For many years, torsion was not considered explicitly in design, its influence being absorbed in the overall factor of safety of rather conservatively designed structures. Reinforced concrete members are commonly subjected to bending moments, transverse shears associated with those bending moments, and, in the case of columns, to axial forces often combined with bending and shear. In addition, torsional forces may act, tending to twist a member about its longitudinal axis. Such torsional forces seldom act alone, but are almost always concurrent with bending moment and transverse shear, and sometimes with axial force as well.
In recent years, however, it has become necessary to take account of torsional effects in member design in many cases and to provide reinforcement to increase torsional strength. There are two reasons for this change. First, improved methods of analysis and design, such as the strength design approach now favored, have permitted a somewhat lower overall factor of safety through more accurate appraisal of load capacity, and have led to somewhat smaller member sizes. Second, there is increasing use of structural members for which torsion is a central feature of behavior, examples including curved bridge girders, ecentrically load box beams, and helical stairway slabs. Consequently, there has been a greater increase, since the 1960s, in research activity relating to torsion in reinforced concrete, and prctical desing rulels have been formulated.
Primary and Secondary Torsion
Primary torsion is sometimes called equilibrium torsion or statically determinate torsion. It exists when the external load has no alternative but must be supported by torsion. For such cases, the torsion required to maintain static equilibrium can be uniquely determined.