# Section modulus

Section modulus is a geometric property for a given cross-section used in the design of beams or flexural members. Other geometric properties used in design include area for tension, radius of gyration for compression, and moment of inertia for stiffness. Any relationship between these properties is highly dependent on the shape in question. Equations for the section moduli of common shapes are given below. There are two types of section moduli, the elastic section modulus (S) and the plastic section modulus (Z).

## Notation

North American and British/Australian convention reverse the usage of S & Z. Elastic modulus is S in North America,[1] but Z in Britain/Australia,[2] and vice versa for the plastic modulus. Eurocode 3 (EN 1993 - Steel Design) resolves this by using W for both, but distinguishes between them by the use of subscripts - Wel and Wpl.

## Elastic section modulus

For general design, the elastic section modulus is used, applying up to the yield point for most metals and other common materials.

The elastic section modulus is defined as S = I / y, where I is the second moment of area (or moment of inertia) and y is the distance from the neutral axis to any given fibre.[3] It is often reported using y = c, where c is the distance from the neutral axis to the most extreme fibre, as seen in the table below. It is also often used to determine the yield moment (My) such that My = S × σy, where σy is the yield strength of the material.[3]

Section modulus equations[4]
Cross-sectional shape Figure Equation Comment
Rectangle $S = \cfrac{bh^2}{6}$ Solid arrow represents neutral axis
doubly symmetric I-section (strong axis) $S = \cfrac{BH^2}{6} - \cfrac{bh^3}{6H}$ NA indicates neutral axis
doubly symmetric I-section (weak axis) $S = \cfrac{B^2(H-h)}{6} + \cfrac{(B-b)^3h}{6B}$ NA indicates neutral axis
Circle $S = \cfrac{\pi r^3}{4} = \cfrac{\pi d^3}{32}$[4] Solid arrow represents neutral axis
Circular tube $S = \cfrac{\pi\left(r_2^4-r_1^4\right)}{4 r_2} = \cfrac{\pi (d_2^4 - d_1^4)}{32d_2}$ Solid arrow represents neutral axis
Rectangular tube $S = \cfrac{BH^2}{6}-\cfrac{bh^3}{6H}$ 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 where (irreversible) plastic behaviour is dominant. The majority of designs do not intentionally encounter this behaviour.

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. So, for sections with constant yielding stress, the area above and below the PNA will be equal, but for composite sections, this is not necessarily the case.

The plastic section modulus is then the sum of the areas of the cross section on each side of the PNA (which may or may not be equal) multiplied by the distance from the local centroids of the two areas to the PNA:

$Z_P=A_Cy_C + A_Ty_T$

Description Figure Equation Comment
Rectangular section $Z_P = \cfrac{bh^2}{4}$
Hollow rectangular section $Z_P = \cfrac{bh^2}{4}-(b-2t)(\cfrac{h}{2}-t)^2$ where: b=width, h=height, t=wall thickness
For the two flanges of an I-beam with the web excluded $Z_P = b_1t_1y_1+b_2t_2y_2\,$ ,[5]

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 $Z_{P} = bt_f (d-t_f )+ 0.25t_w (d-2t_f )^2$ [6]
For an I Beam (weak axis) $Z_{P} = (b^2t_f)/2 + 0.25t_w^2(d-2t_f )$
Solid Circle $Z_P = \cfrac{d^3}{6}$
Hollow Circle $Z_P = \cfrac{d_2^3-d_1^3}{6}$

The plastic section modulus is used to calculate the plastic moment, Mp, or full capacity of a cross-section. The two terms are related by the yield strength of the material in question, Fy, by Mp=Fy*Z. Sometimes Z and S are related by defining a 'k' factor which is something of an indication of capacity beyond first yield. k=Z/S

Therefore for a rectangular section, k=1.5