31.3 Seismic Qualification

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Frequently, it is necessary to determine whether a given piece of equipment is capable of withstanding a

pre-established seismic environment in a specific application. This process is known as seismic

qualification. For example, electric utility companies should qualify their equipment for seismic

capability before installing it in earthquake-prone geographic localities. Safety-related equipment in

nuclear power plants also requires seismic qualification. Regulatory agencies usually specify the general

procedures to follow in seismic qualification.

Seismic qualification by testing is appropriate for complex equipment, but in such cases, the

equipment size is a limiting factor. For large systems that are relatively simple to model, qualification by

analysis is suitable. Often, however, both testing and analysis are needed in the qualification of a given

piece of equipment. Seismic qualification of equipment by testing is accomplished by applying a dynamic

excitation by means of a shaker to the equipment, which is suitably mounted on a test table, and then

monitoring structural integrity and functional operability of the equipment. Special attention should be

given to the development of the dynamic-test environment, mounting features, the operability variables

that should be monitored, the method of monitoring functional operability and structural integrity, and

the acceptance criteria used to decide qualification.

In monitoring functional operability, the test facility would normally require auxiliary systems to load

the test object or to simulate in-service operating conditions. Such systems include actuators,

dynamometers, electrical-load and control-signal circuitry, fluid-flow and pressure loads, and thermal

loads. In seismic qualification by analysis, a suitable model is first developed for the equipment, and then

static or dynamic analysis (or computer analysis) is performed under an analytically defined dynamic

environment. The analytical dynamic environment is developed on the basis of the specified dynamic

environment for seismic qualification. By analytically or computationally determining system response at

various locations, and by checking for such crucial parameters as relative deflections, stresses, and strains,

qualification can be established.

31-6 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

31.3.1 Stages of Seismic Qualification

Consider the construction of a nuclear power plant. In this context, the plant owner is the customer.

Actual construction of the plant is done by the plant builder, who is directly responsible to the customer

concerning all equipment purchased from the equipment supplier or vendor. Often, the vendor is also

the equipment manufacturer. The equipment may be purchased by the customer and handed over to the

plant builder or directly purchased by the plant builder. Accordingly, the purchaser could be the plant

builder or the plant owner. A regulatory agency might stipulate seismic-excitation capability

requirements for the equipment used in the plant, or might specify the qualification requirements for

various categories of equipment. The customer is directly responsible to the regulatory agency for

adherence to these stipulations. The vendor, however, is responsible to the plant builder and the customer

for the seismic capability of the equipment. The vendor may perform seismic qualification on the

equipment according to required specifications. More often, however, the vendor hires the services of a

test laboratory, which is the contractor, for seismic qualification of equipment in the plant. A reviewer,

who is hired by the plant builder or the customer, may review the qualification procedure and the report,

which are usually developed by the test laboratory by adhering to the qualification requirements.

Figure 31.3 gives a flowchart for test-object movement and for associated information interactions

between various groups in the qualification program.

A basic step in any qualification program is the preparation of a qualification procedure. This is a

document that describes in sufficient detail such particulars as the tests that will be conducted on the test

object, pretest procedures, the nature of test-input excitations and the method of generating these signals,

inspection and response-monitoring procedures during testing, definitions of equipment malfunction,

and qualification criteria. Where analysis is also used in the qualification, the analytical methods and

computer programs that will be used should be described adequately in the qualification procedure. The

qualification procedure is prepared by the test laboratory (contractor), equipment particulars are

obtained from the vendor or the purchaser, and the purchaser usually supplies the test environment for

which the equipment will be qualified.

Before the qualification tests are conducted, the test procedure is submitted to the purchaser for

approval. The purchaser normally hires a reviewer to determine whether the qualification procedure

satisfies the requirements of both the regulatory agency and the purchaser. There could be several stages

of revision of the test procedure until it is accepted by the purchaser upon the recommendation of the

reviewer.

The approved qualification procedure is sent to the test laboratory and qualification is performed

accordingly. The test laboratory prepares a qualification report that also includes the details of static or

dynamic analysis when incorporated. The qualification report is sent to the purchaser for evaluation. The

purchaser might obtain the services of an authority to review the qualification report. The report might

have to be revised and analysis and tests might have to be repeated before a final decision is made on the

qualification of the equipment. Information flow in a typical qualification program is shown in

Figure 31.4.

Regulatory Agency

Equipment

Supplier

(vendor)

(Manufacturer)

Test

Laboratory

(Contractor)

Customer

(Plant Owner)

Plant

Builder

Equipment

To Be

Qualified

Qualified

Equipment

FIGURE 31.3 Test-object movement and information interactions in seismic qualification.

Seismic Qualification of Equipment 31-7

© 2005 by Taylor & Francis Group, LLC

31.3.2 Test Preliminaries

Seismic qualification tests are usually conducted by

one of two methods depending on whether singlefrequency

or multifrequency excitation inputs are

employed in the main tests. The two test categories

are (1) single-frequency tests and (2) multifrequency

tests. Although the second test method is

more common in seismic qualification by testing,

the first method is used under some conditions

depending on the nature of the test object and its

mounting features (for example, line-mounted vs.

floor-mounted equipment). Typically, multifrequency

excitations are preferred in qualification

tests, and single-frequency excitations are favored

in design-development and quality-assurance tests.

In single-frequency testing, amplitude of the excitation input is specified by a required input motion

(RIM) curve, similar to that shown in Figure 31.5. If single-frequency dwells (e.g., sine dwell, sine beat)

are employed, then the excitation input is applied to the test object at a series of selected frequency values

in the frequency range of interest for that particular test environment. In such cases, dwell times (and

number of beats per cycle where sine beats are employed) at each frequency point should be specified.

If a single-frequency sweep (such as a sine sweep) is employed as the excitation signal, then the sweep

rate should be specified. When the single-frequency test-excitation is specified in this manner, the tests

are conducted very much like multifrequency tests.

Multifrequency test are normally conducted by employing the response spectra method to represent

the test-input environment. Basically, the test object is excited using a signal whose response spectrum,

known as the test response spectrum (TRS), envelops a specified response spectrum, known as the

required response spectrum (RRS). Ideally, the TRS should equal the RRS but it is practically impossible

to achieve this condition. Hence, multifrequency tests are conducted using a TRS that envelops the RRS

so that in significant frequency ranges, the two response spectra are nearly equal (see Figure 31.6).

However, excessive conservatism, which would result in overtesting, should be avoided. It is usually

acceptable to have TRS values below the RRS at a few frequency points.

The RRS is part of the data supplied to the test laboratory prior to the qualification tests being

conducted. Two types of RRS are provided, representing (1) the operating-basis earthquake (OBE), and (2)

the safe-shutdown earthquake (SSE). The response spectrum of the OBE represents the most severe

motions produced by an earthquake under which the equipment being tested would remain functional

without undue risk of malfunction or safety hazard. However, if the equipment is allowed to operate at a

Equipment

Supplier Test

Laboratory

Purchaser (Contractor)

Purchaser

Regulatory

Agency

Purchaser

Reviewer

Test

Laboratory

Equipment

Data

Test

Environment

Qualification

Procedure

Approved

Procedure

Standards

Guides

Specification

Test Report

Qualification

Procedure

Approved

Test Report Test Report

FIGURE 31.4 Information flow in a seismic qualification program.

Frequency

Acceleration Amplitude

FIGURE 31.5 A typical required input motion curve.

31-8 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

disturbance level higher than the OBE level for a

prolonged period then there would be a significant

risk of malfunction.

The response spectrum of the SSE represents the

most severe motions produced by an earthquake

that the equipment being tested could safely

withstand while the entire nuclear power plant is

being shutdown. However, prolonged operation

(i.e., more than the duration of one earthquake)

could result in equipment malfunction. In other

words, equipment is designed to withstand only

one SSE in addition to several OBEs.

A typical seismic qualification test would first subject the equipment to several OBE-level excitations,

primarily for aging the equipment mechanically to its end-of-design-life condition, and then would

subject it to one SSE-level excitation. When providing RRS test specifications, it is customary to supply

only the SSE requirement. The OBE requirement is then taken as a fraction (typically, 0.5 or 0.7) of the

SSE requirement.

Test response spectra corresponding to the excitation signals are generated by the test laboratory

during testing. The purchaser usually supplies the test laboratory with an FM tape containing frequency

components that should be combined in some ratio to generate the test-input signal.

Qualification tests are conducted according to the test procedure approved and accepted by the

purchaser. The main steps of seismic qualification testing are outlined in the following subsections.

31.3.3 Single-Frequency Testing

Seismic ground motions usually pass through

various support structures before they eventually

are transmitted to equipment. For seismic qualification

of that equipment by testing, in theory we

should apply the actual excitations felt by it, and

not the seismic ground motions. In an ideal case,

the shaker-table motion should be equivalent to

the seismic response of the supporting structure

at the point of attachment of the equipment.

The supporting structure would have a particular

frequency-response function between the

ground location and the equipment-support

location (see Figure 31.7). Consequently, it could

be considered a filter that modifies seismic ground

motions before they reach the equipment mounts.

In particular, the components of the ground motion that have frequencies close to a resonant frequency of

the supporting structure will be felt by the equipment at a relatively higher intensity. Furthermore, the

ground motion components at very high frequencies will be almost entirely filtered out by the structure. If

the frequency response of the supporting structure is approximated by a lightly damped simple oscillator,

then the response felt by the equipment will be almost sinusoidal, with a frequency equal to the resonant

frequency of the structure.

When the equipment supporting structure has a very sharp resonance in the significant frequency

range of the dynamic environment (for example, 1 to 35 Hz for seismic ground motions), it follows from

the previous discussion that it is desirable to use a short-duration single-frequency test in seismic

qualification of the equipment. Equipment that is supported on pipelines (valves, valve actuators, gauges,

and so forth) falls into this category and is termed line-mounted equipment.

FIGURE 31.6 The TRS enveloping the RRS in a

multifrequency test.

FIGURE 31.7 Schematic representation of the filtering

of seismic ground motions by a supporting structure.

Seismic Qualification of Equipment 31-9

© 2005 by Taylor & Francis Group, LLC

Resonant frequency of the supporting structure is usually not known at the time of the seismic

qualification test. Consequently, single-frequency testing must be performed over the entire frequency

range of interest for that particular dynamic environment.

Another situation in which single-frequency testing is appropriate is when the test object (equipment)

itself does not have more than one sharp resonance in the frequency range of interest. In this case, the

most prominent response of the test object occurs at its resonant frequency, even when the dynamic

environment is an arbitrary excitation. Consequently, a single-frequency excitation would yield

conservative test results. Equipment that has more than one predominant resonance may employ singlefrequency

testing provided that each resonance corresponds to a dynamic DoF (e.g., one resonance along

each dynamic principal axis), and that cross coupling between these DoF is negligible.

In summary, single-frequency testing may be used where one or more of the following conditions are

satisfied:

1. The supporting structure has one sharp resonance in the frequency range of interest (linemounted

equipment is included).

2. The test object does not have more than one sharp resonance in the frequency range of interest.

3. The test object has a resonance in each DoF, but the DoF are uncoupled (for which adequate

verification should be provided in the test procedure).

4. The test object can be modeled as a simple dynamic system (such as a simple oscillator), for which

adequate justification or verification should be provided.

Usually, the required SSE excitation level for a single-frequency test over a frequency range is specified

by a curve such as the one shown in Figure 31.5. This curve is known as the RIM magnitude curve. The

OBE excitation level is usually taken as a fraction (typically, 0.5 or 0.7) of the RIM values given for the

SSE. For a sine-sweep test, the sweep rate and the number of sweeps in the test should also be specified.

Typically, the sweep rate for seismic qualification tests is less than 1 octave/min. One sweep, from the state

of rest to the maximum frequency in the range and back to the state of rest, is normally carried out in an

SSE test (for example, 1 – 35 to 1 Hz). Several sweeps (typically, five) are performed in an OBE test.

In an SSE sine-dwell test, the dwell time for each dwell frequency should be specified. The dwellfrequency

intervals should not be high (typically, a half-octave or less). For an OBE test, the dwell times

are longer (typically, five times longer) than those specified for an SSE test.

For an SSE test using sine beats, the minimum number of beats and the minimum duration of

excitation (with or without pauses) at each test frequency should be specified. In addition, the pause time

for each test frequency should be specified when sine beats with pauses are employed. For an OBE test,

the duration of excitation should be increased (as in a sine-dwell test).

The dwell time at each test frequency should be adequate to perform at least one functional-operability

test. Furthermore, a dwell should be carried out at each resonant frequency of the test object as well as at

those frequencies that are specified. Total duration of an SSE test should be representative of the duration

of the strong-motion part of a standard safeshutdown

earthquake.

Sometimes, narrow-band random excitations

may be used in situations where single-frequency

testing is recommended. Narrowband random

signals are those that have their power concentrated

over a narrow frequency band. Such a signal

can be generated for test-excitation purposes by

passing a random signal through a narrow-bandpass

filter. By tuning the filter to different center

frequencies in narrowbands, the test-excitation

frequency can be varied during testing. This center

frequency of the filter should be swept up and

down over the desired frequency range at a

FIGURE 31.8 A typical RRS for a narrow-band

excitation test.

31-10 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

reasonably slow rate (e.g., 1.0 octave/min) during the test. Thus, a multifrequency test with a sharp

frequency-response spectrum (the RRS), as illustrated in Figure 31.8, is adequate in cases where singlefrequency

testing is recommended. In this case, a requirement that has to be satisfied by the testexcitation

signal is that its amplitude should be equal to or greater than the zero-period acceleration of

the RRS for the test.

31.3.4 Multifrequency Testing

When equipment is mounted very close to the ground under its normal operating conditions, or if its

supporting structure and mounting can be considered rigid, then seismic ground motions will not be

significantly filtered before they reach the equipment mounts. In this case, the seismic excitations that are

felt by the equipment will retain broadband characteristics. Multifrequency testing is recommended for

seismic qualification of such equipment.

Whereas single-frequency tests are specified by means of an RIM curve along with the test duration at

each frequency (or sweep rates), multifrequency tests are specified by means of an RRS curve. The test

requirement in multifrequency testing is that the response spectrum of the test excitation (the TRS) felt

by the equipment mounts should envelop the RRS. Note that all frequency components of the test

excitation are applied simultaneously to the test object. This is in contrast to single-frequency testing, in

which only one significant frequency component is applied at a given instant.

When random excitations are employed in multifrequency testing, enveloping of the RRS by the TRS

may be achieved by passing the random signal produced by a signal generator through a spectrum shaper.

As the analyzing frequency bandwidth (e.g., one-third octave bands, one-sixth octave bands) decreases,

the flexibility of shaping the TRS improves. A real-time spectrum analyzer (or a personal computer) may

be used to compute and display the TRS curve corresponding to the control accelerometer signal (see

Figure 31.9). By monitoring the displayed TRS, it is possible to adjust the gains of the spectrum-shaper

filter to obtain the desired TRS that would envelop the RRS.

Most test laboratories generate their multifrequency excitation signals by combining a series of sine

beats that have different peak amplitudes and frequencies. Using the same method, many other signal

types (such as decaying sinusoids) may be superimposed to generate a required multifrequency

excitation signal. A combination of signals of different types could also be employed to produce a desired

test input. A commonly used combination is a broadband random signal and a series of sine beats. In this

combination, the random signal is adjusted to have a response spectrum that will envelop the broadband

portion of the RRS without much conservatism. The narrow-band peaks of the RRS, which generally will

not be enveloped by such a broadband response spectrum, will be covered by a suitable combination

of sine beats.

Random

Signal

Generator

Spectrum

Shaper

Power

Amplifier

Test

Table

Random

Signal

Spectrum

Analyzer

Shaped

Signal

Test Input

Control Excitation

Accelerometer

Output

TRS

Spectrum

Control

FIGURE 31.9 Matching of the TRS with the RRS in multifrequency testing.

Seismic Qualification of Equipment 31-11

© 2005 by Taylor & Francis Group, LLC

By employing such mixed composite signals, it is possible to envelop the entire RRS without having to

increase the amplitude of the test excitation to a value that is substantially higher than the ZPA of the

RRS. One important requirement in multifrequency testing is that the amplitudes of the test excitation be

equal to or greater than the ZPA of the RRS.

31.3.5 Generation of RRS Specifications

Seismic qualification of an object is usually specified in terms of an RRS. The excitation input that is used

in seismic qualification analysis and testing should conservatively satisfy the RRS; that is, the response

spectrum of the actual excitation input should envelop the RRS (without excessive conservatism, of

course).

For equipment to be installed in a building or on some other supporting structure, the RRS generally

cannot be obtained as the response spectrum of a modified seismic ground-motion time-history. The

supporting structure usually introduces an amplification effect and a filtering effect on seismic ground

motions. This amplification factor alone could be as high as three. Some of the major factors that

determine the RRS for a particular seismic qualification test are as follows:

1. Nature of the building that will be qualified

2. Dynamic characteristics of the building or structure and the location (elevation and the like)

where the object is expected to be installed

3. In-service mounting orientation and support characteristics of the object

4. Nature of the seismic ground motions in the geographic region where the object would be installed

5. Test severity and conservatism that is required by the purchaser or the regulator agency

The basic steps in developing the RRS for a specific seismic qualification application include the

following:

1. Development of representative safe-shutdown earthquake (SSE) ground-motion time histories for

the building (or support structure) location

2. Development of a suitable building (or support structure) model

3. Response analysis of the building model, using the time histories that are obtained in Step 1

4. Development of response spectra for various critical locations in the building (or support

structure), using the response time histories obtained in Step 3

5. Normalization of the response spectra obtained in Step 4 to unity ZPA (that is, dividing by their

individual ZPA values)

6. Identification of the similarities in the set of normalized response spectra that are obtained in Step

5 and grouping them into a small number of groups

7. Representation of each similar group by a response spectrum consisting of straight-line segments

that envelop all members in the group, giving a normalized RRS for each group

8. Determination of scale factors for various locations in the building for use in conjunction with the

corresponding normalized RRS curves

Representative strong-motion earthquake time histories (SSEs) are developed by suitably modifying

actual seismic ground-motion time histories that have been observed in that geographic location (or a

similar one), or by using a random-signal-generation (simulation) technique or any other appropriate

method. These time histories may be available as either digital or analog records, depending on the

way in which they are generated. If computer simulation is used in their development, then a

statistical representation of the expected seismic disturbances in the particular geographic region

(using geological features in the region, seismic activity data, and the like) should be incorporated in

the algorithm. The intensity of the time histories can be adjusted, depending on the required test

severity and conservatism.

The normalized response spectra are grouped so that the spectra that have roughly the same shape are

put in the same group. In this manner, relatively few groups of normal response spectra (normalized) are

31-12 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

obtained. Then, the response spectra that belong to each group are plotted on the same graph paper.

Next, straight-line segments are drawn to envelop each group of response spectra. This procedure results

in a normalized RRS for each group of analytical response spectra.

The RRS that is used for a particular seismic qualification scheme is obtained as follows. First, the

normalized RRS corresponding to the location in the building where the object would be installed is

selected. The normalized RRS curve is then multiplied by the appropriate scaling factor. The scaling

factor normally consists of the product of the actual ZPA value under SSE conditions at that location (as

obtained from the analytical response spectrum at that location, for example) and a factor of safety that

depends on the required test severity and conservatism.

In fact, three RRS curves corresponding to the vertical, east – west and north – south directions might

be needed, even for single-degree-of-freedom (single-DoF) seismic qualification tests because, by

mounting three control accelerometers in these three directions, triaxial monitoring could be

accomplished. If only one control accelerometer is used in the test, then only one RRS curve is used.

In this case, the resultant of the three orthogonal RRS curves should be used. One way to obtain the

resultant RRS curve is to apply the square root of the sum of squares (SRSS) method to the three

orthogonal components. Alternatively, the envelope of the three orthogonal RRS curves is obtained and

multiplied by a safety factor (greater than unity).

Note that more than one building (or even many different geographic locations) could be included in

the described procedure for developing RRS curves. The resulting RRS curves are then valid for the

collection of buildings or geographic locations considered. When the generality of an RRS curve is

extended in this manner, the test conservatism increases. This will also result in an RRS curve with a

much broader band.

In practice, in a particular seismic qualification project, only a few normalized RRS curves are

employed. In conjunction with these RRS curves, a table of data is provided that identifies the proper

RRS curves and the scaling factors that should be used for different physical locations (for example,

elevations) in various buildings that are situated at several geographic locations.

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