Keywords: gravity dams, reliability, safety, seismic, pseudo-static, pseudo-dynamic.

Evaluation of the dynamic effects on structures is a subject of great interest in Brazil. Each year, in the country, on average about twenty earthquakes occur with a magnitude higher than 3, two earthquakes with magnitude higher than 4, 0.2 higher than 5 (one every 5 years), 0.02 higher than 6, etc. [1]. Recently, an earthquake with a low magnitude was felt next to the Tucuruí dam. It is a cause for concern because this dam is the biggest in Brazil (100% Brazilian) and the fourth biggest world wide [2,3].

In general dams are huge structures which should be able to resist large forces. They are also expected to carry new forces caused by an earthquake and to continue resisting eventual increases in applied forces induced by this phenomenon. Therefore these structures must maintain acceptable security levels under the action of seismic forces; otherwise important dysfunctions can put human life at risk in the downstream direction.

Dam analysis can be carried at one of five levels. Level 0 - Preliminary analysis considers only a static evaluation. The next four levels take into account seismic analysis: pseudo-static method or seismic coefficient (Level I), it is also called the Westergaard method [4]; pseudo-dynamic method or Chopra method (Level II) [5]; Analysis of linear response in time and, or frequency domains (Level III) and analysis of nonlinear response in the time domain (Level IV).

In this context, this paper presents a probabilistic analysis of concrete gravity dam profiles considering usual loads: dead load, upstream and downstream hydrostatic pressures, and uplift pressures; and also seismic loads. Reliability assessment allows determining safety assured by undamaged dam profiles considering usual geometries. Probabilistic analysis gives a reliability index for a set of typical concrete gravity dam profiles that are determined using the first order reliability method [6]. In this kind of analysis variables are randomly described. Dam sliding stability is investigated through a suitable safety margin, since seismic loads contribute to the sliding between the dam and foundation. Furthermore, this safety criterion characterizes one of the main failure modes identified in dams.

Among the levels of analysis cited above, this paper considers levels I and II. The main results show that it is important expected to define differences in safety for dam profiles considering different levels of seismic analysis (pseudo-static and pseudo-dynamic methods) and geometry shapes.

1

Assumpção M.S., Neto C.M.D, "Sismicidade e Estrutura Interna da Terra", In Teixieira W., Toledo M.C.M., Fairchild T.R., Taioli F., "Decifrando a Terra", São Paulo, Oficina de Textos, 2000.

2

http://portalamazonia.globo.com/noticias.php?idN=54200&idLingua=1

3

http://blogdowaldyr.blogspot.com/2007/06/tremor-de-terra-assusta-moradores-de.html

4

Westergaard H.M., "Water Pressures on Dams during Earthquakes", in "Proceedings of the American Society of Civil Engineering", paper 1835, 1931.

5

Chopra A.K., "Earthquake Resistant Design of Concrete Gravity Dams", in "Proceedings of the American Society of Civil Engineering", Journal of the Structural Division, vol. 104, no. ST6, June, 1978.

6

Melchers R.E., (Editor), "Structural Reliability Analysis and Prediction", Wiley, 1999.

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