Читать книгу Earthquake Engineering for Concrete Dams - Anil K. Chopra - Страница 16

Koyna Dam in India was designed by the traditional static analysis procedure using a seismic coefficient of 0.05. Even though a “no‐tension” criterion was satisfied in the design procedure, as mentioned earlier, the earthquake of December 11, 1967 caused significant horizontal cracks in the upstream and/or downstream faces of a number of the taller non‐overflow monoliths near the elevation at which there is an abrupt change in slope of the downstream face (Figure 1.1.2).

To understand why the damage occurred, the dynamic response of the tallest non‐overflow monolith to the recorded ground motion was computed assuming linear behavior. The results indicated large tensile stresses on both faces, with the greatest values near the elevation where the slope of the downstream face changes abruptly. These computed stresses (shown in Figure 1.3.1), which exceeded 600 psi on the upstream face and 1000 psi on the downstream face, were about two to three times the estimated tensile strength – 350 psi – of the concrete at that elevation. Hence significant cracking consistent with what was observed after the earthquake could have been anticipated. A similar analysis of the overflow monoliths indicated that cracking should not have occurred there, which is also consistent with the observed behavior.