Earthquake Engineering for Concrete Dams Design, Performance, and Research Needs (1990) / Chapter Skim
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6. Recommended Criteria for Evaluating Seismic Performance
6. Recommended Criteria for Evaluating Seismic Performance
Pages 104-126
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From page 104... ...
Consequently, concrete dams were considered to be safe during earthquake loading conditions if computed tensile stresses were small or nonexistent, if resultant forces in two-dimensional sections through gravity dams fell within the central onethird of their bases, and if compressive stresses were computed to be less than an allowable working stress (usually 1,000 psi or less)
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From page 105... ...
More applicable criteria began to be developed partly as a result of dynamic finite element analyses conducted for concrete dams that successfully withstood the relatively large ground motions associated with the 1971 San Fernando earthquake in California. Criteria were eventually set forth whereby computed tensile stresses large enough to indicate the initiation of cracking, and compressive stresses larger than the allowable working stress level, were understood to not necessarily indicate structural instability (6-1~.
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From page 106... ...
However, some preliminaries concerning initial conditions and analytical procedures must be addressed before these criteria can be considered. Inappropriate recognition of initial conditions or misapplication of analytical procedures may produce numerical results that cannot be meaningfully related to expected seismic performance regardless of the criteria used.
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From page 107... ...
However, for concrete dams whose behavior is three-dimensional, including some concrete gravity dams (see section below titled Two-Dimensional Versus Three-Dimensional Analytical Models) , the effects of temperature changes must be assessed in order to adequately evaluate seismic performance.
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From page 108... ...
However, more research concerning the creep behavior of the mass concrete used to construct dams is needed, particularly for multiaxial loading conditions. Uplift Another factor that can have a significant influence on the seismic performance of a concrete dam is uplift.
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From page 109... ...
Pore pressures, based on appropriate considerations regarding the presence and effectiveness of drainage systems within the dam and its foundation, can then be summed with total stresses to calculate effective stresses (6-4, 6-5~. For seismic loading conditions, where a dam oscillates rapidly, the present state of practice for linear elastic analyses is to not attribute any additional significance to pore pressures beyond their effects during static loading conditions.
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From page 110... ...
For example, in the case of a relatively narrow site where additional restraint is provided by the abutments, the restraint can increase compressive stresses near the heel of the dam along the upstream face during static loading conditions, thereby reducing the magnitude of tensile stresses developing during seismic conditions. Two-dimensional analyses will almost always indicate the development of high tensile stresses during large earthquakes at these locations, whereas three-dimensional analyses may indicate much lower stresses (6-4~.
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From page 111... ...
thermal strains. GUIDELINES FOR EVALUATING RESULTS FROM LINEAR ANALYSES At present, seismic safety evaluations of concrete dams are usually based on numerical results from linear dynamic finite element response analyses.
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From page 112... ...
However, for the range of compressive strengths common for concretes used to construct dams, the intact uniaxial static tensile strength is approximately 10 percent of the static uniaxial compressive strength (4-12~. When increased 50 percent for rapid loading conditions, intact uniaxial concrete tensile strengths approximately equal to 15 percent of static uniaxial compressive strengths are presently considered appropriate for evaluating the seismic performance of concrete dams.
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From page 113... ...
This results in apparent rapid loading tensile strengths that are approximately 20 percent of static uniaxial compressive strengths for the range of compressive strengths common for concretes used to construct dams. While the concept of an apparent tensile strength is valid for evaluating the results from linear elastic numerical analyses for severe seismic conditions, it is important to remember that for all concrete dams the limiting tensile strength of the concrete is that which exists across lift surfaces (the horizontal surfaces between concrete placements that are typically spaced at intervals of 1 to 10 ft over the height of the dam, depending on the type of dam and the type of construction)
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From page 114... ...
Evaluating Seismic Performance At present the process of evaluating the seismic performance of concrete dams using results from linear elastic numerical analyses, finite element or other, is in most cases deterministic. The process involves comparing computed levels of stress with levels that are considered acceptable based on considerations of concrete strength and the likelihood of significant earthquakes occurring.
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From page 115... ...
Computed compressive stresses will usually be well below this compressive strength limit; however, it is not unusual for computed tensile stresses to exceed the concrete's rapid-loading tensile strength when ground motion is severe. Unless the numerical model developed to perform the seismic analysis accounts for the existence of vertical contraction joints, horizontal tensile
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From page 116... ...
In the case of arch dams analyzed assuming linear elastic material behavior, the structure may exhibit a tendency toward developing a partial failure, usually resembling the shape of a semicircular or rectangular notch in the upper central portion of the dam, if the calculated seismic stresses are large. Whether such partial failures could actually occur is unknown, since they have not actually been observed.
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From page 117... ...
If extensive cracking and a significant reduction in sliding resistance are indicated, sliding stability can be checked using time histories of nodal point forces, which are available as part of the output from most finite element programs presently used to perform time history dynamic analyses. If the particular finite element program used does not provide time histories of forces directly, relatively simple modifications to the program are usually possible to obtain them.
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From page 118... ...
Because of this reinforcement and because so few buttress dams have been constructed during the past 20 to 30 years, there are no updated criteria intended specifically for such dams that are comparable to available criteria for arch and gravity dams. Consequently, to evaluate the seismic performance of concrete buttress dams based on results from numerical analyses, it is recommended that the most recent criteria developed by the American Concrete Institute (6-11)
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From page 119... ...
Additionally, there can be cases where the geometry of the abutment surfaces is not conducive to sliding stability, adequate drainage is not provided along the contact, and the concrete is not thoroughly bonded to the foundation rock. In such cases the results obtained from time history finite element analyses, together with results from static analyses that include the effects of uplift, can be used to calculate factors of safety against sliding along the abutments.
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From page 120... ...
Depending on the number of time intervals when instability is indicated by a linear time history analysis and the extent of the instability, a time history sliding-block analysis may be required to fully evaluate stability. While load transfers involving significant blocks within the foundation may be acceptable for infrequent, extreme loading conditions, they should generally be considered unacceptable for design loading conditions.
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From page 121... ...
However, there are portions of the presently used criteria that should be reviewed and possibly revised. For example, past research and observations of prototype behavior have clearly shown that pseudostatic stability analyses using simple seismic coefficients will not realistically predict the response of concrete gravity dams to strong earthquakes.
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From page 122... ...
Most well-designed, well-constructed concrete gravity dams analyzed two dimensionally for their response to significant earthquake excitation will exhibit the tendency to develop tensile stresses near and along the foundation contact. If the dam is well proportioned, if an appropriate amount of foundation preparation has been performed, and if appropriate construction techniques have been employed, sufficient tensile capacity across the interface may exist to resist these tensile stresses.
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From page 123... ...
Probability-Based Criteria It is evident from the discussions in the preceding sections that the criteria presently used to evaluate the seismic performance of concrete dams are deterministic. Since characterizing the occurrence of earthquakes is most meaningful in terms of probabilities of occurrence, use of probability-based criteria to evaluate the seismic performance of concrete dams is an appropriate approach.
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From page 124... ...
Therefore, the following research is also recommended: —further characterization of the creep behavior of the mass concrete used to construct dams, together with analytical improvements in accounting for creep in numerical analyses, studies of stress-strain relationships during rapid multiaxial loading conditions, especially including tension, —identification of failure mechanisms for concrete dams and Heir foundations during earthquakes and delineation of the conditions that would cause various failure mechanisms to develop, and Development of defensive design measures to safely accommodate fault displacements.
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From page 125... ...
125 concrete during multiaxial loading conditions would be of particular value for developing deformation or strain-based criteria. Such criteria should be a priority for research and development because it offers the potential for more realistic evaluation of mass concrete behavior under a variety of loading conditions, especially those including seismic events.
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