ML19309H509
| ML19309H509 | |
| Person / Time | |
|---|---|
| Site: | Vallecitos File:GEH Hitachi icon.png |
| Issue date: | 04/30/1980 |
| From: | ENGINEERING DECISION ANALYSIS CO., INC. |
| To: | |
| Shared Package | |
| ML19309H504 | List: |
| References | |
| EDAC-117-254.02, NUDOCS 8005130425 | |
| Download: ML19309H509 (10) | |
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EDAC-ll7-254.02 8 0 0 513 0 'MS CONSERVATISMS IN THE SEISMIC EVALUATIONS s
0F THE GETR REACTOR BUILDING prepared for j
GENERAL ELECTRIC COMPANY Vallecitos, California A WW r*, <h yQ
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ENGINEERING DECISIC.9 At ALYS'S COMPANY,INC.
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TABLE OF CONTENTS Page I NT R O DU C T I ON..............................
I CHARACTERISTICS OF EARTHQUAKES.....................
1 POSTULATED VERONA FAULT........................
4 ANALYTICAL MODELS...........................
5 STRENGTH AND CAPACITY.........................
7 CONCLUSIONS..............................
7 REFERENCES...............................
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CONSERVATISMS IN THE SEISMIC EVALUATIONS OF THE GETR REACTOR BUILDIhG INTRODUCTION This report has been prepared to document the many conservatisms which exist in the seismic evaluations of the GETR Reactor Building. These conservatisms are inherent in the selection of seismic criteria which quantify postulated seismic events: the analytical procedures used to determine the response of the structure to the postulated events, and the acceptance criteria for the structure.
Each of these conservatisms tends to over-estimate response and under-estimate capacities, and the actual overall safety margin is, in fact, quite substantial.
It is not the objective of this document to quantify each of the individual conservatisms but to point out the conservatisms which exist, and illustrate the likely influence of these conservatisms on the total safety margin. This permits, as a minimum, the qualitative conclusion that the total safety margin is substantially above the values determined by the conservative seismic evaluations of the GETR Reac:.or Building.
The following text lists the major areas of conservatisms in the seismic evaluation procedures used for the GETR Reactor Building.
(Table 1, page 2, presents 2. summary of the major areas.)
CHARACTERIZATION OF EARTHQUAKES 1.
Selection of a Low Probability Extreme Event The severity of the earthquakes selected is based on conservative assumptions and a very low probability event.
The use of such extreme events results in a criterion that is much more severe than that used for conventional structures in California.
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SUMMARY
LIST OF AREAS OF CONSERVATISM CHARACTERIZATION OF EARTHQUAKES 1.
Selection of a Low Probability Extreme Event 2.
Use of Wiae-Dand Ground Response Spectra 3.
Conservative Mplification Factors in Response Spectra 4.
Duration of Tin History of Input Motions 5.
Decrease of Grcund Motions With Depth 6.
Effective Ground Acceleration and Damage 7.
Propagation of Seismic Waves Beneath the Base of a Building of Finite Width (" Tau Effect")
POSTULATED VERONA FAULT 8.
Postulated Surface Rupture Offset 9.
" Unsupported Length" in Surface Rupture Offset Case ANALYTICAL MODELS
- 10. Modeling Assumptions -- Response Models
- 11. Modeling Assumptions -- Stress Analysis Model 12.
Embedment Effects
- 13. Additional Nceilinear Effects STRENGTH AND CAPACITY 14.
Static Versus Dynamic Strength
- 15. Concrete Strength 16.
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Use of Wide-Band Ground Respons-Spectra The ground response spectra used for definition of the seismic input are smooth, wide band spectra which conservatively eliminate the areas of less severe amplification in response spectra of actual earthquakes. The l
wide-band response spectra do not represent any single earthquake, but represent response spectra from many earthquakes; thus, the seismic input contains large accelerations over a wide frequency range, which is conservative.
3.
Conservative Amplification Factors in Response Spectra The amplification factors that define the criterion response spectra for the site are conservatively chosen based on statistical stu' dies of past earthquakes.
They are based on the mean recorded amplification plus one standard deviation, which is conservative.
4.
Duration of Time History of Input Motions.
Another factor which contributes to the conservatisms in the seismic
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evaluations is the duration of the time history used in the analyses.
A seismic event on the Verona Fault would most likely produce a time lj (. ! i
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Such high frequency spikes are not damaging and have
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Decrease of Ground Motions With 01pth si i
w'^v It is generally accepted that seismic motions decrease as a function of depth below the ground surface. This effect results in less severe motions at the elevation of the base slab of an embedded structure than the specified free-field criterion motions.
The concrete portion of the GETR Reactor Building is about one-third embedded.
(The ground surface elevation is at approximately 567 ft, the top of the interior concrete j
core is at elevation 611.6 ft, and the base of the foundation mat is at approximately elevation 546.3 ft. Thus, the fraction embedded is (20.7 ft)/(65.3 ft) = 0.32.) The effect of this embedment on the decrease in ground mntions below those specified for free-field has been neglec
4 the seismic evaluations, and the free-field motions were conservatively used as input for the seismic analyses of the GETR Reactor Building.
6.
Effective Ground Acceleration and Damage It is generally recognized that the response of a structure and the resulting potential for damage are not well correlated to maximum instrumental ground motions as recorded during seismic events.
The effective ground acceleration, which correlates with the response of the structure, is actually much less than the maximum instrumental ground motion.
The instrumental values, per se, are of little interest to the structural engineer, and tend to bias upwards the effective values used for structural evaluations.
7.
Propagation of Seismic Waves Beneath the Base of a Building of Finite width (" Tau Effect")
The analytical procedures used in the evaluation of the GETR Reactor are consistent with conventional techniques in which the building is, in effect, assumed to be a vertical cantilever beam.
This modeling
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the response of the reactor building.
Reference 1 presents the results of the investigations of this effect on response spectra shapes.
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POSTULATED VERONA FAULT 8.
Postulated Surf ace Rupture Off set
'^%4 It has been repeatedly stated that the probability of a surface rupture offset occuring under the Reactor Bui M ing is so remote that the structure should not even be evaluated for this condition.
The evaluations for this phenomenon have been performed only in response to requests from the NRC.
The fact that the evaluations have been performed does not in any way imply that it is believed that this event should be an evaluation condition for the GETR Reactor Building.
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5 9.
" Unsupported Length" in Surface Rupture Offset Case As explained in previous documents, it is postulated that the surface rupture offset will tend to lift the Reactor Building and thus create a certain " unsupported length" of the building.
This postulated loss of support would produce stresses within the building.
It is highly unlikely that such a condition can physically exist.
In addition, evaluations performed have ignored the constraining effects of the surrounding soil which would tend to resist lifting up of the building as postulated for this severe analysis case.
ANALYTICAL MODELS 10.
Modeling Assunptions -- Response Models The analyses of the GETR Reactor Building for seismic events on the Calaveras and Verona faults were performed in two steps.
The first step was to use linearly elastic lumped mass " stick" models to obtain dynamic response, and the second step was to perform detailed stress analyses using a three-dimensional finite element model of the concrete core structure of the Reactor Building.
The linearly clastic response model neglects the considerable energy dissipation characteristic of the structure due ta nonlinear behavior, and thus conservatively over-estimates response.
- 11. Modeling p sumptions -- Stress Analysis Model The analyses used to datermine the internal stresses within the Reactor core structure are based on a conservative model.
For example, as mentioned later, embedment constraining effects were ignored.
Other conservatisms arise from the following:
A.
The model is extremely severe in that it only considers the core of the concrete structure, when in fact there is considerable additional strength from the remainder of the structure which includes the circular wall around the structure between the l
basement and first floors, as well as additional columns, beams, and slabs.
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The inertial forces were assumed to be concentrated in the regions of the floors, rather than distributed over the height of the structure. This assumption was made to simplify the computations, but results in conservative analyses in terms of local element stresses.
C.
The stress analysis model for the Verona event includad the consideration of a totally immobilized wall between the basement and first floor levels, which was assumed to fail due to soil pressures. The likelihood that this wall will fail is remote.
The f act that the structure is still adequate without this wall demonstrates that there is tremendous reserve strength within the building.
In addition, the original stress analyses for the Calaveras case were performed for the same model which excluded this wall, which also is an extremely severe and conservative assumption.
12.
Embeament Effects As mentioned under Item 5, it should be recognized that the GETR Reactor Building is a partially embedaed structure, and is embedded roughly one-third of the height of the main concrete structure of tb.c building.
It is clear that this embedment will have a significan*, effect in reducing the stresses in the Reactor Building.
The linearly elastic stress analysis model, as well as the nonlinear response models, have ignored this constraining embedment effect, which will effectively reduce the height of the building due to the resisting soil pressures and due to the vertical loads imposed by the soils, which will tend to resist the overturning moments produced by the seismic response.
Detailed treatment of this embedment effect would reduce the response and stresses in the building by a significant percentage.
13.
Additional Nonlinear Effects Analyses performed to date assume that there is a perfectly rigid connection between the base slab and the soil medium.
It is probable that, for high levels of excitation and thus high base shears, there will be a very thin soil layer in which there is likely to be frictional-type f ailures of the soil and perhaps slight movements at this soil-concrete interface.
This slight movement will permit substantial energy dissipation, and thus decreased response in the structure, and will
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j STRENGTH AND CAPACITY 14.
Static Versus Dynamic Strength The strength of structural materials tends to be greater under dynamic loading conditions, such as those encountered during earthquakes, than under static loading conditions -- the normal method of determining capacities of materials. This potential increase in strength has been neglected in the seismic evaluations of the GETR.
- 15. Concrete Strength As stated in Reference 3, the strength at initiation of cracking was conservatively selected.
The capacity of the structure is thus greater than calculated.
16.
Energy Dissipation Capacity A major contributor to ultimate structural capacity is in the inelastic range beyond the yield point where capacity to dissipate energy enables structures to withstand very severe ground motions. A very small amount of yielding (or vacking and resultant frictional motion between cracked surfaces) can dissipate a large amount of energy.
This energy dissipation means that the inherent reserve strength of the structure to withstand collapse is significantly greater than predicted by conventional analytical methods which do not consider such energy dissipation.
CONCLUSIONS In conclusion, it is evident that there are numerous conservatisms in the procedures used to evaluate the adequacy of the GETR Reactor Building to resist postulated seismic events. Also, it must be noted that the conservatisms are cumulative; the total safety margin is a product of the many individual margins described above.
It is not the objective of this OFFICIAL SEAL T
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8 document to quantify each of the individual margins. The likely influence on total safety margin can be illustrated, however, by the following example.
If it is assumed that the loads (L) calculated by conventional procedures are roughly equal to the capacities (C) calculated by conventional A
procedures, (i.e. L = C = 1.0), then the actual loads (L'), if they were developed on the basis of procedures which eliminated the conservatisms as described above, could clearly equal seventy percent or less of the loads obtained by conventional procedures (i.e. L' = 0.7L = 0.7).
Similarly, the capacities as obtained by procedures which eliminated conservatisms could be at least thirty percent greater than obtained by conservative procedures (i.e. C' = 1.3C = 1.3).
Thus, the actual safety margin would be on the order of (1.3)/(0.7) = 1.9.
It is clear that a safety margin at least on the order of that obtained from this example can be realistically expected.
Thus, if all individual margins were quantified, the result would be a total margin of safety significantly above (and likely on the order of at least two times) that conservatively determined by the seismic evaluations of the GETR Reactor Building.
REFFRENCES 1.
Engineering De-ision Analysis Company, Inc., " Review of Seismic Design Criteria for the GETR Site," EDAC-117-254.03, prepared for General Electric Company, 30 April 1980.
2.
Engineering Decision Analysis Company, Inc., " Seismic Analysis of Reactor Building, General Electric Test Reactor - Phase 2,"
EDAC-ll7-217.03, prepared for General Electric Company, 1 June 1978.
l 3.
General Electric Company, " Response to NRC Questions, Structural Issues, Part I, General Electric Test Reactor," submitted to NRC 24 April 1980.
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