Beam under torsion

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For common buildings in areas with high seismic activity, torsion is a quite dangerous stress condition, because:

a) It is a statically determined stress. This means that if a failure occurs there is no possibility for transition to another static state.

b) The rotations caused to the beam due to creep are relatively large thus leading to large slab deflections.

c) Failure is caused by shear stresses therefore, it is of a brittle nature and consequently there are no signs for the impending failure.

d) It is not easy for the technicians to understand that the formwork of the elements under torsion must not be removed in the usual time. On the contrary, the formwork must remain in place until the following-phase-casted-concrete that affects these elements has adequately hardened.

e) Reinforcing of beams subjected to torsion follows special rules. In order to apply these rules, one does not only require thorough knowledge but also extreme care during the construction.

In order to behave in the required way, all rebars placed at the upper, lower and side parts of the beam must be properly anchored inside the column.

Regardless whether the beam section is hollow or solid, it counteracts with the torsional moment and maintains a state of equilibrium developing a closed flow of torsional shear stresses, in a peripheral zone.
Due to the high shear strength requirements, beams under torsion, are usually constructed with a large width. That way the beam’s cross section approximates the square shape.
Torsion causes diagonal tensile stresses along the entire perimeter of the beam and in order these stresses to be carried both longitudinal and vertical reinforcement is required. The longitudinal reinforcement at the upper and lower side of the beam is provided by the upper and lower rebars, while the longitudinal reinforcement of the lateral faces is provided by the rebars placed along the stirrups legs depth.

In order for the vertical reinforcement, i.e. the stirrups, to carry the required stresses they must be properly closed therefore, it is mandatory to be formed with a double hook.

The rebars of the slab supported by a beam, must be anchored at least with a double bend, as shown at the above detail.

In the slab area where torsion appears, the stirrup and the slab rebar are formed with one single bar, as shown at the figure.