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Estimation of Torsional stiffness
Observed behavior of reinforced concrete members under torsion (see also section 7.3) shows that the torsional stiffness is little influenced by the amount of torsional reinforcement in the linear elastic phase, and may be taken as that of the plain concrete section. However, once torsional cracking occurs, there is a drastic reduction in the torsional stiffness. The post -cracking torsional stiffness is only a small fraction (less than 10 percent) of the pre -cracking stiffness, and depends on the amount of torsional reinforcement, provided in the form of closed stirrups and longitudinal bars. Heavy torsional reinforcement can, doubt, increase the torsional resistance (strength) to a large extent, but this can be realized only at very large angles of twist (accompanied by very large cracks).
Hence, even with torsional reinforcement provided, in most practical situations, the maximum twisting moment in a reinforced concrete member under compatibility torsion is the value corresponding to the torsional cracking
of the member. The „cracking torque? is verydentical nea plain concrete section.
In the usual linear elastic analysis of framed structures, the torsional stiffness kt (torque per unit beam of length l is expressed as
KT = GC / l
Where GC is the torsional rigidity, obtained as a product of the shear modulus G and the geometrical parameter C of the section (Ref. 7.1). It is recommended in the Explanatory Handbook to the code (Ref.7.2) that G may be taken as 0.4 times the c is a property of the section having the same relationship to the torsional stiffness of a rectangular section as the polar moment of inertia has for a circular section.
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