Steel Section Properties Explained

A practical guide to understanding structural steel section types, their geometric properties, and how to choose the right section for beams, columns, and truss members. StructureCalcs includes 2,600+ sections from six international standards.

What Are Steel Sections?

Structural steel sections are standardised cross-sectional shapes produced by hot rolling or cold forming steel. Rather than designing a unique cross-section for every beam and column, engineers select from a catalog of pre-defined shapes manufactured to tight tolerances. This standardisation dramatically reduces fabrication cost and lead time, and ensures predictable structural performance.

Each section shape is optimized for a specific structural role. I-shaped sections (also called H-shapes or wide flanges) are efficient in bending because they place material far from the neutral axis. Hollow sections are efficient in compression and torsion because they distribute material evenly around the centroid. Angles are used for bracing and connections where simplicity is more important than structural efficiency.

The StructureCalcs calculators include a built-in library of over 2,600 section profiles from six international standards: AS4100 (Australia), AISC (USA), EN10365 (Europe), BS4 (UK), JIS (Japan), and IS808 (India). When you select a section, all relevant properties are loaded automatically, eliminating manual data entry errors.

I-Beams and Wide Flanges (UB, UC, W, IPE, HEA)

I-shaped sections are the workhorse of structural steel construction. They consist of two parallel flanges (the horizontal plates at top and bottom) connected by a vertical web. The flanges resist bending moment (one in compression, the other in tension), while the web resists shear force.

Universal Beams (UB) — also called wide flange W shapes in American practice or IPE in European practice — are deeper than they are wide. The large depth gives them a high second moment of area (Ix) about the strong axis, making them very efficient as beams. A 310UB40.4 (an Australian section weighing 40.4 kg/m) has Ix = 86.4 × 10⁶ mm⁴, allowing it to span significant distances with minimal deflection.

Universal Columns (UC) — also called wide flange W shapes with flange widths approximately equal to the depth, or HEA/HEB in European practice — are stockier. Their wider flanges give a larger Iy (second moment of area about the weak axis) and radius of gyration, which improves resistance to column buckling. A 310UC96.8 has nearly the same depth as a 310UB40.4 but weighs more than twice as much because its flanges are much wider.

When to use I-beams: Floor beams, roof beams, lintels, transfer beams, crane runway beams, and any member that primarily resists bending about one axis. Use UB shapes for beams (maximise Ix per kg) and UC shapes for columns (maximize weak-axis resistance).

Figure 1.A universal beam at 310UB40.4 nominal dimensions, with its centroid marked. Ix = 84.3×10⁶ mm⁴ against Iy = 7.6×10⁶ mm⁴ — the same steel, 11× stiffer one way than the other.

Channels (PFC, C, MC, UPE)

Channel sections have a C-shaped cross-section: a web with flanges projecting from one side only. They are called Parallel Flange Channels (PFC) in Australian/British practice and C or MC shapes in American practice.

The key characteristic of channels is their asymmetry. The centroid and the shear center do not coincide, and both are offset from the web. This means that a load applied through the centroid will cause the channel to twist unless it is restrained laterally. For this reason, channels are often used in pairs (back-to-back to form a symmetric section), or they are connected to other elements that provide torsional restraint.

When to use channels: Purlins and girts (roof and wall framing members), stair stringers, edge beams, light framing, and as components of built-up sections. Their flat back makes them easy to bolt to walls and columns. In trusses, back-to-back channels are sometimes used for chord members.

Figure 2.A 250 PFC. The centroid sits toward the web rather than at the middle of the flanges, which is what makes a channel asymmetric to work with.

Hollow Sections (RHS, SHS, CHS)

Hollow structural sections (HSS) come in three shapes: rectangular (RHS), square (SHS), and circular (CHS). They are formed by rolling flat steel plate into a tube and welding the seam.

Hollow sections have several unique advantages over open sections:

Excellent compression resistance: The closed shape distributes material evenly around the centroid, giving a high radius of gyration in all directions. This eliminates the weak-axis buckling problem that plagues open sections. CHS and SHS have the same radius of gyration about every axis.
Superior torsional resistance: The closed cross-section resists twisting much better than I-beams or channels. This is critical for members subjected to eccentric loads or lateral forces.
Aesthetic appearance: The clean, enclosed shape is visually appealing for exposed structural members in architectural applications.
Reduced maintenance: The enclosed shape has less surface area to paint and no internal surfaces where moisture can collect, reducing corrosion maintenance.

When to use hollow sections: Columns (especially where buckling about both axes governs), truss members (particularly compression members), architectural exposed steelwork, outdoor structures, and handrails. In the Truss Calculator, hollow sections are ideal for member properties because their high radius of gyration prevents buckling in compression members.

Figure 3.A 150×150×6 SHS: Ix = Iy = 12.0×10⁶ mm⁴, so it has no weak axis to get wrong.

Angles (EA, UA, L)

Angle sections have an L-shaped cross-section with two legs meeting at a right angle. Equal angles (EA or L) have both legs the same length; unequal angles (UA) have one leg longer than the other.

Angles are the simplest and most economical steel sections. They are easy to connect using bolts through one leg, making them ideal for secondary structural members where speed and economy of fabrication are priorities. However, they are structurally inefficient because of their highly asymmetric shape — the principal axes are rotated 45° from the leg directions, and they have a very low radius of gyration about the minor principal axis.

When to use angles: Bracing members in trusses and frames (where they carry primarily axial force), lintels over small openings, shelf angles supporting masonry or cladding, connection components (cleats, gussets), and light framing. In truss analysis, single or double angles are commonly used as web members.

Figure 4.A 150×90×10 unequal angle with its principal axes drawn — rotated 20.1° from the legs. Its minimum I is 0.9×10⁶ mm⁴ about that rotated axis, and that is the value buckling cares about.

Key Section Properties: Ix and Iy

The second moment of area (I), also called the moment of inertia, is the single most important property for beam design. It measures how effectively the cross-section resists bending.

Ix (about the strong axis) determines the beam’s resistance to bending in the vertical plane (the usual loading direction). A beam loaded vertically deflects less and develops lower stresses when Ix is large. Mathematically, I = ∫y² dA — it is the sum of each tiny area element multiplied by the square of its distance from the neutral axis. Material far from the neutral axis contributes much more than material near it.

Iy (about the weak axis) determines resistance to bending in the horizontal plane and is critical for lateral-torsional buckling checks and column design. For I-beams, Iy is typically 5 to 20 times smaller than Ix, reflecting the section’s high efficiency about one axis.

Practical implication: If your beam deflects too much, you need a section with higher Ix. Doubling the beam depth roughly quadruples Ix because of the y² relationship. This is why deep, lightweight beams (like 530UB82 or W21x44) are often more efficient than shallow, heavy ones (like 310UC96.8) when used as floor beams.

Key Section Properties: the elastic and plastic section moduli

The section modulus links the bending moment to the maximum bending stress in the cross-section. There are two versions:

You are reading AISC 360 notation, which names S the elastic modulus and Z the plastic one. ⚠ Australian (AS 4100) and Japanese tables use these two letters the other way round, so if you are reading one of those, swap them. Change the design code at the top of the page and every symbol below follows.

Elastic section modulus, S: Equals I divided by the distance from the neutral axis to the extreme fiber (ymax). The maximum elastic bending stress is σ = M / S. The beam first yields when this stress reaches the material yield strength fy. A larger value means the beam can carry a larger moment before yielding. (Written Z in AS 4100, S in AISC, W_el in Eurocode and Z_e in IS 800.)

Plastic section modulus, Z: Represents the full plastic capacity of the cross-section, where the stress has reached fy everywhere (above and below the plastic neutral axis). The plastic moment capacity is Mp = fy × Z. For I-sections it is typically 10–15% larger than the elastic modulus, and their ratio is the shape factor. Modern design codes (AS 4100, AISC LRFD, Eurocode 3) use the plastic section modulus for compact sections. (Written S in AS 4100, Z in AISC, W_pl in Eurocode and Z_p in IS 800.)

Practical implication: When sizing a beam for strength you need the plastic modulus Z to be at least the design bending moment divided by the product of the capacity reduction factor and the yield strength — M* / (φ fy) in AS 4100, Mu / (φ fy) in AISC LRFD. Look it up in the section property tables, or draw or place the section in the Section Calculator, which computes both moduli, and verify this inequality.

Cross-Sectional Area and Radius of Gyration

Cross-sectional area (A) is the total area of the steel cross-section, measured in mm² or in². It determines:

Axial capacity: For truss members and columns, the maximum axial force before yielding is Ny = fy × A.
Self-weight: Mass per unit length = A × steel density (7,850 kg/m³). This determines the beam’s self-weight load.
Stiffness under axial load: Axial stiffness = EA/L, used in truss stiffness matrices.

Radius of gyration (r) equals √(I/A) and is measured in mm or inches. It is the key property for column buckling design. The slenderness ratio of a column is L/r, where L is the effective buckling length. A higher slenderness ratio means the column is more prone to buckling and has lower capacity.

For columns, the critical radius of gyration is typically ry (about the weak axis), because buckling occurs about the axis with the lowest I (and hence lowest r). This is why UC sections (with wider flanges and higher ry) are preferred as columns over UB sections (which have low ry due to narrow flanges).

In the Truss Calculator, the cross-sectional area A is the primary section property because truss members carry axial force only. When you select a steel section, A is automatically populated.

Choosing the Right Section: A Decision Framework

Selecting the appropriate steel section depends on the member’s structural role:

Floor beams (bending governs): Use UB / W / IPE shapes. Maximize Ix per kg of steel. Start with the lightest section that satisfies the deflection limit, then verify moment and shear capacity.
Columns (buckling governs): Use UC / HEA / HEB shapes or SHS/CHS. Maximize the minimum radius of gyration. For short columns (low slenderness), almost any shape works; for tall slender columns, CHS or SHS are most efficient.
Truss chord members (axial + some bending): Use UB, double angles, or hollow sections depending on the truss type. For heavy trusses, UB chords are common; for lighter trusses, hollow sections are preferred.
Truss web members (axial only): Use angles (for economy), CHS/RHS (for compression efficiency), or flat bars (for tension-only members like bracing).
Bracing (tension only): Use angles, rods, or flat bars. Since there is no buckling risk, the lightest section that provides enough area is optimal.

The Beam Calculator and Truss Calculator let you rapidly compare sections by changing the selection dropdown and watching the results update instantly.