Design for torsion can be neglected if¹

T_u < T_u,n

where

T_u = factored torsional moment at section

T_u,n = φ * 0.083 * λ * f`_c^0.5 * A_cp²/ p_cpmetric-units

T_u,n = φ * λ * f`_c^0.5 * A_cp²/ p_cp US-units

φ = 0.75 for torsion

λ = 1.0 for normal weight concrete

p_cp = 2 * (b+h) outside perimeter of concrete cross section

A_cp = b * h area of by outside perimeter of concrete cross section

f`_c = specified compressive strength of concrete

Otherwise torsion must be considered as follows.

Firstly a check for crushing of the concrete compression struts² is performed. If this fails then the torsion design process fails since the section size is inadequate for torsion.

Provided the crushing of the concrete compression struts check passes, a check is then carried out to ensure the design torsional strength is equal to or greater than the required torsional strength³.

φ * T_n ≥ T_u

The nominal torsional moment strength in terms of stirrup yield strength is given by;⁴

T_n = 1.7*A_oh * A_t*f_yt/s

When links are required to resist torsion, the spacing of the link assemblies along the length of the beam is given by;⁵

s_t ≤ MIN[p_h/8, 300mm (12in.), s]

where

s = Maximum stirrup spacing for shear

The additional longitudinal torsion reinforcement is always considered when torsional stirrups are required.

The basic requirement is:

A_prov ≥ [A_s + A_l]

where

A_s = required reinf. from design for bending

A_l = additional longitudinal torsion reinforcement

The additional longitudinal torsion reinforcement is calculated as per the code requirement.⁶

There should be at least one longitudinal bar in each corner of the stirrups. The additional longitudinal torsional reinforcement is distributed in each of the four faces of the beam in proportion to the length of the face of the cross-section.

Footnotes