Materials #04: A Summary of STKR Square Steel Tube Standards

This article summarizes the standards related to STKR square steel tubes, including dimensions, standard lengths, strength and basic properties. The content focuses on STKR structural steel, which is widely used in mechanical and structural applications.

Main content

Standard dimensions and lengths commonly available on the steel materials market.
Tensile strength, yield point or yield strength.
Cross-sectional area (断面積).
Second moment of area (断面二次モーメント).
Section modulus (断面係数).
Radius of gyration of the section (断面二次半径).

1. STKR square steel tube standards and common sizes

STKR square steel tube is cold-rolled or hot-rolled steel tube manufactured to JIS standards for steel structures. In actual trade in Vietnam and Japan, the commonly encountered sizes include:

10 × 10 mm, 15 × 15 mm, 19 × 19 mm
25 × 25 mm, 30 × 30 mm, 40 × 40 mm
50 × 50 mm, 60 × 60 mm, 75 × 75 mm
80 × 80 mm, 90 × 90 mm, 100 × 100 mm

Wall thickness commonly comes in these values:

1.0 mm, 1.2 mm, 1.5 mm
2.0 mm, 2.3 mm, 2.7 mm, 3.2 mm

These sizes are standards easily bought from a steel supplier, especially STKR square tubes for light structures and mechanical frames.

Standard lengths

Standard length depends on the distributor's stock and cutting requirements, but you can refer to these common lengths:

6 m, 8 m, 9 m, 10 m, 12 m

In many cases the store can pre-cut or machine to order, but the lengths above are the commercial lengths most often kept in stock.

2. Basic strength of STKR square steel tube

For STKR structural steel tube, the basic parameters to pay attention to are:

Tensile strength (引張強さ): the maximum tensile force the material can withstand before rupture.
Yield point / yield strength (降伏点 or 耐力): the stress at which the steel begins to deform plastically.

In practice, STKR steel tube is usually manufactured in strength grades suitable for light structures and frames, for example:

STKR400: minimum yield strength around 400 MPa.
STKR490: minimum yield strength around 490 MPa.

On the market, manufacturers also state the tensile limit and elongation for each specification.

3. Sectional properties of a square tube

When designing a structure, a square steel tube requires calculating the following sectional quantities:

3.1. Cross-sectional area (断面積)

The cross-sectional area A of an STKR square tube depends on the side a and the thickness t:

A = a² - (a - 2t)²

Where:

a: the outer side width of the tube
t: the wall thickness

The cross-sectional area is the basis for calculating basic compression, tension and shear loads.

3.2. Second moment of area (断面二次モーメント)

The second moment of area I reflects the section's resistance to bending.

For a hollow square tube, the general formula is:

I = (a⁴ - b⁴) / 12

Where:

a: outer side
b: inner side (b = a - 2t)

The larger the value of I, the better the tube resists bending under an applied bending moment.

3.3. Section modulus (断面係数)

The section modulus Z is calculated by:

Z = I / y

Where:

I: second moment of area
y: the distance from the neutral axis to the outer edge of the section

For a square tube, y = a / 2.

The section modulus is the parameter used directly to calculate bending stress:

σ = M / Z

3.4. Radius of gyration of the section (断面二次半径)

The radius of gyration i is:

i = √(I / A)

This is an important coefficient when evaluating axial compression capacity and the overall stability of a member.

4. Summary from JIS G 3466 and manufacturers

Information on the values of I, Z and i is usually taken from JIS G 3466 tables and steel manufacturer documentation. Below are a few points to note:

JIS G 3466: the standard for square and rectangular steel tubes for structures, providing dimensional specifications, wall thickness and sectional parameters.
JIS G 3446: a complementary standard for round steel tubes and other technical requirements.
Steel manufacturers usually provide published tables of sectional parameters, second moment of area and section modulus for each STKR square tube specification.

If you need to check detailed values for each size, you should look directly at the JIS G 3466 document or the Excel table provided by the manufacturer.

5. Notes on choosing a square steel tube for a structure

Choose the side size a and thickness t suited to balance load-bearing section against weight.
Prioritize specifications available on the market to reduce cost and fabrication time.
For structures under bending and compression, pay more attention to the second moment of area I and the section modulus Z than to area A alone.
For columns, frames or open beams, the radius of gyration i must be controlled to evaluate resistance to buckling and stability.

Conclusion

This article has summarized the dimensional standards, standard lengths, tensile strength parameters and basic sectional properties of STKR square steel tubes. These parameters are the design foundation when you work with square steel tubes for mechanical and construction structural frames.

For further details not covered here, check JIS G 3446 and JIS G 3466, or ask your steel supplier for the specific datasheet of the tube specification you need.

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