๐Ÿ‘‹ Introduction

Section classification decides how much of a section's strength we are allowed to use in design. For a CHS, there is only one check โ€” the D/t ratio โ€” because the tube has no flat plate elements and no separate flange and web. Importantly, the D/t limits are the same in bending and compression, so a CHS falls into the same class regardless of the stress distribution โ€” unlike I-sections and RHS, where the web's limits depend on whether it is in bending or uniform compression. In this article we classify the same CHS (355.6x8.0) under both pure bending and pure compression (both give Class 2).

The concept behind this and its corresponding structural meaning can be found in another article: Classification of Steel Sections - I-Section, CHS, and RHS Explained.

โ„น๏ธ Class Naming Difference between Eurocode 3, IS 800 and AISC

The classification concept is the same worldwide, but the class names and the limiting ratios differ slightly between codes:

Design Capability Eurocode 3 IS 800 AISC 360
Plastic design (full hinge rotation) Class 1 Plastic Compact
Full plastic moment, limited rotation Class 2 Compact Compact
Elastic capacity (yield at extreme fibre) Class 3 Semi-compact Noncompact
Elastic with reduced (effective) section Class 4 Slender Slender

Note that the material factor ฮต also uses a different constant: EN 1993-1-1 uses ฮต = โˆš(235/fy), while IS 800 uses ฮต = โˆš(250/fy).

๐Ÿ“ Design Data

Below are the design information of the CHS (355.6x8.0), which we will check under both pure bending and pure compression.

Steel Section
Type CHS
Standard 355.6x8.0
Diameter, D 356mm
Thickness, t 8.0mm
Material
Steel Grade S355
Yield Strength, fy 355 N/mmยฒ


d = 356 mm t = 8 mm Fig. 1 Circular Hollow Section (355.6x8.0)

๐Ÿ”ข Calculation Steps

*Below is using Eurocode symbols and limits Table 5.2, EN 1993-1-1. For IS 800, use ฮต = โˆš(250/fy) and the limits in its Table 2.

Common check (the single D/t ratio)

For CHS, classification is based on a single D/t ratio, and the limits are the same in bending and compression โ€” so this check is computed once and applies to both stress states.

Material factor โ“˜
=
=
=
Diameter-to-thickness ratio โ“˜
=
=
=
Classification check (whole tube โ€” same in bending and compression) โ“˜
=
=
=

Under bending

Section class โ€” under bending โ“˜
=
=
=
Moment capacity to use โ€” under bending โ“˜
=
=
=

Under compression

Section class โ€” under compression โ“˜
=
=
=
Compression capacity to use โ€” under compression โ“˜
=
=
=
๐Ÿ’ก Key takeaway:

For CHS, the D/t limits are identical in bending and compression, so the section class is unchanged (Class 2) under both โ€” unlike I-sections and RHS, where the web's limits tighten under uniform compression and can drop the section into a lower class.

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#steel design #work example #structural
CivilSimple Team

CivilSimple Team

The CivilSimple Team writes practical engineering guides for the profession and the curious. All articles are reviewed for technical accuracy before publication.