2022 – Earthquake Guidelines and Intake Towers: Small Shortcomings with Serious Implications

Francisco Lopez – Chief Technical Principal, SMEC Australia and Michael McKay – Associate Engineer, SMEC Australia

Seismic performance of intake towers is fundamental for dam safety, as they would allow post-earthquake dewatering of reservoirs in case a dam need repairing. Likewise, the collapse of an intake tower may result in an uncontrolled water release from the reservoir.

The recently published ANCOLD Guidelines for Design of Dams an Appurtenant Structures for Earthquake (2019) do not present a detailed procedure for seismic assessment of intake towers. Instead, the Guidelines refer to other guidelines and manuals from ICOLD and USACE for more detailed advice.

One aspect of particular importance contained in the guidelines and manuals is the treatment of inelasticity of the intake towers when analysed by means of linear elastic methods such as the response spectral approach. Unfortunately, the guidelines and manual do not expand in the explanation of the inelasticity phenomenon, and end up recommending an indiscriminate use of a response reduction factor (also referred to as Demand-Capacity Ratio, DCR) of 2.0 to account for inelastic behaviour and energy dissipation under large earthquakes. This factor, in practice, halves the demand of flexural moment at the base of intake towers under large earthquakes.

The ductility factor concept used by the guidelines and manuals for intake tower is based on the work of Newmark (1960, 1982), who postulated that the energy stored by an elastic system at maximum displacement is the same as that stored by an inelastic system. However, Newmark’s relation between displacement and stored energy is highly dependent on the fundamental period of the structure, an essential detail that the guidelines and manuals seem to forget.

This paper explores the recommendations presented by ANCOLD and other related guidelines and manuals for analysis of intake towers, and explains Newmark’s same energy and same displacement concepts and their association with the structural period. The paper also notes the implications of an indiscriminate use of the response reduction factor of 2.0 prescribed by guidelines, and how the lack of a proper scrutiny of the specific structural characteristics of intake towers may lead to the erroneous conclusion that they are structurally inadequate and therefore may require complex and expensive strengthening works.

Finally, in order to illustrate the impact of the guideline’s ductility prescriptions versus Newmark’s actual principles, the paper examines the intake towers from five reservoirs in Australia, each with different geometry and structural configuration.

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