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Adaptation of torsional constant

See Ref.[1], Chapter 10.1.5., Ref.[2],3.5 and Ref.[3],3.3.4..

When diaphragms (steel sheeting) are used, the torsional constant It is adapted for symmetric/asymmetric I sections, channel sections, Z sections, cold formed U, C , Z sections.

The torsional constant It is adapted with the stiffness of the diaphragms:

with

l	the LTB length
G	the shear modulus
vorhC_q	the actual rotational stiffness of diaphragm
C_q_M,k	the rotational stiffness of the diaphragm
C_q_A,k	the rotational stiffness of the connection between the diaphragm and the beam
C_q_P,k	the rotational stiffness due to the distortion of the beam
k	numerical coefficient = 2 for single or two spans of the diaphragm = 4 for 3 or more spans of the diaphragm
EI_eff	bending stiffness of per unit width of the diaphragm
s	spacing of the beam
b_a	the width of the beam flange (in mm)
C₁₀₀	rotation coefficient - see table
h	beam height
t	thickness beam flange
s	thickness beam web

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