Section modulus

Section modulus is a geometric property of a given cross-section used in the design of beams or flexural members. Other geometric properties used in design include: area for tension and shear, radius of gyration for compression, and second moment of area and polar second moment of area for stiffness. Any relationship between these properties is highly dependent on the shape in question. There are two types of section modulus, elastic and plastic:

The elastic section modulus is used to calculate a cross-section's resistance to bending within the elastic range, where stress and strain are proportional.

The plastic section modulus is used to calculate a cross-section's capacity to resist bending after yielding has occurred across the entire section. It is used for determining the plastic, or full moment, strength and is larger than the elastic section modulus, reflecting the section's strength beyond the elastic range^[1].

Equations for the section moduli of common shapes are given below. The section moduli for various profiles are often available as numerical values in tables that list the properties of standard structural shapes.^[2]

Note: Both the elastic and plastic section moduli are different to the first moment of area. It is used to determine how shear forces are distributed.

Notation

Different codes use varying notations for the elastic and plastic section modulus, as illustrated in the table below.

Section Modulus Notation
Region	Code	Section Modulus
Region	Code	Elastic	Plastic
North America	USA: ANSI/AISC 360-10^[3]	S	Z
North America	Canada: CSA S16-14^[4]	S	Z
Europe	Europe (inc. Britain): Eurocode 3^[5]	W_el	W_pl
Europe	Britain (obsolete): BS 5950 ^a ^[6]	Z	S
Asia	Japan: Standard Specifications for Steel and Composite Structures^[7]	W	Z
	China: GB 50017^[8]	W	W_p
	India: IS 800^[9]	Z_e	Z_p
	Australia: AS 4100^[10]	Z	S
Notes: a) Withdrawn on 30 March 2010, Eurocode 3 is used instead ^[11].

The North American notation is used in this article.

Elastic section modulus

The elastic section modulus is used for general design. It is applicable up to the yield point for most metals and other common materials. It is defined as^[1]

S = I/c

where:

I is the second moment of area (or area moment of inertia, not to be confused with moment of inertia), and

c is the distance from the neutral axis to the most extreme fibre.

It is used to determine the yield moment strength of a section^[1]

M_y = S ⋅ σ_y

where σ_y is the yield strength of the material.

The table below shows formulas for the elastic section modulus for various shapes.

Elastic Section Modulus Equations^[12]
Cross-sectional shape	Equation	Comment
Rectangle	$S={\cfrac {bh^{2}}{6}}$	Solid arrow represents neutral axis
doubly symmetric Ɪ-section (major axis)	$S_{x}={\cfrac {BH^{2}}{6}}-{\cfrac {bh^{3}}{6H}}$ $S_{x}={\tfrac {I_{x}}{y}}$ , with $y={\cfrac {H}{2}}$	NA indicates neutral axis
doubly symmetric Ɪ-section (minor axis)	$S_{y}={\cfrac {B^{2}(H-h)}{6}}+{\cfrac {(B-b)^{3}h}{6B}}$ ^[13]	NA indicates neutral axis
Circle	$S={\cfrac {\pi d^{3}}{32}}$ ^[12]	Solid arrow represents neutral axis
Circular hollow section	$S={\cfrac {\pi \left(r_{2}^{4}-r_{1}^{4}\right)}{4r_{2}}}={\cfrac {\pi (d_{2}^{4}-d_{1}^{4})}{32d_{2}}}$	Solid arrow represents neutral axis
Rectangular hollow section	$S={\cfrac {BH^{2}}{6}}-{\cfrac {bh^{3}}{6H}}$	NA indicates neutral axis
Diamond	$S={\cfrac {BH^{2}}{24}}$	NA indicates neutral axis
C-channel	$S={\cfrac {BH^{2}}{6}}-{\cfrac {bh^{3}}{6H}}$	NA indicates neutral axis

Plastic section modulus

The plastic section modulus is used for materials and structures where limited plastic deformation is acceptable. It represents the section's capacity to resist bending once the material has yielded and entered the plastic range. It is used to determine the plastic, or full, moment strength of a section^[1]

M_p = Z ⋅ σ_y

where σ_y is the yield strength of the material.

Engineers often compare the plastic moment capacity against factored applied moments to ensure that the structure can safely endure the required loads without significant or unacceptable permanent deformation. This is an integral part of the limit state design method.

The plastic section modulus depends on the location of the plastic neutral axis (PNA). The PNA is defined as the axis that splits the cross section such that the compression force from the area in compression equals the tension force from the area in tension. For sections with constant, equal compressive and tensile yield strength, the area above and below the PNA will be equal^[14]

$A_{C}=A_{T}$

These areas may differ in composite sections, which have differing material properties, resulting in unequal contributions to the plastic section modulus.

The plastic section modulus is calculated as the sum of the areas of the cross section on either side of the PNA, each multiplied by the distance from their respective local centroids to the PNA.

$Z=A_{C}y_{C}+A_{T}y_{T}$

where:

A_C and A_T are the areas in compression and tension respectively, and

y_C and y_T are the distances from the PNA to their centroids.

Plastic section modulus and elastic section modulus can be related by a shape factor k

$k={\cfrac {M_{p}}{M_{y}}}={\cfrac {Z}{S}}$

This is an indication of a section's capacity beyond the yield strength of material. The shape factor for a rectangular section is 1.5^[1].

The table below shows formulas for the plastic section modulus for various shapes.

Plastic Section Modulus Equations
Description	Equation	Comment
Rectangular section	$Z={\cfrac {bh^{2}}{4}}$ ^[15]^[1]	$A_{C}=A_{T}={\cfrac {bh}{2}}$ , $y_{C}=y_{T}={\cfrac {h}{4}}$
Rectangular hollow section	$Z={\cfrac {bh^{2}}{4}}-(b-2t)\left({\cfrac {h}{2}}-t\right)^{2}$	where: b = width, h = height, t = wall thickness
For the two flanges of an Ɪ-beam with the web excluded^[16]	$Z=b_{1}t_{1}y_{1}+b_{2}t_{2}y_{2}\,$	where: $b_{1},b_{2}$ =width, $t_{1},t_{2}$ =thickness, $y_{1},y_{2}$ are the distances from the neutral axis to the centroids of the flanges respectively.
For an I Beam including the web	$Z=bt_{f}(d-t_{f})+0.25t_{w}(d-2t_{f})^{2}$	^[17]
For an I Beam (weak axis)	$Z=(b^{2}t_{f})/2+0.25t_{w}^{2}(d-2t_{f})$	d = full height of the I beam
Solid Circle	$Z={\cfrac {d^{3}}{6}}$
Circular hollow section	$Z={\cfrac {d_{2}^{3}-d_{1}^{3}}{6}}$

Use in structural engineering

Though generally section modulus is calculated for the extreme tensile or compressive fibres in a bending beam, often compression is the most critical case due to onset of flexural torsional (F/T) buckling^[18]. Generally (except for brittle materials like concrete) tensile extreme fibres have a higher allowable stress or capacity than compressive fibres.

In the case of T-sections if there are tensile fibres at the bottom of the T they may still be more critical than the compressive fibres at the top due to a generally much larger distance from the neutral axis so despite having a higher allowable stress the elastic section modulus is also lower. In this case F/T buckling must still be assessed as the beam length and restraints may result in reduced compressive member bending allowable stress or capacity.

There may also be a number of different critical cases that require consideration, such as there being different values for orthogonal and principal axes and in the case of unequal angle sections in the principal axes there is a section modulus for each corner.

For a conservative (safe) design, civil structural engineers are often concerned with the combination of the highest load (tensile or compressive) and lowest elastic section modulus for a given section station along a beam, although if the loading is well understood one can take advantage of different section modulus for tension and compression to get more out of the design. For aeronautical and space applications where designs must be much less conservative for weight saving, structural testing is often required to ensure safety as reliance on structural analysis alone is more difficult (and expensive) to justify.

References

^ ^a ^b ^c ^d ^e ^f Young, Warren C. (1989). Roark's Formulas for Stress and Strain (PDF). McGraw Hill. p. 217.
^ "'Blue Book' home - Blue Book - Steel for Life". www.steelforlifebluebook.co.uk. Retrieved 2024-08-25.
^ "Specification for Structural Steel Buildings (ANSI/AISC 360-10) - 2010 | American Institute of Steel Construction". www.aisc.org. Retrieved 2024-08-23.
^ S16-14 (R2019) Design of steel structures. Canadian Standards Association. 2024-08-23.
^ Eurocode 3: Design of steel structures - Part 1-1: General rules and rules for buildings. ISBN 978 0 539 13167 3.
^ BS 5950-1 Structural use of steelwork in building, BSI British Standards, retrieved 2024-08-23
^ Standard Specifications for Steel and Composite Structures (First ed.). Japan: Japan Society of Civil Engineers (published December 2009). 2024-08-24.
^ GB 50017 Code for Design of Steel Structures. China: Ministry of Construction of the People's Republic of China (published 2003-04-25). 2003.
^ IS800:2007 General Construction in Steel - Code of Practice (Third ed.). India: Bureau of Indian Standards (published 2017). 2007.
^ AS 4100- 2020 Steel Structures. Australia: Standards Australia Ltd. 2020. ISBN 978 1 76072 947 9.
^ "British Standards Institute". 2024-08-23. Retrieved 2024-08-23.
^ ^a ^b Gere, J. M. and Timnko, S., 1997, Mechanics of Materials 4th Ed., PWS Publishing Co.
^ "Section Modulus Equations and Calculators Common Shapes".
^ "Plastic Modulus" (PDF).
^ "Calculating the section modulus".
^ American Institute of Steel Construction: Load and Resistance Factor Design, 3rd Edition, pp. 17-34.
^ Megson, T H G (2005). Structural and stress analysis. elsever. pp. 598 EQ (iv). ISBN 9780080455341.
^ Brockenbrough, Roger L.; Merritt, Frederick S., eds. (1999). Structural steel designer's handbook. McGraw-Hill handbooks (3 ed.). New York: McGraw-Hill. p. 3.96. ISBN 978-0-07-008782-8.

[:0-1] ^ ^a ^b ^c ^d ^e ^f Young, Warren C. (1989). Roark's Formulas for Stress and Strain (PDF). McGraw Hill. p. 217.

[2] "'Blue Book' home - Blue Book - Steel for Life". www.steelforlifebluebook.co.uk. Retrieved 2024-08-25.

[3] "Specification for Structural Steel Buildings (ANSI/AISC 360-10) - 2010 | American Institute of Steel Construction". www.aisc.org. Retrieved 2024-08-23.

[4] S16-14 (R2019) Design of steel structures. Canadian Standards Association. 2024-08-23.

[5] Eurocode 3: Design of steel structures - Part 1-1: General rules and rules for buildings. ISBN 978 0 539 13167 3.

[6] BS 5950-1 Structural use of steelwork in building, BSI British Standards, retrieved 2024-08-23

[7] Standard Specifications for Steel and Composite Structures (First ed.). Japan: Japan Society of Civil Engineers (published December 2009). 2024-08-24.

[8] GB 50017 Code for Design of Steel Structures. China: Ministry of Construction of the People's Republic of China (published 2003-04-25). 2003.

[9] IS800:2007 General Construction in Steel - Code of Practice (Third ed.). India: Bureau of Indian Standards (published 2017). 2007.

[10] AS 4100- 2020 Steel Structures. Australia: Standards Australia Ltd. 2020. ISBN 978 1 76072 947 9.

[11] "British Standards Institute". 2024-08-23. Retrieved 2024-08-23.

[Timo-12] Gere, J. M. and Timnko, S., 1997, Mechanics of Materials 4th Ed., PWS Publishing Co.

[13] "Section Modulus Equations and Calculators Common Shapes".

[14] "Plastic Modulus" (PDF).

[15] "Calculating the section modulus".

[16] American Institute of Steel Construction: Load and Resistance Factor Design, 3rd Edition, pp. 17-34.

[17] Megson, T H G (2005). Structural and stress analysis. elsever. pp. 598 EQ (iv). ISBN 9780080455341.

[18] Brockenbrough, Roger L.; Merritt, Frederick S., eds. (1999). Structural steel designer's handbook. McGraw-Hill handbooks (3 ed.). New York: McGraw-Hill. p. 3.96. ISBN 978-0-07-008782-8.

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

Notation

Elastic section modulus

Plastic section modulus

Use in structural engineering

See also

References