Beam theory: Torsion

Engineering Fundamentals


When a straight beam is subjected to an axial moment, each cross-section twists around its torsional center. Shear stresses occur within the cross-sectional planes of the beam.

Angular twist

For a torsionally loaded beam, the angular twist is described by:

$$\theta=\frac{T\cdot L}{G\cdot J_T}$$

Torsion of beam

$J_T$ is the torsion constant. It is equal to the polar moment of inertia $I_z$ if the cross section is circular. For non-circular cross sections warping occurs which reduces the effective torsion constant. For these shapes, approximate solutions of the torsion constant and maximum stress are given in the table below. When no warping can occur see the equations on page Torsion of leaf springs: Restrained warping.

$G$ is the shear modulus. The relation between the shear modulus $G$ and the elastic modulus $E$ is defined by the following formula:

$$G=\frac E{2\left(1+v\right)}\approx 0.38E \textsf{   (For most metals)}$$

Rotational stiffness

The rotational stiffness of a torsionally loaded beam is:

$$K_z=\frac{T}{\theta}=\frac{G\cdot J_T}{L}$$

Maximum shear stress

For a torsionally loaded beam with a circular cross-section, the maximum shear stress can be calculated with:

Precision Point - Beam Theory Torsion

For non-circular cross-sections the equations in the table below can be used.

For pure shear, the Von Mises stress is then equal to: 


Cross section ConditionTorsion constant $J_T$Maximum shear stress $\tau_{max}$
Precision Point - Beam Theory Torsion$$J_T=I_Z=\frac{\pi}{2}r^4$$$$\tau_{max}=\frac{2T_{max}}{{\pi}r^3}$$
Precision Point - Beam Theory Torsion$$J_T=I_Z=\frac{\pi}{2}(r_o^4-r_i^4)$$$$\tau_{max}=\frac{2T_{max}r_o}{\pi\left(r_o^4-r_i^4\right)}$$
Precision Point - Beam Theory Torsion$$h=w$$$$J_T=\frac{9}{64}w^4$$$$\tau_{max}=\frac{4.808T}{w^3}$$
$$w\geq\ h$$$$J_T=\frac{1}{16}wh^3\left(\frac{16}{3}-3.36\frac{h}{w}\left(1-\frac{h^4}{12w^4}\right)\right)$$
$$\approx\frac{1}{3}wh^3\textsf{ for } w\gg h$$
$$\approx\frac{3T}{wh^2}\textsf{ for } w\gg\ h$$
Precision Point - Beam Theory TorsionFor thin walled structures$$J_t=\frac{2t^2\left(w-t\right)^2\left(h-t\right)^2}{\left(w+h\right)t-2t^2}$$$$\tau_{average}=\frac{T}{2t(w-t)(h-t)}$$

Note: stress is nearly uniform if t is small. There will be higher stresses at inner corners unless fillets of fairly large radius are provided.

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