Round Bars - Section Properties

What is in the Round Bar table

This page lists every Round Bars size to EN 10058 · Continental Steel, with the full set of section properties for each one. The table is sortable on any column and searchable by designation, and each size links to its own page with the complete property set, a worked capacity check and dimensions for detailing.

Sizes in this table

31

Mass range (kg/m)

0.2 to 246.6 - from RND 6 up to RND 200

Major-axis stiffness, I (cm⁴)

0.00636 to 7,854 - a factor of 1,234,517 across the range

Published to

EN 10058 · Continental Steel

The spread is worth noticing: the heaviest Round Bar weighs 1121.0x the lightest but carries roughly 1,234,517x the major-axis stiffness. Bending stiffness grows far faster than weight, which is the whole reason the range is shaped this way - going deeper buys stiffness much more cheaply than going heavier.

How to read a Round Bar designation

A Round Bar designation is serial size then mass per metre - RND 30. The first two figures are the NOMINAL serial size in millimetres (depth × width); the last is the real mass per metre in kg/m.

The nominal size is shared across a serial group: every section in one group rolls from the same set of rolls, so the outside depth and width stay near-constant while the web and flange thicken to add mass. That is why two sections with the same first two numbers have different real depths in the table above, and why a heavier section is usually a drop-in for a lighter one in the same group.

What each column in the table means

The table above publishes 20 properties for every Round Bar size. These are the ones that decide a design:

Mass per metre

Self-weight of the section, and the basis on which steel is bought - so it is the first cost signal when comparing two sections that both pass.

Cross-sectional area, A

Governs axial capacity and, with mass, tells you how efficiently the shape uses its steel.

Second moment of area, I (major axis)

Resistance to bending about the strong axis. It is what deflection is proportional to, so a serviceability check is driven by this number rather than by strength.

Second moment of area about the minor axis

The weak-axis stiffness. It sets lateral-torsional buckling behaviour and matters whenever the compression flange is not continuously restrained.

Elastic section modulus, Wel

Moment capacity while the extreme fibre stays elastic - the value to use for a Class 3 section, and for any check where yielding is not permitted.

Plastic section modulus, Wpl

Moment capacity once the whole section has yielded - the value used for Class 1 and 2 sections, and typically 10-20% above the elastic modulus.

Radius of gyration, i

The slenderness term: effective length divided by i gives the slenderness that drives buckling. The minor-axis value normally governs a column.

Torsion constant, J

St Venant torsional stiffness. Closed shapes carry torsion through a shear flow around the perimeter and have a J orders of magnitude above an open shape of the same weight.

Warping constant, Cw

The second half of torsional resistance in an open section, and an input to lateral-torsional buckling. It is what makes an I-section usable in torsion at all.

Buckling parameter, u

A Blue Book shortcut that condenses the section geometry used in the lateral-torsional buckling check.

Torsional index, x

Approximately depth divided by flange thickness; a high value means a slender, torsionally soft section.

Surface area per metre

Drives the cost of painting, galvanising and fire protection - often the deciding factor between two sections of similar capacity.

Every one of these is a property of the shape alone. None of them depends on the steel grade - grade enters only when a property is multiplied by a design strength to give a capacity, which is what the Capacity tab above does.

Choosing a Round Bar size

Which property governs depends on what is being checked, and it is rarely the one people reach for first:

Bending strength

Compare plastic modulus Wpl for Class 1 and 2 sections, elastic Wel for Class 3. Depth buys this cheaply.

Deflection

Compare second moment of area I. A section that passes strength can still fail serviceability, and this is usually the check that decides the size.

Compression

Compare the MINOR-axis radius of gyration. A deep section with a narrow flange is efficient in bending and poor as a column.

Torsion

Compare J. If the load is genuinely torsional, a closed section beats an open one so decisively that the comparison is rarely close.

Cost

Compare mass per metre first, then surface area per metre - protective treatment is charged by area, not weight.

Elastic section modulus

$W_{el} = \frac{I}{c}$

Radius of gyration

$i = \sqrt{\frac{I}{A}}$

Slenderness

$\lambda = \frac{L_{cr}}{i}$

Open the Capacity tab above with a size selected to run these as a full EC3 check rather than comparing the raw properties by eye.

Common questions about Round Bar sections