2022 – Structural Considerations for Anchoring Design of Energy Dissipation Structures

Maz Mahzari – Principal Structural Engineer, Stantec, North Sydney, Australia
Shayan Maleki (Barjastehmaleki) – Ph.D, Principal Dams Engineer, Stantec, Brisbane, Australia
Virgilio Fiorotto – Professor, formerly, Dept. of Engineering and Architecture – University of Trieste, Italy

The structural design of the energy dissipation system of dams poses a crucial risk to the scour protection downstream of the large dams. The structural stability of the protection/dissipator is ensured by its weight. However, if a fraction of the uplift force is sustained by anchors, the dynamic behaviour of the system (slabs plus anchors) has to be considered. Whether it is an anchored reinforced concrete liner in a plunge pool or a baffle block in a stilling basin, stochastic and turbulence driven pressures apply significant dynamic actions on the energy dissipation structures. These actions are often transferred to the foundation through an anchoring system. Despite the simplicity of the structural system, these dynamic actions introduce unique challenges which make the design far more complex than a straightforward structural analysis and design exercise.

This study focuses on the critical design aspects of the anchors and their detailing. It is assumed that the forces developed in the anchors are represented in a stochastic sense, in line with the stochastic nature of the turbulence. This is a general case; a deterministic anchor force approach, often used in the industry, can be deemed as a special case of this formulation.

Two frequent governing failure modes of the anchors and anchor heads, namely anchor breaking and fatigue
failure modes, are addressed through a rigorous but practical approach. Well-developed methodologies are
employed to determine the probability of failure corresponding to each one of these modes. The breaking
failure is addressed through estimation of the probability of the anchor force reaching a prescribed breaking failure limit.

For the fatigue failure mode, deterministic S-N curves of the anchor material along with the counting of stress cycles at different stress levels is implemented to determine the fractional damage index and subsequently the fatigue failure assessment. Various operational scenarios of the energy dissipation structure can be considered to better estimate the fatigue service life of the anchors.

A design example is provided to better clarify the methodology and details based on a practical case. Furthermore, some practical aspects regarding the post-tensioned anchors of energy dissipation structures are discussed. These anchors are subject to cyclic turbulence pressure loading, which differentiate them from the post-tensioned anchors often used for stabilising gravity dams. Hence, special fatigue considerations shall be considered in their design.