Earthquake Engineering for Concrete Dams Design, Performance, and Research Needs (1990) / Chapter Skim
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5. Experimental and Observational Evidence
5. Experimental and Observational Evidence
Pages 80-103
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From page 80... ...
Ground motions were not recorded, but with the epicenter in close proximity they probably reached significant intensity. Perhaps the strongest shaking experienced by a concrete dam to date was that which acted on Lower Crystal Springs Dam, a curved gravity structure with the modest height of 42 m (shown in Figure 5-2)
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From page 81... ...
However, the stability of this structure exceeds that of typical gravity dams due to its curved plan and a cross section that was designed thicker than normal in anticipation of future heightening, which was never completed. Another example of a concrete dam subjected to strong shaking is the 103-m-high Pacoima (arch)
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From page 82... ...
82 FIGURE 5-2 Lower Crystal Springs Dam, California; the San Andreas fault lies under the reservoir (5-3)
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From page 83... ...
is impressive, it falls short of providing complete confidence that large concrete dams with full reservoirs are safe against strong seismic shaking. Cases with Recorded Responses Actual recorded time histories of the response of concrete dams during earthquakes are scarce for events causing other than very small shaking.
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From page 84... ...
However, in the United States only Lower Crystal Springs and Pacoima dams are well enough instrumented to record motions at both the abutments and the toe (2-69, S-1S) , and it may be that the three accelerographs used on the damfoundation interface at each of these sites are not sufficient to define the spatial distribution of earthquake motions.
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From page 85... ...
Finite element models of the dam and foundation region (the latter assumed massless and truncated at a far boundary) with the reservoir water (assumed incompressible)
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From page 86... ...
Linearly elastic finite element analysis (3-6) in which dam-water interaction effects were approximately included showed that net tensile stresses near the point of slope change on the downstream face and at a similar elevation on the upstream face significantly exceeded the tensile strength of concrete when the recorded ground motion was used as input.
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From page 87... ...
The linearly elastic analyses of Yuda Dam employed a two-dimensional finite element model of a 67-m-high monolith with reservoir water included; this monolith was chosen because its resonant frequency matched the observed predominant frequency of response of the three-dimensional darn. With modal damping taken to be 3 percent of critical and using accelerations recorded on one abutment during the two earthquakes as input, the computed response time histories of the crest showed similarities to those recorded on the crest of the 90-m-high monolith.
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From page 88... ...
The elastic moduli of the dam and foundation were adjusted to match measured resonant frequencies from previous forced-vibration field tests. The solution procedure defined seismic input in terms of free-field motions at the foundation interface; these motions, which were applied to both dam and reservoir, were taken as those recorded on the foundation rock a short distance from the dam at both abutments at crest level and at the canyon bottom.
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From page 89... ...
Contraction joints may also affect the forcedvibration behavior of arch dams, as at Talvacchia Dam (5-13) , where differences in resonant frequencies were observed between different tests at the same water level.
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From page 90... ...
dams, comparisons of the computed results with those measured during forced-vibration field tests were made for resonant frequencies; for resonating shapes and amplitudes of the dam, including those at the foundation interface; for frequency-response curves of the dam crest near the resonant frequencies; and for profiles and amplitudes of the dynamic water pressure at the resonant frequencies. The agreement obtained for Xiang Hong Dian Dam, in all comparisons, was quite remarkable and represents a significant step forward in understanding the dynamic behavior of concrete dams; the mode shape comparison is shown in Figure 5-5.
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From page 91... ...
91 o it ~ ~ Hi ~ ~ ~ -- -l o l I!
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From page 92... ...
Good agreement was obtained between computed eigenfrequencies and measured resonant frequencies and between computed mode shapes and shapes measured under ambient conditions. Additional comparisons with the resonant frequencies from other ambient data at water depths equal to 85 and 95 percent of the dam height were good for the first resonance, but the finite element model with incompressible water predicted changes in the next three resonant frequencies for the variation in water level that exceeded the measured changes.
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From page 93... ...
were good for resonant frequencies, resonating shapes of the dam crest, and frequency-response curves of the dam crest near the resonant frequencies. However, less-than-satisfactory agreement was obtained for amplitudes of the dynamic water pressures, including a too-rapid decay with distance from the dam in the numerical results.
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From page 94... ...
Water compressibility effects were exhibited in the forced vibration data through larger measured damping ratios for the symmetric resonances and by the presence of a "cantilever" crossover in the dynamic water pressures at the second symmetric resonance, where the resonating shape of the dam, with only a slight cantilever crossover, closely resembled the shape at the first resonance. An attempt to isolate the fundamental symmetric resonance of the reservoir water domain apparently succeeded by extracting the frequency-dependent amplitude of the water mode from the measured pressure data and normalizing it with the corresponding frequency-dependent measured motion of the dam.
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From page 96... ...
Certainly, further work is needed in evaluating mathematical models using experimental data for significantly different reservoir water depths. Another approach for evaluating a mathematical model is to compare resonant frequencies computed using experimentally determined values of elastic moguls ot the dam and foundation, either by in situ geophysical means or from core tests, with those derived from forced-vibration field tests or ambient measurements.
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From page 97... ...
Best results were obtained for Bai-Shan and Hendrik Verwoerd using a finite element model of the dam and reservoir water; good agreement with measured dynamic water pressure distributions also was obtained at BaiShan. A recent experiment was designed to demonstrate water compressibility effects by employing a steel arch for the dam and was accompanied by numerical analysis that included water compressibility (5-57~.
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From page 98... ...
, very little detailed information from these sources is available in the earthquake engineering literature. What there is, though, indicates that nonlinear behavior is a very important feature in the response of concrete darns to strong ground shaking and that a dam undergoing nonlinear behavior can retain considerable stability.
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From page 99... ...
....-~ -- ~9~ CRACKED FIGURE 5-8 Model of single monolith of Koyna Dam; crack resulted from shaking-table test, including two-dimensional reservoir model (4-3)
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From page 100... ...
to represent a horizontal cross section of an arch dam (shown in Figure 5-9) , it was confirmed that opening of the contraction joints is an important response mechanism.
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From page 101... ...
101 FIGURE 5-10 Setup for shaking-table test of scaled model of Pine Flat Dam monolith, including two-dimensional reservoir (5-65)
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From page 102... ...
Records of dynamic water pressure response would be very useful; practically none exist. Consideration should also be given to measuring movements associated with contraction joints, such as joint openings for arch and gravity dams and relative tangential motions across unkeyed joints in gravity and buttress dams.
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From page 103... ...
3. Nonlinear Phenomena Nonlinear phenomena play an important role in the response to strong shaking of a large concrete dam with full reservoir.
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