application of cumulative absolute velocity (cav) to real-time earthquake damage assessment

7/30/2019 Application of Cumulative Absolute Velocity (Cav) to Real-time Earthquake Damage Assessment

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Application of Cumulative Absolute Velocity (CAV) to Real-timeEarthquake Damage Assessment

Robert L. Nigbor, Ph.D. P.E.

Civil Engineering Department

University of Southern California

Abstract

The CAV parameter, developed as a rational estimator of earthquake damage potential for nuclear power plantapplications, can be used as a damage estimation parameter for many other applications. Modern real-timedigital seismic instrumentation, such as Earthquake Safety Systems' SIRC system, can incorporate CAVcalculations to provide immediate indication of the severity of earthquake shaking. Coupled with site-specificdamage potential thresholds, this provides a rational decision tool for automatic or manual response. This paperdescribes real-time CAV calculations and gives a general procedure for determining appropriate site-specific

damage potential thresholds.

Introduction

Strong shaking due to earthquakes can now be monitored and analyzed in real-time by modern digitalinstrumentation. Such real-time monitoring systems can instantly provide parameters which describe the shakingand can then be used to automatically or manually trigger an appropriate mitigation response. An appropriateparameter for real-time estimation of the damage potential of earthquake shaking is Cumulative Absolute Velocity('CAV').

Simple electronic devices called seismic triggers have been available for more than 20 years to perform this

earthquake monitoring and alarm function. However, these devices use the Peak Acceleration ("PA") parameteronly to trigger an alarm or other response to earthquake shaking. PA is not a good estimator of damage potential asit does not include information about shaking duration. A better estimator is needed for critical real-timeearthquake monitoring applications.

Recently the CAV parameter was developed by the Electric Power Research Institute ('EPRI') in response to aneed within the nuclear power community for a rational criterion for earthquake shutdown of nuclear power plants(see References I and 2). CAV correlates well with earthquake damage potential, and can be applied to otherapplications in addition to nuclear power plants. In digital instrumentation with real-time processing capability,CAV can be calculated in real-time to provide immediate indication of earthquake damage potential.

With appropriate site-specific earthquake damage thresholds, real-time earthquake monitoring systems such asEarthquake Safety Systems' SIRC can apply CAV-based criteria to many different applications from nuclearpower plants to industrial facilities to office buildings.

Background

It is now recognized that PA alone is not a good measure of the damage potential of earthquake shaking.Reference 3 states that

"this (PA) is a poor parameter for evaluating damage potential For instance, a large recorded peak accelerationmay be associated with a short-duration impulse of high frequency. In this case, most of the impulse is absorbedby the inertia of the structure with little deformation. On the other hand, a more moderate acceleration may beassociated with a long-duration impulse of low frequency which results in a significant deformation of thestructure."

There can be considerable variation in the value of PA unrelated to the severity of shaking. For example, the 1985Mexico City Earthquake produced PA values of about 0.2 g. Damage was considerable. PA=0.2g has also been

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measured during nearby small earthquakes (Magnitude 4 Northridge aftershocks, for example) with no reporteddamage. PA only describes the maximum value of earthquake shaking. It does not represent shaking duration,which is equally important to damage potential.

Nuclear power plants have long used the PA parameter to determine if shutdown is required after an earthquake.There are several examples (see Reference 1) where shaking from nearby small earthquakes exceeded PA criteriaand caused the plants to shutdown unnecessarily. To prevent this expensive occurrence, EPRI studied a range ofearthquake shaking parameters to determine which was the most appropriate as a shutdown criterion. CAV waschosen as the most representative of damage potential (Reference 1).

A standardized calculation method for CAV is defined in Reference 2. The units of CAV are g-sec. Thiscalculation method compensates for the limited resolution of the monitoring instrumentation by including in theCAV calculation only those one-second intervals of earthquake shaking for which PA >0.025g.

CAV values for historical earthquakes range from less than 0.1 g-sec to as large as 3 g-sec. In the NorthridgeEarthquake, C,4V values of 1.8 g-sec were measured (Calculated by the author from the USGS Sepulveda VAfree-field strong ground motion). For nuclear power plant applications, EPRI recommends a shutdown threshold ofCAV =0.16 g-sec (Reference 2). This is about 1/3 of the smallest CAV value corresponding to documenteddamage in engineered structures.

A Real-time Earthquake Monitoring Application

Earthquake Safety Systems' SIRC (Reference 4) is a real-time digital earthquake monitoring system whichincorporates CAV calculations. It measures three components (x y, z) of ground or structure acceleration from anexternal accelerometer. Data are digitally processed and parameters for detected events are displayed as shown inFigure 1. Included are PA values for x, y and z and the maximum CAV value. CAV is also displayed in integerunits of "EPRI". with EPRI =20*CAV.

For CAV applications, the acceleration sensor will be located on or below ground level to measure the earthquakeinput to a structure or facility.

This monitoring system can have up to four different thresholds. For example, there might be thresholdscorresponding to Minor, Moderate, and Major earthquake intensities. When a threshold is exceeded, programmedactions occur. These can include audible alarms, visible alarms, automatic paging, or automatic shutdown ofequipment or valves. Thresholds can incorporate a combination of C,4V and PA values, depending upon theapplication.

Damage Threshold Determination

The threshold levels for a specific site are determined by combining general site information, economic/legalissues, and fragility analysis. This should be done by an experienced earthquake engineer in cooperation withpersonnel from the site.

Required Information

CAV

CAV

Ver tambin, ANALYSIS of CUMULATIVE ABSOLUTE

VELOCITY (CAV) and JMA INSTRUMENTAL SEISMIC

INTENSITY... (PACIFIC EARTHQUAKE RESEARCH


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For a particular application, determination of damage potential thresholds should first consider the followingquestions:

1. What are the critical components of the facility or structure in regards to earthquake damage?2. What actions will be taken in response to threshold exceedance?3. What is the cost (direct cost, business interruption cost, other indirect costs) of these actions?4. What is the cost of a false negative (damage occurs but threshold not exceeded)?5. What are the liability issues?

Question 1 requires understanding of the operation of the facility or structure. The answer will vary considerablyfrom site to site. For example, the critical components of an office building will probably be the entire buildingand the parking structure servicing the building. Critical components of a semiconductor manufacturing facilitymay be the physical plant, the chemical tanks, or the air handling system. Critical components for an oil refinerymay be the piping or large tanks.

For Question 2, the site personnel must carefully consider what they will do with the knowledge that earthquakeshaking has exceeded a given threshold. Actions in response to threshold exceedance can be either automatic ormanual. The monitoring system can provide a variety of electrical outputs or alarms for automatic actions. Forexample, valves can be closed, audio messages played over a public address system, or personnel can be paged.Manual actions can include evacuation of buildings, inspection of a structure, or distribution of emergency

response resources.

The actions from Question 2 will likely have direct and indirect financial consequences. These must be consideredin the development of a threshold appropriate for a given site. The cost of a false alarm (Question 3) must beweighed against the cost of a false negative (no alarm but damage occurs, Question 4). For example, automaticallyshutting down a chemical pipeline may cost $ 100,000 in lost revenue. However, not closing the valve beforeearthquake damage occurs and hazardous material is released may cost $1,000,000.

Finally, there are important liability issues surrounding the automatic or manual actions prompted by anearthquake alarm system. Some of these issues are related to the financial implications of either false alarms orfalse negatives. Some are related to human response to earthquakes and earthquake alarms. Although these issues

are mostly intangible, the general issue of liability must be considered when determining appropriate thresholdswhich require action if exceeded.

Fragility Analysis

The fragility levels of each of the components identified in Question 1 will be determined using engineeringanalysis, experience data, or a combination of both. The fragility level can be expressed by CAV alone or by alogical combination of CAV and PA values.

There is a large body of earthquake experience data from recent and historic earthquakes in the United States. This

can provide the most reliable damage threshold values for common structural types and equipment types. CAVvalues are available in Reference I for a large number of earthquakes. CAV values can also be calculated frommore recent recordings of strong ground motions, such as those from the Northridge Earthquake. These can becorrelated with observed earthquake damage near the earthquake instrument locations. Reference I uses suchexperience data to justify the 0.16 g-see CAV threshold for damage to nuclear power plant components andsystems.

For modern engineered structures or components, the earthquake design criterion can be used to develop a damagethreshold criterion based upon PA. CAV can be included as well if design earthquake time histories or spectra areavailable.

For older structures or non-engineered equipment or systems, it may be necessary to determine a fragility levelbased upon further analysis. This could include finite element analysis or experimental testing.

Final Level Determination


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There will always be some uncertainty in the estimated fragility level for a particular site. This is the nature ofengineering calculations and of earthquake shaking.

The financial and legal issues surrounding a particular application of real-time earthquake damage estimation mustbe used to modify the estimated fragility level within the range of uncertainty. If the costs of a false alarm are verygreat (financial or liability), perhaps the final threshold level will be set at the upper end of the range ofuncertainty. If the costs of a false negative are high, the final threshold value will be set at the lower end of therange. This is a "gray" decision, to be made by the engineer in cooperation with the site personnel.

CONCLUSIONS

CAV is a parameter which provides good correlation with damage due to earthquake shaking. It can be cal-culated in real-time by monitoring systems such as Earthquake Safety Systems' SIRC and used to trigger manytypes of automatic and manual responses.

For a particular application of real-time earthquake damage estimation, threshold values of CAV or PA must becarefully determined by an experienced earthquake engineer in cooperation with site personnel. Physical fragilitylevels weighed with economic and legal issues will determine final damage threshold levels

REFERENCES

1. Electric Power Research Corporation, A Criterion for Determining Exceedance of the Operating BasisEarthquake, EPRI Report 2848-16, July 1988.

2. Electric Power Research Corporation, Standardization of the Cumulative Absolute Velocity, EPRI ReportTR-100082, December 1991.

3. Naeim, Farzad and James Anderson, Classification and Evaluation of Earthquake Records for Design,University of Southern California Report CE93-08, July 1993.

4. Model SIRC Real-Time Seismic Intensity Indicator Controller Product Specification, Earthquake SafetySystems, Los Osos, California.

Submitted January 20, 1995

EXPLANATORY NOTES TO FIGURE 1 (By ESS)

1. Cumulative Absolute Velocity - CAV - Cumulative Absolute Velocity (CAV) - Developed by the Electric PowerResearch Institute (EPRI) of Palo Alto, CA, for application in the Nuclear Power Industry. Defined as the integral ofthe absolute value of ground acceleration over the seismic time-history record. The CAV value correlates highly withphysical property damage, however, the fractional nature of the scale is difficult to understand and requiresnormalization.

2. Seismic Intensity Scale - EPRI - This scale (0-100) is a normalized CAV scale, (1 CAV = 20 EPRI). Both are based

on real engineering units and express total energy input over the seismic event.

After identification of structure type, a definitive EPRI scale number is established and programmed, indicating thepoint at which a structure is likely to have sustained various degrees of damage.

3. Action Bulletin - The SIRC is equipped with factory programmed action bulletins which are displayed when userprogrammed setpoint thresholds are exceeded.

These bulletins can be reprogrammed by user if required with an external keyboard. These commands are typicallyused to instruct staff in the required response to a specific seismic intensity recorded, e.g.:- MINOR EVENT,

MODERATE EVENT, MAJOR EVENT, CATASTROPHIC EVENT, etc.

4. User Programmable Threshold Levels (indicates setpoints selected on each scale). - There are a total of seven(7)user-selectable threshold levels available. Four (4) on the Seismic intensity (EPRI) Scale and three (3) on the PGA


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(acceleration) scale. Each setpoint selected, is independently associated with a control relay and or an indicator lightabove the screen.

5. Calendar and Time Clock - A calendar clock records current date and time.

6. 24 hour Clock and Duration Record - A 24 hour clock records exact time at which event started and duration of the

seismic event in seconds.

7. Peak Ground Acceleration - PGA - Peak Ground Acceleration on three axes (x, y, z), are recorded simultaneouslywith the Seismic Intensity scale.

application of cumulative absolute velocity (cav) to real-time earthquake damage assessment

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