SHAKER TABLE SEISMIC TESTING OF EQUIPMENT USING HISTORICAL
STRONG MOTION DATA SCALED TO SATISFY A SHOCK RESPONSE SPECTRUM
Revision B
By Tom Irvine
Email:
March 6, 2015
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The purpose of this paper is to apply the author’s aerospace shock testing experience to earthquake engineering. The resulting method can be used either for shaker table testing or for finite element modal transient analysis. The method can also be used for other base input shock events such as pyrotechnic shock.
Introduction
Consider the following type of equipment:
1.Telecommunication
2.Medical life-support
3.Network servers
4.Nuclear power plant control consoles
Now consider that this equipment is to be installed in buildings in an active seismic zone. The equipment must be designed and tested accordingly to withstand the dynamic loads.
A typical specification format for the loading is the shock response spectrum (SRS). The testing is performed on a shaker table. A control computer applies a drive signal to the shaker table to meet the SRS.
The SRS models the responses of individual single-degree-of-freedom systems to a common base input. The natural frequency of each system is an independent variable. The damping value is usually held constant. The SRS calculation retains the peak response of each system as a function of natural frequency. The resulting SRS is plotted in terms of peak acceleration (G) versus natural frequency (Hz).
A given time history has a unique SRS. On the other hand, a given SRS may be satisfied by a variety of base inputs within prescribed tolerance bands.
The SRS format thus offers flexibility. A common testing approach is to use a burst of wavelets such that the synthesized time history resembles a transient sine sweep with descending frequency.
The drawback is that the time history used to satisfy the SRS may be very dissimilar from the measured source data. This brings us a number of concerns including linearity and multi-modal response. Some of these concerns can be quantified in terms of the scalar temporal moments which quantify the energy, RMS, skewness and kurtosis.
These concerns have led to a desire to synthesize a time history which “resembles” the real-world event while still satisfying the SRS
The purpose of this paper is present a method which uses a measured reference time history as a basis for synthesizing a time history to meet the SRS specification
Method Description
The following steps use trial-and-error-random number generation with some built-in convergence. The method is implemented as a function in the Vibrationdata Matlab GUI package.
The first step is to decompose the reference time history into a series of wavelets using the method in Reference 1. An acceleration wavelet has zero net velocity and zero net displacement. A series of wavelets likewise has these properties. The wavelet equation is given in Appendix A. Wavelets are very amenable to shaker table shock testing. They are also convenient for analysis. The examples in the paper use a series of 200 wavelets to model a reference time history.
The second step is to randomly vary the wavelet amplitudes so that the modified wavelet series will have an SRS that matches the specification as closely as possible. The number of iterations may be 16000 or so. The modified time history will thus have some distortion relative to the reference, but this is needed to shape the time history so that its SRS meets the specification.
The second step yields an SRS that has some peaks and dips relative to the specification. This is a consequence of trying to adapt a measured time history to a smoothed SRS.
The third step is to add wavelets so that the resulting SRS meets the specification within, say, + 3 dB tolerance limits.
The third step also adds some distortion. The amount of distortion depends largely on how much the SRS specification differs from that of the reference data.
These steps are demonstrated in the following example, as well as in another example in Appendix B.
Example
Figure 1. El Centro Earthquake Time History
The measured 1940 El Centro North-South time history from Reference 2is used as a basis. This time history will be modeled with a series of wavelets.
The magnitude of this earthquake was 7.1. It was the first earthquake for which useful “strong motion data” was measured in terms of engineering purposes. Note that the North-South component was horizontal with respect to the ground.
Figure 2. El Centro North-South Shock Response Spectrum
Note that the amplification factor Q=50 is equivalent to 1% damping. The fundamental mode of a launch vehicle mounted on a pad at Vandenberg AFB might have 1% damping.
Figure 3. SRS Specification Curves for Central California
The specification in Figure 3 is from Reference 3. It can be used for ground support equipment and launch vehicles at Vandenberg AFB.
The 1% curve is used in the following example.
Figure 4. Time History Evolution
The top time history is the measured El Centro NS data. The middle time history is the wavelet series model. The bottom time history has additional wavelets to improve the SRS match.
The Modified time history appears reasonably similar to the Reference, although no attempt is made to quantify the “similarity.”
Figure 5. Modified Acceleration Time History with Corresponding Velocity and Displacement
The velocity and displacement time histories are well-behaved which is important for both testing and analysis.
Figure 6. Shock Response Comparison
The SRS of the modified, or synthesized, time history is within + 3 dB of the nominal specification. The method is thus successful in generating an El Centro-like time history to meet the specification.
The SRS was calculated using the method in Reference 4.
Future Work
Additional specification should be considered, including the Bellcore specifications.
Additional historical strong motion accelerograms should also be considered.
A standard library of scaled time histories could then be established with all the permutations.
The synthesis process may be more appropriate for the velocity rather than the time history given that geophones are often used for source measurements. The resulting acceleration could then be obtained by differentiation.
It may also be possible to meet all of the goals while still optimizing the pulse for minimum displacement, which could be a concern for the shaker table displacement limits.
References
1.R.C. Ferebee, T. Irvine, et al., An Alternative Method of Specifying Shock Test Criteria, NASA/TM-2008-215253, 2008.
2.Pacific Earthquake Engineering Research Center
3.NASA HDBK-7005, Dynamic Environmental Criteria, 2001.
4.D. Smallwood, An Improved Recursive Formula for Calculating Shock Response Spectra, Sandia National Laboratories, New Mexico.
5.C. Harris, Shock and Vibration Handbook, Fourth Edition; W.J. Hall, Chapter 24, Vibrationdata of Structures Induced by Ground Motion, McGraw-Hill, New York, 1996.
APPENDIX A
Wavelet Equation
The equation for an individual wavelet is
(A-1)
where
/ = / acceleration of wavelet m at time t/ = / wavelet acceleration amplitude
/ = / wavelet frequency
/ = / number of half-sines
/ = / wavelet time delay
Note that must be an odd integer greater than or equal to 3. This is required so that the net velocity and net displacement will each be zero.
The total acceleration at time t for a set of n wavelets is
(A-2)
Figure A-2.
A sample, individual wavelet is shown in Figure A-2. This wavelet was a component of a previous analysis for an aerospace project.
APPENDIX B
Another Example
Figure B-1. Specification
The specification SRS in Figure B-1 is taken from Reference 5. The initial slope is 26.5 dB/octave, which seems unrealistically steep with respect to measured data. Perhaps the reason was to limit shaker table displacement.
Note that measured shock time histories typically have initial SRS slopes between 6 and 12 dB/octave, corresponding to constant velocity and constant slope respectively.
Figure B-2. Time History Evolution
The El Centro NS data is again used as a basis. The modified time history departs somewhat from the Reference in order to meet the SRS specification with its steep initial slope.
Figure B-3. Modified Acceleration Time History with Corresponding Velocity and Displacement
The velocity and displacement time histories are again well-behaved.
Figure B-4. Shock Response Comparison
The SRS of the modified, or synthesized, time history is within + 3 dB of the nominal specification. The method was thus successful in generating a time history to meet the specification, loosely based on the El Centro earthquake.
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