Torsional resistance and stiffness

Torsional stiffness and resistance are characteristics of building structures which significantly Clause influence their response to seismic actions. Responses in which translational motion is dominant are preferable to those in which torsional motion is significant because they tend to stress the different structural elements in a more uniform way.

To counteract the torsional response of buildings, the fundamental modes of vibration of the structure should be translational (or mainly translational in non-purely symmetrical buildings). To this end, the torsional stiffness of the structure must be sufficiently large to ensure that the first torsional vibration mode has a frequency higher than the translational modes.

In fact this is implicit in condition 4.1b oiclause, which establishes the criteria for in-plan regularity of a building. Such a condition corresponds to the objective that in regular buildings the first torsional mode has a frequency higher than the translational modes, thus ensuring that its importance in the global seismic response of the building is relatively minor.

It should be noted that this concern with the poorer behaviour of buildings with small torsional stiffness is also present in the classification of reinforced concrete buildings, for which a class of Horsionally flexible systems' is introduced (see clause 5.1.2 of EN 1998-1). In line with this concern, these systems are given smaller values for their behaviour factor (see clause of EN 1998-1).

For the purpose of ensuring adequate torsional stiffness and resistance, the main elements resisting the seismic action should be well distributed in plan or, even better, they should be close to the periphery of the building and oriented along the two main directions. Buildings with their main lateral resisting elements located at the centre of the building in plan should be avoided because, even in the case of symmetrical structural arrangements, they may be prone to large uncontrolled torsional motions.

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  • victoria
    What is torsional vibration in building?
    8 years ago

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