2024 – Torsion, The Silent Killer of Intake Towers

Francisco Lopez, Stella Harrison, Michael McKay, Sam Lalli

Torsional effects are often ignored in the seismic analysis of intake towers (both existing and new designs) due to their complexity and the rather vague recommendations to this aspect provided by guidelines and manuals for intake towers. This paper explains the expected torsional behaviour of typical freestanding intake towers and the implications of ignoring torsion in these types of structures, from both torsional capacity and demand perspectives.

The main source of torsional demand in intake towers is due to earthquake loading combined with eccentric masses (such as those from an access bridge, trashracks, bulkheads, valves and other equipment) as well as irregular cross sections of the shaft including inlet port voids and trashrack guides. In this paper a worked example demonstrates how cross sections of intake shafts with intake ports or other types of voids result in a torsional capacity reduction of approximately half of that for the full shaft section without ports. The example also illustrates how the torsional capacity reduction is significantly more sensitive to the presence of intake ports and other voids than the more commonly assessed flexural and shear actions.

The worked example, which includes the mass of an access bridge, shows that the torsional demand is not proportional to ratio of the mass times the bridge span. It is instead the result of the location and magnitude of the mass triggering different frequency behaviours in the tower, in each case engaging different amounts of torsional mass, resulting in different fundamental periods for torsion, and consequently different spectral accelerations. These results highlight the importance of conducting a proper FEA with a rigorous scrutiny of the results when assessing the demand of torsion in intake towers. In the absence of explicit recommendations for torsion by guidelines for dams and appurtenant structures, and based on the theoretical torsional behaviour of freestanding intake towers and the presented worked example, this paper proposes a step-by-step torsional assessment methodology for these types of structures.