Steel Channel Sizes — C-Shape and MC-Shape Section Properties

Steel channels (C-shapes and MC-shapes) are asymmetric sections used for purlins, girts, ledger members, framing rails, and as chord members in light trusses. This reference provides AISC dimension and property tables for standard channels (C) and miscellaneous channels (MC), with area, Ix, Sx, rx, Iy, and shear center location.

Standard C-Shape (American Standard Channel) Properties

Designation: C d × w (e.g., C12×20.7 = 12 in depth, 20.7 lb/ft)

eo = shear center distance from back of web. Channels are singly-symmetric — torsion develops when load does not pass through shear center.

Shear Center and Torsion in Channels

A key property of channels is the shear center located outside the cross-section, at distance eo from the back of the web. When a transverse load is applied through the centroid (not the shear center), torsion develops.

For a C-shape:
  eo ≈ bf²×tf / (2×Ix / d) for thin-walled approximation

Torsion moment: T = V × (eo - e_load)

Where e_load = distance from centroid to load application point

To avoid torsion:

1. Connect purlins at both flanges (Z-purlins are torsionally balanced)
2. Use sag rods to provide lateral support at mid-span
3. Apply loads through the shear center (difficult in practice)
4. Include St. Venant + warping torsion if torsion cannot be avoided

Channels as Purlins and Girts

Channels are widely used as roof purlins and wall girts in metal building systems:

Note: In most modern metal building systems, C/Z purlins are cold-formed sections (per AISI S100), not hot-rolled AISC channels. Check which standard applies to your project.

Back-to-Back Channels (Double Channel)

Two channels placed back-to-back (2C) act as an I-shaped section with excellent bending properties in both directions. Used for:

• Built-up beams where W-shapes are unavailable
• Truss top/bottom chords
• Built-up columns in light structures

Properties for 2C (back-to-back, 3/8 in gap):

Metric Channel Sizes — AS/NZS 3679.1 (Parallel Flange Channels)

Frequently Asked Questions

What is the difference between a C-shape and an MC-shape? C-shapes (American Standard channels) are produced to ASTM A36 with sloped inner flanges at approximately 16.67% taper. MC-shapes (Miscellaneous channels) are produced to A36 as well but have non-standard proportions — typically wider or deeper flanges than the standard C series for the same weight. MC channels offer more design flexibility.

Why do channels experience torsion when loaded? The shear center of a channel is located to one side of the web, outside the section. When a vertical load is applied to the centroid (which does not coincide with the shear center), a torsional moment develops equal to V × eccentricity. For uniform loading on a simply-supported purlin, the combined bending + torsion must be checked.

Can I use hot-rolled channels as purlins? Yes, but cold-formed Z-purlins (AISI S100) are generally more efficient for that application because their doubly-symmetric geometry about the strong axis reduces torsion. For heavy-duty purlins or where deflection is critical, hot-rolled C or W shapes may be appropriate.

What does the shear center distance eo mean and how does it affect design? The shear center is the point through which a transverse load must pass to produce bending without torsion. For C-shape channels, the shear center lies outside the web, at distance eo from the back face. Values of eo typically range from 0.40 to 0.80 inches for common sizes. When a uniformly distributed load acts on the top flange — as in a roof purlin carrying deck — the load eccentricity relative to the shear center generates a torsional moment that must be resisted by St. Venant torsion and warping torsion acting together. AISC Design Guide 9 provides the combined bending-torsion interaction approach for this case.

When should I use back-to-back channels instead of a W-shape? Back-to-back channels (2C) are typically used when W-shapes of the required weight class are not available or when the connection geometry favors a double-channel built-up section. They are common as truss chord members, built-up lintels over large openings, and transfer beams in retrofit work. A 2C built-up section has approximately twice the Ix of a single channel and provides a flat bearing surface on both sides, which simplifies framing connections. However, the fabrication cost of stitching two channels together usually exceeds the material cost difference compared to a rolled W-shape, so W-shapes are preferred when available in the required size range.

What is the section modulus difference between the x-axis and y-axis for a channel? Channels are highly asymmetric about the y-axis. For a C8×11.5, Ix = 32.6 in⁴ and Iy = 1.32 in⁴ — a ratio of approximately 25:1. This means channels are far stiffer bending about their strong axis than their weak axis. In practice this limits channel use to applications where strong-axis bending dominates and weak-axis stability is provided by continuous lateral support from decking or bridging. For biaxial bending applications, W-shapes, HSS tubes, or back-to-back channel assemblies are more efficient because they provide better weak-axis stiffness.

Run This Calculation

→ Beam Capacity Calculator — moment and shear capacity for C-shape or MC-shape channel beams per AISC 360.

→ Section Properties Calculator — Ix, Sx, rx, Iy, Sy, and shear center location for any channel size in this table.

Related Calculators and References

• Steel Angle Sizes — L-shape section properties for bracing and framing
• Steel Section Types — when to use channels vs. other sections
• Beam Capacity Calculator — verify moment and shear capacity for your channel size
• Steel Beam Span Guide — span ranges for roof and floor framing
• structural steel weight per foot table
• Steel Roof Framing Reference
• Wide Flange Beam Sizes
• steel member weight calculator
• Section Comparison Reference

Section properties from AISC Steel Construction Manual 16th Ed., Part 1. Shear center locations are approximate. For combined bending and torsion, use AISC Design Guide 9 (Torsional Analysis of Structural Steel Members).

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