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Structural Engineering and Mechanics
  Volume 9, Number 6, June 2000, pages 615-636

Seismic torsional vibration in elevated tanks
Dutta SC, Murty CVR, Jain SK

    Some elevated water tanks have failed due to torsional vibrations in past earthquakes. The overall axisymmetric structural geometry and mass distribution of such structures may leave only a small accidental eccentricity between centre of stiffness and centre of mass. Such a small accidental eccentricity is not expected to cause a torsional failure. This paper studies the possibility of amplified torsional behaviour of elevated water tanks due to such small accidental eccentricity in the elastic as well as inelastic range; using two simple idealized systems with two coupled lateral-torsional degrees of freedom. The systems are capable of retaining the characteristics of two extreme categories of water tanks namely, a) tanks on staging with less number of columns and panels and b) tanks on staging with large number of columns and panels. The study shows that the presence of a small eccentricity may lead to large displacement of the staging edge in the elastic range, if the torsional-to-lateral time period ratio (tau) of the elevated tanks lies within a critical range of 0.7 < tau < 1.25. Inelastic behaviour study reveals that such excessive displacement in some of the reinforced concrete staging elements may cause unsymmetric yielding. This may lead to progressive strength deterioration through successive yielding in same elements under cyclic loading during earthquakes. Such localized strength drop progressively develop large strength eccentricity resulting in large localized inelastic displacement and ductility demand, leading to failure. So, elevated water tanks should have tau outside the said critical range to avoid amplified torsional response. The tanks supported on staging with less number of columns and panels are found to have greater torsional vulnerability. Tanks located near faults seem to have torsional vulnerability for large tau.
Key Words
    lateral-torsional coupling, elevated water tanks, stagings, reinforced concrete, frame-type, inelastic, strength deterioration
Dutta SC, Deemed Univ, Dept Appl Mech, Bengal Engn Coll, Howrah 711103, India
Deemed Univ, Dept Appl Mech, Bengal Engn Coll, Howrah 711103, India
Indian Inst Technol, Dept Civil Engn, Kanpur 208016, Uttar Pradesh, India

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