentally change its character. Alloy cast irons serve demanding applications:

• Automotive: cylinders, pistons, piston rings, crankcases, brake drums
• Machine tools: structural castings requiring vibration damping with higher strength
• Dies: forming and stamping dies requiring heat resistance
• Heavy industry: crushing and grinding machinery, high-temperature scaling resistance

Machinable alloy cast irons can achieve tensile strengths up to 70,000 psi or higher.

Malleable Iron: Toughness from Transformation

Malleable iron is produced by annealing (graphitizing) white iron castings. The process converts cementite into temper carbon—graphite in the form of compact, rounded aggregates rather than flakes. This microstructural change delivers strength, ductility, machinability, and shock resistance that white iron alone cannot provide.

Pearlitic Malleable Iron (ASTM A 220-79) — Tension Test Requirements:

Notice the inverse relationship: as strength increases, elongation decreases. The grade numbering system itself encodes this: Grade 40010 means 40 ksi yield, 10% elongation. Grade 90001 means 90 ksi yield, 1% elongation. This tradeoff is fundamental to all metallurgy.

Applications of pearlitic malleable iron: axle housings, differential housings, camshafts, crankshafts, machine parts, ordnance equipment, and tools.

Ductile Cast Iron (Nodular / Spheroidal Graphite Iron)

This is the cast iron that changed the game. By adding small amounts of magnesium- or cerium-bearing alloys with special processing, the graphite forms spheroidal (ball-like) nodules instead of flakes. The result: a casting with the castability and machinability of gray iron but with dramatically superior mechanical properties.

ASTM A 536-80 — Five Standard Grades:

Grade nomenclature: The three numbers represent minimum tensile strength, 0.2% yield strength, and percent elongation in 2 inches. Grade 60-40-18 means 60,000 psi tensile, 40,000 psi yield, 18% elongation.

Key capabilities of ductile iron:

• Toughness intermediate between cast iron and steel
• Shock resistance comparable to mild carbon steel
• Can be surface hardened by flame, induction, or chilling methods
• Machines with the same ease as gray cast iron
• Weldable and brazeable
• Pressure tight under high stress

Industrial applications: crankshafts, pistons, and cylinder heads (automotive); forging hammer anvils, cylinders, guides, and control levers (heavy machinery); wrenches, clamp frames, face-plates, chuck bodies, and dies (tooling).

Steel Castings — When Nothing Else Is Strong Enough

Why Steel Castings Exist

Steel castings fill a critical niche: they handle shocks and heavy loads that would destroy cast iron, wrought iron, or malleable iron. They are stronger and tougher than all three. Unlike wrought steel, steel castings do not exhibit directionality effects, meaning properties are consistent regardless of orientation.

Production methods: open-hearth, electric arc, side-blow converter, or electric induction furnaces. The choice depends on production volume—open-hearth for large tonnages, electric arc for small lots with widely differing analyses, high-frequency induction for small quantities of expensive specialty compositions.

Typical applications: hydroelectric turbine wheels, forging presses, gears, railroad car frames, valve bodies, pump casings, mining machinery, marine equipment, engine casings.

Carbon Steel Castings: The Three Classes

Structural and Engineering Grades — Complete Mechanical Properties

Structural Grades of Carbon Steel Castings:

Engineering Grades of Low-Alloy Steel Castings: