Читать книгу Earthquake Engineering for Concrete Dams - Anil K. Chopra - Страница 13

The ability to evaluate the effects of earthquake ground motion on concrete dams is essential in order to assess the safety of existing dams, to determine the adequacy of modifications planned to improve existing dams, and to evaluate proposed designs for new dams to be constructed. However, the prediction of performance of concrete dams during earthquakes is one of the most complex and challenging problems in structural dynamics because of the following factors:

1 Dams and the impounded reservoirs^† are of complicated shapes, as dictated by the topography of the site (see Figures 1.2.1 and 1.2.2).

2 The response of a dam is influenced greatly by the interaction of the motions of the dam with the impounded water and the foundation, both of which extend to large distances. Thus the mass, stiffness, material damping, radiation damping of the foundation (see Section 1.6), and the earthquake‐induced hydrodynamic pressures must be considered in computing the dynamic response.

3 During intense earthquake motions, vertical contraction joints may slip or open; concrete may crack; and separation and sliding may occur at lift joints in concrete, dam–foundation interface, and fissures in foundation rock. These phenomena are highly nonlinear and extremely difficult to model realistically.

4 The response of dams is affected by variations in the intensity and frequency characteristics of the ground motion over the width and height of the canyon; however, this factor cannot be fully considered at present for lack of instrumental records to define the spatial variations of the ground motion.

Considering all these factors, analytical and computational procedures to determine the response of dam–water–foundation systems subjected to ground shaking are presented in this book. A substructure method for linear analysis of two‐dimensional (2D) models, usually appropriate for gravity dams, is the subject of Chapters 2–6; and of three‐dimensional (3D) models – required for arch dams, buttress dams, and gravity dams in narrow canyons – is the subject of Chapter 8. The Direct Finite‐Element Method (FEM) for nonlinear analysis of 2D or 3D dam–water–foundation systems is presented in Chapter 11.

Figure 1.2.1 Olivenhain Dam, California, USA. Completed in 2003, this is a 318‐ft‐high roller‐compacted concrete dam with a crest length of 2552 ft.

Figure 1.2.2 Morrow Point Dam, Colorado, USA, a 465‐ft‐high single‐centered arch dam.