Yanjie Xu

Non‐linear seismic response of arch dams with contraction joint opening and joint reinforcements

A comprehensive study of non-linear seismic response of arch dams with contraction joint opening and joint reinforcements has been conducted. A numerical model of contraction joint reinforcements is presented for optimization control of the joint opening. The objective of this control is to reduce the joint opening and expectantly to balance the sustained loads between the horizontal and the vertical components of the dam, thus avoiding an overstress in the cantilever while retaining the release of arch tensile stresses to some extent. Several parameter studies such as critical element size and required number of joints to be simulated for convergence are also performed. As an engineering application, a 292-m high arch dam (the Xiaowan arch dam) and the Big Tujunga dam are analysed in detail. The results demonstrate that the joint opening and the corresponding load transfer from the arch to cantilever components of the dam during strong earthquakes are substantial. It is also evident that by providing sufficient strength and reinforcement flexibility, the joint opening can be controlled to some extent. However, the stress redistribution due to reinforcement control is not sufficient to avoid the overstress in the cantilever for the Xiaowan arch dam. Thus, alternative measures are discussed.

Seismic performance assessment of arch dams using incremental nonlinear dynamic analysis

In this paper, an approximately incremental dynamic analysis (IDA) is presented for seismic performance assessment of arch dams. The nonlinear seismic analysis of arch dams involves the effects of contraction joint opening, cracking of concrete and foundation radiation damping. Three damage measures, i.e. the maximum joint opening, the cracking depth on the dam–foundation interface and the extent of cracking in the upper portion of the dam, associated with the IDA curves are suggested to identify the performance levels of arch dams. The Dagangshan arch dam (210 m high) under construction in China is used as the case study. Although there is slight cracking on the dam–foundation interface during earthquake with the peak ground acceleration (PGA) up to .30 g, the dam exhibits nearly linear behaviour with contraction joints closing and can be approximately considered to have serviceability performance. The dam suffers severe damage on the dam–foundation interface and in the upper portion, and it is repairable under the design earthquake (PGA = .56 g) or the check earthquake (PGA = .66 g). The collapse prevention performance of the dam is achieved when the PGA reaches 1.10 g, in which case, the horizontal cracking in the upper portion penetrates the dam blocks and generates partially free cantilevers that control the dynamic stability of the structure.

A SEISMIC FREE FIELD INPUT MODEL FOR FE-SBFE COUPLING IN TIME DOMAIN

A seismic free field input formulation of the coupling procedure of the finite element (FE) and the scaled boundary finite-element (SBFE) is proposed to perform the unbounded soil-structure interaction analysis in time domain. Based on the substructure technique, seismic excitation of the soil-structure system is represented by the free-field motion of an elastic half-space. To reduce the computational effort, the acceleration unit-impulse response function of the unbounded soil is decomposed into two functions; linear and residual. The latter converges to zero and can be truncated as required. With the prescribed tolerance parameter, the balance between accuracy and efficiency of the procedure can be controlled. The validity of the model is verified by the scattering analysis of a hemi-spherical canyon subjected to plane harmonic P, SV and SH wave incidence. Numerical results show that the new procedure is very efficient for seismic problems within a normal range of frequency. The coupling procedure presented herein can be applied to linear and nonlinear earthquake response analysis of practical structures which are built on unbounded soil.

Seismic damage analysis of the outlet piers of arch dams using the finite element sub-model method

This study aims to analyze seismic damage of reinforced outlet piers of arch dams by the nonlinear finite element (FE) sub-model method. First, the dam–foundation system is modeled and analyzed, in which the effects of infinite foundation, contraction joints, and nonlinear concrete are taken into account. The detailed structures of the outlet pier are then simulated with a refined FE model in the sub-model analysis. In this way the damage mechanism of the plain (unreinforced) outlet pier is analyzed, and the effects of two reinforcement measures (i.e., post-tensioned anchor cables and reinforcing bar) on the dynamic damage to the outlet pier are investigated comprehensively. Results show that the plain pier is damaged severely by strong earthquakes while implementation of post-tensioned anchor cables strengthens the pier effectively. In addition, radiation damping strongly alleviates seismic damage to the piers.

Seismic stability assessment of an arch dam-foundation system

A seismic stability assessment of arch dam-foundation systems is presented using a comprehensive approach, in which the main factors that significantly influence the seismic response of an arch dam-foundation system are considered. A large scale finite element model with over 1 million degrees of freedom is constructed for the Baihetan arch dam (289 m high), which is under construction in the Southwest of China. In particular, the complicated geological conditions with faults intersecting interlayer shear weakness zones at the dam base and the dam abutment resisting force body is modeled in the analysis. Three performance indices are adopted to assess the seismic stability of the arch dam. The results demonstrate that the opening of the joints of the Baihetan arch dam is small and the water stop installed between the joints would not be torn during a design earthquake. The yielding formed in the interface between the dam and foundation does not reach the grouting curtain that would remain in an elastic state after an earthquake. The yielding zones occurring on the upper portion of the dam faces extend 1/8 thickness of block section into the dam body and thus cantilever blocks need not be concerned with sliding stability. The faults and interlayer shear weakness zones in the near field foundation exhibit severe yielding, and a potential sliding surface is penetrated. Although the factor of safety against sliding of the surface fluctuates with a decreased trend during an earthquake, the minimum instantaneous value reaches 1.02 and is still larger than 1.0. Therefore, a conclusion is drawn that the Baihetan arch dam-foundation system will remain stable under the design earthquake.

Chapter 13 – A Comparative Study of the Different Procedures for Seismic Cracking Analysis of Concrete Dams1

Different procedures are compared for the three-dimensional seismic cracking analysis of gravity and arch dams during strong earthquakes. The fracture procedures include the extended finite element (FE) method with cohesive constitutive relations, crack band FE method with plastic-damage relations and the FE Drucker–Prager elastoplastic model. These procedures are used to analyze the nonlinear dynamic response of Koyna dam to the 1967 Koyna earthquake and the seismic cracking of the Dagangshan arch dam subjected to design earthquakes. The cracking process and profiles of the two dams using the three different procedures are compared. The applicability and suitability of the three procedures for seismic cracking analysis of gravity and arch dams are discussed.

Chapter 12 – Seismic Damage-Cracking Analysis of Arch Dams Using Different Earthquake Input Mechanisms1

A nonlinear model is presented for analysis of damage-cracking behavior in arch dams during strong earthquakes using different seismic input mechanisms. The nonlinear system includes a plastic-damage model for cyclic loading of concrete considering strain softening and a contact boundary model of contraction joint opening. Two earthquake input mechanisms are used: a massless foundation input model and a viscous-spring boundary model considering radiation damping due to infinite canyon. The results show significant effects of seismic input mechanism and radiation damping on nonlinear response and damage-cracking of the dam. Compared with the results using the massless foundation input model, the damage-cracking region and contraction joint opening are substantially reduced using the viscous-spring boundary model taking into account radiation damping. If the damping ratio of the dam is artificially increased to ∼10–15% for the massless foundation input model, the joint opening and damage-cracking of the dam are comparable to results obtained from the viscous-spring boundary model.