earthquake requirements and seismic capabilities for eaton’s … · 2019-12-23 · 2 seismic...

Table of contents

Description

Part I Abstract and general overview . . . . . . . . . 2Part II Seismic terminology and earthquake

engineering . . . . . . . . . . . . . . . . . . . . . . . 3Part III Seismic requirements . . . . . . . . . . . . . . . 9Part IV Test facility and test methodology . . . . . 16Part V Shared responsibilities . . . . . . . . . . . . . . 18Part VI Typical Eaton seismic equipment

specifications . . . . . . . . . . . . . . . . . . . . . 18Part VII References . . . . . . . . . . . . . . . . . . . . . . . 20

Effective August 2009Seismic White Paper SA12501SE

Earthquake requirements and seismic capabilities for Eaton’s electricaldistribution and control equipment

2

Seismic White Paper SA12501SEEffective August 2009

Earthquake requirements and seismic capabilities for Eaton’s electrical

distribution and control equipment

EATON CORPORATION www.eaton.com

About the authorsMr. Eddie Wilkie graduated from North Carolina State University, earning a Bachelor of Science degree in Mechanical Engineering. Eddie has been employed with Eaton for 19 years. During that time, he has held a variety of engineering and management positions. Eddie has worked as a design engineer, design engineering man-ager, business operations manager and most recently as the division engineering manager for Eaton’s Power Distribution Operations–Americas. Eddie is currently responsible for coordinating Eaton’s electrical equipment seismic program, which includes annual testing and equipment certification.

Mr. Frederick M. Paul has spent over 31 years in various aspects of the electrical industry. The last 11 have been as an application engi-neer with Eaton, covering the central valley of California, USA. Prior to Eaton, he was involved in the sales and application of electrical control and automation equipment. Earlier in his career, he was vice president of operations for 11 years at an electrical contracting firm in Southern California, USA.

Dr. Mostafa A. Ahmed has 37 years of extensive experience in structural and mechanical design and construction of power generating stations. He is a fellow engineer with the Westinghouse Electric Company Nuclear Service division. He is skilled in civil and mechanical engineering practices, dynamic analysis of equipment and structures, and finite element analyses. Dr. Ahmed is an expert in equipment seismic qualification and seismic testing. He earned his bachelor of science degree in civil engineering from Cairo University in 1971, and received his master of science and doctorate in structural mechanics from the University of Pittsburgh in 1981 and 1991. He is currently working in Shanghai, China, as a technical advisor for the Westinghouse On-Shore Engineering Organization.

Mr. Nathan M. Glenn, P.E., is a practicing Mechanical Engineer specializing in equipment qualification. He is experienced in shock and vibration testing, structural dynamics, electro-mechanical analysis, and design. Nathan earned his Bachelor of Science degree in Mechanical Engineering Technology from The Pennsylvania State University, and received a Master of Science in Engineering Mechanics specializing in Explosives Engineering from New Mexico Institute of Mining and Technology. He is a registered Professional Engineer in the state of Pennsylvania. Nathan is a Senior Engineer with Westinghouse Electric Company. Currently he is responsible for the qualification of nuclear power plant components and systems. In addition to nuclear power plant equipment qualification, Nathan provides seismic certification for electrical equipment used in building code applications.

Part IAbstract

Eaton Corporation is a global diversified industrial manufacturer consisting of two sectors: Industrial and Electrical. Throughout this document, all references to Eaton are in regards to Eaton’s Electrical Sector.

Beginning with qualification testing in 1985, Eaton has led the industry in seismic certification of electrical equipment for use in facilities across the continental United States. Eaton was the first electrical equipment supplier to employ seismic simulation testing for equipment seismic certification.

For more than 20 years, Eaton has had a comprehensive program focused on designing and manufacturing electrical distribution and control equipment capable of meeting and exceeding the seismic load requirements of the Uniform Building Code� (UBC), the California Building Code (CBC), and the Building Officials and Code Administrators� (BOCA) International, Inc. The entire pro-gram has been updated to demonstrate compliance with the 2006 International Code Council (ICC) and the 2006 International Building Code (IBC) unified seismic requirements. This also includes the 2007 CBC.

Eaton recognized that the most direct and proven method of assuring seismic performance of electrical equipment is through simulation testing via triaxial or biaxial shake tables. Representative configurations for each of Eaton’s product lines were designed and built for seismic testing. Considerable attention was given to selecting test units that conservatively represented the entire family of products being certified.

Test units were initially subjected to independent 0.2g resonant searches in each of the three principal axes prior to being subjected to a series of seismic simulation tests. The test assemblies were proven to meet or exceed the seismic performance requirements and remain operational immediately after the seismic event. This paper provides a summary of the efforts that were involved in the achievement of this objective.

Background

Although the need for seismic-capable electrical equipment is known, there is a lack of understanding of how to comply with current code requirements. The 2006 International Building Code (IBC) and the 2007 California Building Code (CBC) both emphasize building design requirements with limited information for seismic certifications of equipment. Electrical equipment and distribution system components are treated as non-structural attachments to the building.

Since seismic testing contains many special terms and formulations, this paper begins with the basics of seismic terminology and earthquake engineering, then proceeds with addressing the specific factors involved with meeting the requirements of the IBC and CBC.

The 2006 IBC seismic requirements, along with associated codes derived from the IBC, will be addressed, explaining how they relate to Eaton’s previous and current test programs. The most stringent requirements of these codes (IBC and CBC) will be presented as they apply to electrical distribution and control equipment and will be combined to formulate a single reference for purposes of evaluation.

To properly define the acceptability of the equipment to the specified codes, it is necessary to present the equipment seismic requirements and the equipment seismic capability data on the same technical basis. For this purpose, the use of the “response spectrum” concept will be introduced. To simplify the application for the user, the seismic capability of all of Eaton’s equipment has been established to the same basic levels and requirements.

The equipment is considered acceptable, or granted a “seismic certificate,” if it can withstand the seismic event and perform its function immediately afterward. Eaton participates in a cooperative effort with the user, building designer, and installer to ensure that the equipment is mounted properly to a foundation that can withstand the effects produced by an earthquake.

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Earthquake requirements and seismic capabilities for Eaton’s electrical distribution and control equipment


General overview

Eaton’s electrical distribution and control equipment has undergone seismic simulation tests and meets or exceeds performance requirements as identified in the 2006 IBC1 and the 2007 CBC2. It is important to note that Eaton has tested its equipment using the most typical mounting methods. All Eaton floor-mounted equipment has been seismically tested as free-standing units, with no lateral supports at the top that are affixed to adjacent walls or structures. This allows users to either secure it from the base alone or in com-binations of base and top lateral supports. It must be recognized that equipment tested with top lateral supports is not certified to the same levels as free-standing items. Eaton has shown by simulation testing that equipment seismic capabilities are reduced by more than a factor of two when mounted at the base only as compared to securing with lateral supports located at the top of the equipment. Complete and proper certification of equipment to achieve maxi-mum flexibility for the user must include testing of equipment as stand-alone items. Eaton equipment that is certified to higher levels when installed with top lateral supports is specifically indicated on Eaton’s seismic certificates.

Eaton is highly experienced in the design, manufacture, and seismic certification of electrical distribution equipment to meet the most rigorous seismic standards. As new products are developed, or existing products are modified, Eaton continues to verify the seismic acceptability of nearly all lines of electrical equipment for applica-tions requiring certification to the IBC and CBC.

Over a period of more than 20 years, over 100 different assemblies, representing many product lines, have been successfully tested and verified to seismic levels higher than the maximum seismic requirements specified in the IBC and CBC. The equipment main-tained structural integrity and demonstrated the ability to function immediately after the seismic simulation tests. This achievement, an industry first, is consistent with the Eaton commitment to produce the most reliable equipment that exceeds both present and future requirements. Testing was performed on simulation tables at Wyle Test Laboratory in Huntsville, Alabama, along with the former Westinghouse Advanced Energy Systems Division in Pittsburgh, Pennsylvania.

The general concepts for seismic test methodology, ANSI/IEEE� Standard 344-1987, and the applicable procedures from ANSI/IEEE C37.81—Guide for Seismic Qualification of Class 1E Metal-Enclosed Power Switchgear Assemblies—were consulted.

The equipment was subjected to the following vibration excitation and seismic simulation testing:

1. Initial resonance searches in all three principal directions in the frequency range of 1 to 50 Hz, using sine sweep motion at the base of the test units, with a sweep rate of 1.0 octave per minute. Peak acceleration of the sine wave was designed around 0.2g. (Some resonance sweep tests have been conducted up to 100 Hz.)

2. Seismic simulation testing using 30-second-long random multifrequency inputs imposed simultaneously and measured at the base of the test cabinets in all three principal directions. The base acceleration levels were increased further to encompass the combined code requirements, and additional testing was performed to demonstrate margin beyond code requirements.

Nearly all of Eaton’s electrical assemblies have been tested and were found acceptable when evaluated to IBC and CBC seismic requirements. Eaton continues to lead the industry in usingsimulation testing to ensure conformance of electrical distribution and control equipment to the most current codes. (See Part III.)

Establishing the equipment seismic capability is only the first step. A seismically qualified mounting base with anchors or welds is required to hold the equipment safely to the supporting structure. (See Part V.)

Part II

Seismic terminology and earthquake engineering

Earthquakes occur in most every region around the world.3 (See Figure 1.) As reported by the U.S. Geological Survey, in 2007 alone, 55 earthquakes greater than 6.0 were recorded around the world. These resulted in 681 reported deaths and widespread damage to structures, buildings, and equipment with damage estimates in the billions of dollars.

The May 12th, 2008 earthquake in Chengdu, China, exceeded 6.9 and caused a major tragedy of more than 69,000 deaths and resulted in several thousands of people missing. The problems were further compounded due to the delays in restoring power and service to the affected areas.

To restore function of emergency management facilities as quickly as possible, public officials have revised building codes to mandate improved seismic design. This includes not only buildings, but also the electrical and mechanical equipment contained therein, as well as machinery necessary for safe occupancy and normal operation.

Eaton has taken the unique step of performing seismic simulation tests on various lines of distribution and control products. Users can be sure that Eaton’s electrical equipment has been designed and tested to exceed the requirements as identified by the IBC and CBC. For purposes of understanding, it is important to review a few of the basic principles of earthquake engineering. From an analytical per-spective, it is not easy to quantify the severity of an earthquake. Most news reports refer to the magnitude of the earthquake in terms of the open-ended Richter scale. Although most people have heard of the Richter scale, the understanding is limited. The original definition is:4

Richter magnitude is M, where M = log10(A)

Where A is equal to the trace amplitude (in microns) of a Wood-Anderson Seismograph having magnification of 2800, natural period of 0.8 seconds, a damping coefficient of 80%, located on firm ground, at a distance of 62.5 miles (100 km)from the earthquake epicenter.

Since 1000 microns are equal to 1.0 millimeter, and log10 (1000)is equal to 3, M could then be redefined as:

M = 3 + log10(trace amplitude in mm)

1 International Code Council, International Building Code.

2 California Building Standards Commission, California Building Code.

3 Newmark, Fundamentals of Earthquake Engineering, p. 252.

4 Ibid., p. 217.

4





Table 1. Relationship of Earthquake Magnitude to Other Parameters

Earthquake Magnitude M (Richter Scale)

Maximum7 Ground Acceleration (% g)

Duration of7 Strong Motion (Seconds)

Length of8 Fault Slip (Miles)

Equivalent9 Energy (Tons of TNT)

8.5 50 73 530 70 million

8 50 43 190 13 million

7.5 45 30 70 2.2 million

7 37 24 25 400,000

6.5 29 18 9 70,000

6 22 12 5 13,000

5.5 15 6 3 2200

5 9 2 2 400

Table 2. Modified Mercalli Intensity Scale (abridged and Rewritten by C. F. Richter10)

Intensity11 Definition

1 Not felt. Marginal and long period of large earthquakes.

2 Felt by persons at rest, on upper floors, or favorably placed.

3 Felt indoors. Hanging objects swing. Vibration like passing of light trucks. Duration estimated. May not be recognized as an earthquake.

4 Hanging objects swing. Vibration like passing of heavy trucks; or sensation of a jolt like a heavy ball striking the walls. Standing motor cars rock. Windows, dishes, doors rattle. Glasses clink. Crockery clashes. In the upper range of 4, wooden walls and frames crack.

5 Felt outdoors; direction estimated. Sleepers awakened. Liquids disturbed, some spilled. Small unstable objects displaced or upset. Doors swing, close, open. Shutters, pictures move. Pendulum clocks start, stop, change rate.

6 Felt by all. Many frightened and run outdoors. Persons walk unsteadily. Windows, dishes, glassware broken. Knickknacks, books, and so on, off shelves. Pictures off walls. Furniture moved or overturned. Weak plaster and masonry D cracked. Small bells ring (church, school). Trees, bushes shaken visibly, or heard to rustle.

7 Difficult to stand. Noticed by drivers of motor cars. Hanging objects quiver. Furniture broken. Damage to masonry D including cracks. Weak chimneys broken at roof line. Fall of plaster, loose bricks, stones, tiles, cornices, unbraced parapets, and architectural ornaments. Some cracks in masonry C. Waves on ponds; water turbid with mud. Small slides and caving in along sand or gravel banks. Large bells ring. Concrete irrigation ditches damaged.

8 Steering of motor cars affected. Damage to masonry C; partial collapse. Some damage to masonry B; none to masonry A. Fall of stucco and some masonry walls. Twisting, fall of chimneys, factory stacks, monuments, towers, elevated tanks. Frame houses moved on foundations if not bolted down; loose panel walls thrown out. Decayed piling broken off. Branches broken from trees. Changes in flow or temperature of springs and wells. Cracks in wet ground and on steep slopes.

9 General panic. Masonry D destroyed; masonry C heavily damaged, some-times with complete collapse; masonry B seriously damaged. General damage to foundations. Frame structures, if not bolted, shifted off foundations. Frames cracked. Conspicuous cracks in ground. In alleviated areas, sand and mud ejected, earthquake fountains, sand craters.

10 Most masonry and frame structures destroyed with their foundations. Some well-built wooden structures and bridges destroyed. Serious damage to dams, dikes, embankments. Large landslides. Water thrown on banks of canals, rivers, lakes, and so forth. Sand and mud shifted horizontally on beaches and flat land. Rails bent slightly.

11 Rails bent greatly. Underground pipelines completely out of service.

12 Damage nearly total. Large rock masses displaced. Lines of sight and level distorted. Objects thrown into the air.

To determine the magnitude of an earthquake in terms of Richter Scale M, one could proceed as follows:

1. Measure the amplitude of the Wood-Anderson Seismograph in millimeters at a location 62.5 miles from the earthquake’s epicenter.

2. Take common logarithm of the amplitude.

3. Add 3 to it.

The result is the magnitude (M) of an earthquake on theRichter scale.

ote: N It is important to note that for a change of one unit in the Richter scale, M means a change of 10 in the amplitude of the motion of the earthquake. To relate the magnitude (M) to the energy radiated by the earth-quake, Gutenberg and Richter developed the following relationship5:

log10(Es) = 11.8 + 1.5 M

Where Es is the seismic energy of the earthquake and M is the Richter magnitude.

A one megaton bomb releases about 5x1022 ergs. If all of the energy could be converted into seismic energy (typically only about 2% would be), it would correspond to a magnitude 7.3 earthquake6. Table 1 relates the earthquake magnitude to other relevant param-eters. A change of one unit in the magnitude M means an increase of 1.5 in the right-hand side of the equation, resulting in a change of 32 in the total energy of the earthquake, Es, since log10 (32) = 1.5.

Although the magnitude of the earthquake is a direct measure of its severity, there are a number of difficulties in using it for equipment design. Specifically, they include:

1. Maximum displacement alone does not provide necessary information about the frequency content of the motion. Due to the effects of amplification, equipment is most susceptible to damage when the earthquake motion contains the equip-ment’s inherent natural frequencies.

2. Maximum displacement is not necessarily a good measure of the total amount of energy the equipment is subjected to during the earthquake. Velocity is a better indicator of the energy.

3. Maximum displacement is not a good measure of the force the equipment will experience. Acceleration shows a better correlation to the resultant seismic forces on the equipment mounted inside buildings or structures.

4. Seismographs are normally tuned to frequencies in the 1 to 2 Hz range. This is adequate for measuring the magnitude of the earthquake, but does not provide accurate information about the frequencies typically translated to buildings and equipment.

Figure 1. Seismicity of the Earth, 1961–1967

5 Wiegel, Earthquake Engineering, p. 31 (Ref. 8).

6 Newmark, Fundamentals of Earthquake Engineering, p. 252 (Ref. 7).

7 Wiegel, Earthquake Engineering, p. 79, Table 4.3 (Ref. 8).

8 Wiegel, Earthquake Engineering, p. 77, Table 4.1 (Ref. 8).


10 Newmark, Fundamentals of Earthquake Engineering, Appendix 2 (Ref. 7).

11 Intensity is frequently represented by Roman numerals.

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To eliminate many verbal repetitions in the original scale, the follow-ing convention has been adopted. Each effect is named at that level of intensity at which it first appears frequently and characteristically. Each effect may be found less strongly, or in fewer instances, at the next lower grade of intensity; more strongly or more often at the next higher grade. A few effects are named at two successive levels to indicate a more gradual increase.

Masonry A, B, C, and D. To avoid ambiguity of language, the quality of masonry, brick or otherwise, is specified by the followinglettering (which has no connection with the conventional Class A, B, C construction).

Masonry A. Good workmanship, mortar, and design; reinforced, especially laterally, and bound together by using steel, concrete, and so forth; designed to resist lateral forces.

Masonry B. Good workmanship and mortar; reinforced, but not designed to resist lateral forces.

Masonry C. Ordinary workmanship and mortar; no extreme weaknesses like failing to tie in at corners, but neither reinforced nor designed against horizontal forces.

Masonry D. Weak materials, such as adobe; poor mortar; low standards of workmanship; weak horizontally.

In addition to the magnitude of the earthquake, which measures the amount of energy released, another parameter, the intensity, is used to measure the local destructiveness of earthquakes. Therefore, one earthquake will have a single magnitude, but a number of differ-ent intensities, depending on the location of the observers.12 Most intensity scales are based on personal and subjective observations, including “scary feeling” and the ability (or inability) to remain standing, as well as the sorts of property damage that occurred.

Although quantitative and based on actual damage effects, a review of Table 2, the Modified Mercalli (mm) scale13, reveals that it is too subjective for use in electrical equipment design and qualification. Despite its limitations, the intensity can be quite useful in areas where there are no seismic instruments available to record the earthquake, and it may provide the only consistent way to interpret the diaries and other written accounts of historical earthquakes.14

When available, the most accurate descriptions of actual earth-quake motions are the time history records. A time history record is simply a graphical recording of the earthquake motion (it can be in terms of displacement, velocity, or acceleration) as a function of time. Figures 2 and 3 illustrate time history records for two different earthquakes.15

Although accurate and readily available soon after the event, these records are not ideal for translating requirements to equipment design and seismic certification. For example, note that the El Centro, California, earthquake acceleration magnitude reaches 0.3g several times, and is consistently above 0.1g, while the maxi-mum peak-to-peak displacement is about 30 cm. For the Mexico City earthquake, the acceleration magnitude is generally about 0.01g, with one peak at about 0.02g—only about 10% of the El Centro acceleration levels.

The maximum peak-to-peak destruction displacement of the Mexico City earthquake was 60 cm, or about twice the El Centro displace-ment value. Noting the difference in the time scale, one immediately realizes that the Mexico City earthquake motions are characterized by much lower frequencies than the El Centro event.

Figure 2. El Centro, California, Earthquake of May 18, 1940, NS Component16

Because acceleration is a function of displacement times the square of the natural circular frequency (for sinusoidal motions), the domi-nant frequencies of the El Centro earthquake are four to five times those of the Mexico City earthquake. This illustrates that while the time history is very accurate for any one earthquake, it is difficult to use as the basis for generalizations about other earthquakes.

0 5 10 15 20 25 30Time, seconds

South

North Displacement

t

20cm10

0

10

South

North Acceleration

t

0.3 g0.2 g0.1 g

0-0.1 g-0.2 g-0.3 g

South

North Velocity

t

20cm sec-1

0

20

40


13 Ibid., Appendix 2.

14 Ibid., p. 218.

15 Ibid., p. 227.

16 Ibid., p. 227.

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Finally, the last concern in using time history can be illustrated as follows. Figure 4 shows the time history records (both displacement and acceleration forms) for a shake table test run.

The question arises related to the adequacy of the time history to accurately represent the earthquakes shown in Figures 2 and 3. Because the acceleration record for the test contains several peaks in the range of 2 to 3g, there are enough peak accelerations and inertial forces to satisfy the requirements. Similarly, the peak-to-peak displacements are in the range of 60 to 75 cm for the test, which would appear to be sufficient to meet even the Mexico City displacement levels.

On the basis of amplitude alone, it appears that the shake table time history (Figure 4) meets both actual earthquake events (Figures 2 and 3). However, there are two critical factors not yet addressed: (a) frequencies present in the required motion versus the frequencies present in the test motion, and (b) the inherent equipment damping.

It has already been shown that the frequency content represents a significant difference in contrasting Figures 2 and 3. Similarly, frequency is also critically important in establishing equipment certification. The reason is basic: each piece of electrical equipment has its own natural frequency that produces maximum amplification. For larger assemblies, 4 to 6 Hz is typically the minimum natural frequency. If the earthquake has significant 4 to 6 Hz motion, the equipment will respond accordingly, amplifying or resonating with the earthquake motion. If the earthquake has substantial 10 to 12 Hz motion, the equipment will be too flexible to keep up with the higher frequency, thus, it will tend to sit still or attenuate the earth-quake motion. If the earthquake has a significant amount of 1 to 2 Hz motion, the equipment will rigidly follow the motion of the floor, neither amplifying nor attenuating. Figure 5 is a resonance curve and illustrates the three regions of equipment response as a function of the ratio of the equipment natural frequency to the input motion frequency.

Another important factor that time histories do not address is equipment damping. For simplicity, equipment damping is often expressed as a ratio (C/Cc) of the actual equipment (C) damping to that of a critically damped system (Cc). Frequently, the ratio is expressed as the percent of critical damping. As one can see from Figure 5, the damping property of the equipment limits the total amplification that the equipment will experience at resonance. With no damping, the equipment response amplification at resonance increases without bound. With a damping coefficient of 12.5%, the equipment response will not exceed four times the input motion, as can be seen in Figure 5.

This resonance curve is helpful in understanding the equipment response to earthquakes. However, these curves, based on continu-ous sinusoidal input motions, are too conservative for representation of actual earthquakes, which have not one, but a number of different frequencies. Additionally, these frequencies are discontinuous, starting and stopping several times during the course of the seismic event.

Figure 3. Mexico City Earthquake of July 6, 1964, NS Component17

Figure 4. Shake Table Time Histories for Equipment Test

Figure 5. Resonance Curves for Continuous Sine Motion

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

0.02 g0.01 g

0-0.01 g-0.02 g

10 0-1040

3020100

-10-20-30

cm

cm sec-1

(Typically the ground displacements are incredibly large due to small errors in the base line for the accelerogram)

Acceleration

Displacement

Time, seconds

Velocity

Displacement(cm)

(g)Acceleration

20 Seconds

CAL2.5" Peak/25 Lines5"/Line

CAL1g/1 Line

5

4

3

2

1

00 1 2 3

Eq

uip

men

t R

esp

on

se /

Inp

ut

Mo

tio

n

EquipmentAmplifies

Input Motion

EquipmentFollows

Input Motion

EquipmentAttenuates

Input Motion

Frequency Ratio

Equipment Natural FrequencyInput Motion Frequency

Equipment Response per Unit Input Motion as a Function of Frequency Ratio:

= 0.25

= 0.50

= 0.125

= 1.00

= 0

17 Newmark, Fundamentals of Earthquake Engineering, p. 227.

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Figure 7. Equipment Qualification by the Response Spectrum Method When TRS “Envelopes” the RRS for All Equipment Natural Frequencies

Figure 8. “Q” Curves—Vibration Magnification vs. Percent of Critical Damping

Frequency HzA

ccel

erat

ion

(g

)

Spectrum Dip—Not Important Because

Frequency Is Not anEquipment Natural

Frequency

Required Response Spectrum

(RRS)

Zero PeriodAcceleration= MaximumTable Test

Motion

Zero PeriodAcceleration= MaximumFloor Motion

Test Response Spectrum(TRS)

1 2 3 4 5 6 7 8 9 10 20 30 40 5060

70 9080 100

10987654

3

2

1.00.90.80.70.60.5

0.4

0.3

0.2

0.1

30

40

20

10

00 5 10 15

ContinuousSine MotionQ=100/(2c/cc)

10 Cycles/Beat

5 Cycles/Beat

Random Vibration

X10X6.8

X5.6

X3.1

Q F

acto

r, M

agn

ific

atio

n N

um

ber

c/cc, Percent of Critical Damping

TypicalEarthquake

Ground Motion

Q = 1002c/cc

Figure 6. Basic Vibration Equations18

Because of these difficulties in universally applying the time history form, engineers have developed a method of comparing earthquake response motions as a function of frequency, rather than time. This is called the acceleration response spectrum method. The acceleration response spectrum for any time history is a plot of the maximum responses of a series of linear, single-degree, free-dom oscillators (one spring, one mass, one dashpot that can move linearly along only one axis) mounted on a surface moving according to the time history being studied. Figure 6 depicts one such simple oscillator and its basic equations of motion. Typically, the response spectra are plotted over the 1 to 35 Hz frequency range in no less than 16 steps, not exceeding one-third octave. (For example, 1.0, 1.26, 1.6, 2.0, 2.5, 3.2, 4.0, 5.0, 6.3, 8.0, 10.0, 12.7, 16.0, 20.0, 25.4, and 32.0 Hz.)

The responses of these oscillators are easily determined in real time, with digital computers and fast spectrum analyzers in the test laboratory. However, the complex and difficult task of commu-nicating earthquake requirements and equipment capabilities has become a routine matter of showing that the equipment capability response spectrum, as produced by shake table test, envelops the ground-level seismic requirements. During this test, the response spectrum envelops the applicable portion of the location where the equipment is to be installed. The applicable portion means that enveloping is required at all equipment frequencies. Figure 7 shows a typical test response spectrum (TRS) enveloping the applicable portion of the required response spectrum (RRS). Note that enveloping does not occur at 4.5 Hz, which is accept-able, because this was not a resonant frequency of the equipment. Enveloping is only necessary at the natural frequencies of the equipment. This illustrates the value of the simple frequency sweep test to identify the lowest natural frequencies and damping factors associated with any equipment seismic test certification program.

Figure 8 illustrates a more useful form for engineers. The peak magnification (alignment of the equipment natural frequency with the earthquake frequency), Q, as a function of the damping. Each curve represents a different type of earthquake motion. The “low-est” curve for “random motion” (all frequencies present to an equal extent) is generally the most like the ground motion during an actual earthquake. Because most electrical equipment is mounted on a rigid surface or inside another structure, the original earthquake motion is “filtered” by that structure. The equipment, therefore, experiences so-called “quasi-resonance” effects as the structure to which it is mounted alternately amplifies and attenuates the earth-quake motion according to its inherent characteristics. The result is the equipment peak amplification lies somewhere between the two extremes—“lower level random motion” response and “higher level continuous sine“ response.

Sinusoidal Harmonic Motion X = A sin wt = Displacement

X = dx = A w cos wt = Velocitydt

X = dx = – A w 2 sin wt = Acceleration

dt

Where W = 2 x Frequency

Linear Single-Degree-of-FreedomOscillator

C

M X

k

18 Beer, Vector Mechanics for Engineers: Statics and Dynamics, p. 771.

8





� CAUTION! IT IS IMPORTANT TO VERIFY THAT THE RRS AND THE TRS ARE BASED ON AND PLOTTED AT THE SAME DAMPING FACTOR BEFORE MAKING THE COMPARISON.

Now that the basics of earthquake engineering have been presented, several key elements that are very useful in understanding the nature of earthquakes, time history, the response spectrum curve (RSC), and the potential effect on electrical enclosures can be discussed further.

Although the Richter scale M is of good use in describing earth-quake strength, it does not identify the energy content of the earthquake or its potential to damage structures and equipment. Basically, the Richter scale M is a displacement indicator rather than an energy or acceleration indicator.

The energy content of an earthquake can best be defined via the RSC. This curve must be carefully understood and carefully applied. It is a theoretical curve computed for application to a system or equipment. Typical curve sets are shown in Figure 9.

The only spectral acceleration magnitude that is directly related to the earthquake event is the maximum response of rigid systems. Within earthquake engineering, it is understood that rigid systems are those with no resonance frequencies below 33 Hz and are considered to have a zero period of acceleration. Because a rigid system will not amplify the motion of the earthquake, its maximum response acceleration is equal to the maximum acceleration of the earthquake time history. As a result, the part of the RSC at the higher frequencies, referred to as zero period acceleration (ZPA), directly defines the maximum acceleration of the earthquake time history. It does not depend on the damping properties of the equip-ment. Thus, no matter what the equipment damping, the ZPA is always the same, and is equal to the maximum acceleration in the earthquake time history.

All other spectral accelerations are possible only if the equipment has a dominant resonant frequency that aligns with the frequency on the response spectrum curve (RSC). Thus, this curve tells the engineer that it is possible for a piece of equipment to experience the spectral acceleration defined in the curve, if the equipment has a dominant resonance frequency matching the frequency on the RSC.

It is important to understand that the damping properties of a system are a direct indication of the system’s ability or inability to dissipate the earthquake energy.

To further explain the effect of damping properties on the seismic response of systems, assume that two enclosures were similarly designed and built, but with one exception: One enclosure is a welded structure, while the other is a bolted structure. Aside from this difference, the enclosures are identical in design, mounting, and weight. Should both be subjected to an earthquake motion, the structural elements in the bolted cabinet will move relative to each other, producing friction and noise. Ultimately, these effects within the bolted enclosure result in increased dissipation of the energy produced by the seismic event as compared to the welded enclosure. The bolted enclosure will dampen the energy much quicker than the welded version, resulting in reduced time for the seismic response to build up.

For this reason, the RSC is usually computed and plotted for different damping properties—typically 1%, 2%, 3%, 5%, 7%, and so forth. It must be recognized that all resultant plots on the RSC are produced as a result of the same earthquake time history input motion (see Figure 9). It should be apparent that the result and response will be higher for systems with lower damping properties, and lower for systems with higher damping. A very useful rule is: The higher the damping coefficient of the equipment, the lower its response curve; the lower the damping coefficient of the equipment, the higher its response curve.

Figure 9. Response Spectrum Curve

Figure 10. Triaxial Shake Table

Figure 11. Response Spectrum Curve, 5% Damping Curves

10

1.0

0.10.1

1.0 10 100

2% Damping, 2.4g Peak Acceleration

Acc

eler

atio

n (

g)

Frequency Hz


Zero Period Acceleration (ZPA), Equal to 0.6g


Side-to-SideBase Motion

VerticalBase Motion

Front-to-BackBase Motion

4 x 36"

SwitchgearAssembly

90"

86"

10

1.0

0.10.1

Frequency Hz

1.0 10 100

Front-to-Back Seismic Base Input

Side-to-Side Seismic Base Input

Vertical Seismic Base Input

Acc

eler

atio

n (

g)

9




To further demonstrate the use of the RCS, consider this example: Multiple switchgear sections are mounted to a triaxial shake table (Figure 10). The section of equipment is then subjected to the base seismic response spectrum shown in Figure 11.

The switchgear assembly has a dominant natural frequency of 6 Hz in the side-to-side direction, 10 Hz in the front-to-back direction, and 55 Hz in the vertical direction. The assembly is a bolted structure with a 5% damping coefficient.

If the switchgear assembly is subjected to test levels that could produce the RRS, shown in Figure 11, one can quickly determine that the enclosure will amplify the base motion and could experience a 1.5g acceleration in the side-to-side, a 2.0g in the front-to-back, and a 0.9g in the vertical direction. The resultant forces and moments on the structural elements and internal components can now be computed to confirm that the enclosure and components contained therein will maintain their structural integrity.

To design the enclosure foundation, apply these accelerations at the cabinet’s center of gravity (C.G.), multiply them by the total mass of the equipment, and increase them with factors as appropriate to account for design margin. Thus, cross-coupling effects and close-mode contributions are taken into account. Next, determine the resultant moments, forces, and shear on the mounting bolts or welds.

Only the acceleration associated with the dominant natural frequencies of the enclosure need to be selected from the spectrum curves. For example, the front-to-back direction RRS has a peak spectral acceleration (3.2g) in the frequency range 1.5 to 3 Hz. This acceleration has little or no effect on the enclosure, because the front-to-back frequencies of 10 Hz do not coincide with this frequency range (1.5 to 3 Hz).

A beneficial engineering practice is to design equipment with natural frequencies that do not align with the frequencies found in the earthquake time history.

Most earthquakes tend to include low frequencies (1 to 3 Hz). Eaton understood this phenomenon and designed equipment with resonance frequencies above those levels. All Eaton equipment is designed with frequencies above 3.2 Hz, which serves to minimize the amplification.

This is further discussed in the next section (Part III) where the devel-opment of seismic requirements for electrical equipment is considered.

Part III

Seismic requirements

Consistent with Eaton’s commitment to produce equipment that exceeds present and future code requirements, essentially all engineered-to-order assemblies and standard assembly products have been designed, manufactured, and tested to meet rigorous seismic requirements.

International Building Code (IBC) 2006

On December 9, 1994, the International Code Council (ICC) was established as a nonprofit organization dedicated to developing a single set of comprehensive and coordinated construction codes. The ICC founders—the Building Officials and Code Administrators (BOCA), the International Conference of Building Officials (ICBO), and the Southern Building Code Congress International (SBCCI)— created the ICC in response to technical disparities among the three recognized model codes in use at the time. The ICC offers a single, complete set of construction codes without regional limitations—the International Building Code (IBC).

Since the establishment of the ICC and the issuance of the 2000 IBC (Rev-0), there have been two revisions: the first was published in 2003; the second in 2006. There were no substantial changes in the code that affected the validity of the 2003 IBC Eaton seismic certifications issued prior to the revisions. This paper addresses the requirements of the 2006 IBC, hereafter referred to as the IBC.

According to Chapter 16 of the IBC, “Structure Design,” the seismic requirements of electrical equipment in buildings may be computed with two pieces of information: 1) a determination of the maximum ground motion at the site; 2) an evaluation of the equipment mount-ing and attachment inside the building or structure. This data can then be evaluated to develop the appropriate seismic test require-ments. The ground motion, the in-structure seismic requirements of the equipment, and the seismic response spectrum requirements are discussed below.

A. Ground motion

According to the IBC, the first and most important step in the process is to determine the maximum considered earthquake spectral response acceleration at short periods of 0.2 seconds (Ss) and at a period of 1.0 second (S1). These values are determined from a set of 24 spectral acceleration maps contained in the International Building Code and include the numerous contour lines indicating the severity of the earthquake requirements at a particular location. Great care has been taken in selecting the maximum values for the contour lines.

For example, the maps indicate low to moderate seismic require-ments for most of the continental United States of America (USA) with exceptions being the West Coast (State of California) and the Midwest (New Madrid area). The seismic levels in the New Madrid area are approximately 30% higher than the maximum levels of the West Coast.

The maps also suggest that the high seismic requirements in both regions, West Coast and Midwest, quickly decrease away from the high magnitude fault areas. These high requirements are limited to a relatively local area along the fault lines. Just a few miles away from this area, only a small percentage of the maximum requirements are indicated.

To provide a realistic estimate of the seismic requirements for the continental USA, attention will initially be focused on the West Coast, where the values noted exceed the rest of the continental USA, with the exception of the New Madrid area. The New Madrid area seismic requirements will be addressed separately to prevent imposing unreasonable requirements on the rest of the USA.

The worst-case conditions are formulated by selecting the mapped Maximum Considered Earthquake Spectral Response Acceleration at short periods of 0.2 seconds (Ss), equal to 285% gravity, and at a 1.0 second period (S1), equal to 124% gravity. These accelerations will be used to calculate the Adjusted Maximum Considered Earthquake Spectral Response Accelerations. This combination of Ss and S1 is identified using the contour maps in Figures 12 and 13. These numbers are the maximum values for the entire country, except for the New Madrid area. These particular sites are on the border of California and Mexico (S1) and in Northern California (Ss). Figures 12 and 13 are developed for Site Class B, at 5% of critical damping.

To determine the maximum considered earthquake ground motion for most site classes (A through D), the code introduces site coef-ficients. When these are applied against the location-specific site class, this produces the adjusted maximum considered earthquake spectral response acceleration. The site coefficients are defined as Fa at 0.2 seconds short period and FV at 1.0 second period. From the tables in the IBC, the highest adjusting factor for SS (≥ 1.25) is equal to 1.0 and 1.5 for S1 (> 0.5). It is important to note that the CBC mandates the use of site class D for California.

Therefore, the adjusted maximum considered earthquake spectral response for 0.2 second short period (S

MS) and 1.0 second period

(SM1

), adjusted for site class effects, is determined from the following equations:

SMS

= Fa S

S = 1.0 x 2.85g = 2.85g

SM1

= FV S

1 = 1.5 x 1.24g = 1.86g

ASCE 7-05 (American Society of Civil Engineers) provides a plot showing the final shape of the design response spectra of the ground (Figure 14). ASCE 7-05 is referenced throughout the IBC as the source for numerous structural design criteria.

10





Figure 12. Maximum Considered Earthquake Ground Motion for Region 1 of 0.2 sec. Spectral Response Acceleration (5% of Critical Damping), Site Class B, S

S

11




Figure 13. Maximum Considered Earthquake Ground Motion for Region 1 of 1.0 sec. Spectral Response Acceleration (5% of Critical Damping), Site Class B, S

1

12





Figure 14. Specific Response Spectrum Curve—Ground

The design spectral acceleration curve can now be computed. The peak spectral acceleration (SDS) and the spectral acceleration at 1.0 second (SD1) may now be computed from the following formulas in the code:

SDS

= 2/3 x SMS

= 2/3 x 2.85g = 1.90g

SD1

= 2/3 x SM1

= 2/3 x 1.86g = 1.24g

SDS, the peak spectral acceleration, extends between the values of TS and T0. TS and T0 are defined in the codes as follows:

TS = S

D1/S

DS = 1.24/1.90 = 0.653 seconds (1.53 Hz)

T0 = 0.2 S

D1/S

DS = 0.2 x 1.24/1.90 = 0.131 seconds (7.63 Hz)

According to the IBC and ASCE 7-05, the spectral acceleration (Sa) at periods less than 0.131 seconds may be computed by using the following formula:

Sa = S

DS (0.6 T/T

0 + 0.4)

where T is the period where Sa is being calculated.

For example, the acceleration at 0.0417 seconds (24 Hz) is equal to:

Sa = 1.90 (0.6 [0.0417/0.131] + 0.4) = 1.12g

The acceleration at 0.03 seconds (33 Hz) is equal to:

Sa = 1.90 (0.6 (0.03/0.131) + 0.4) = 1.02g

At zero period (infinite frequency), T = 0, the acceleration (ZPA) is equal to:

Sa = 1.90 (0.6 [0.0/0.131] + 0.4) = 0.76g (ZPA)

The acceleration to frequency relationship in the range of 1.0 Hz to TS is stated equal to:

Sa = S

D1 /T

where Sa is the acceleration at period T.

At 1.0 Hz (T = 1.0) this equation yields the following acceleration:

Sa = 1.24/1.0 = 1.24g

Testing has demonstrated that the lowest dominant natural frequency of Eaton’s electrical equipment is above 3.2 Hz. This indicates that testing at 1.24g at 1 Hz is not necessary. In addition, having the low end of the spectra higher than realistically required forces the shake table to move at extremely high displacements to meet the spectral acceleration at the low frequencies.

0.131 (7.63 Hz)

1.0(1.0 Hz)

0.653 (1.53 Hz)

Period T (frequency)

(SDS)1.90g

1.24g

0.76g

SD1

Spectral Response Acceleration Sg

It is common to over test by factors of two to three times if the low end of the spectra accommodates this acceleration component. Through testing experience and data analysis, the seismic accel-eration at 1.0 Hz is taken equal to 0.7g, which will ensure that the seismic levels are achieved well below 3.2 Hz. This yields a more vigorous test over a wider range of seismic intensities.

In developing the seismic requirements above, it is important to recognize the following:

TS and T0 are dependent on SMS and SD1. If SM1 is small relative to SMS then TS and T0 will be smaller and the associated frequencies will shift higher. The opposite is also true. This must be realized in develop-ing the complete RRS. Therefore, it is not adequate to stop the peak spectral acceleration at 7.35 Hz. There are other contour line combi-nations that will produce different values for TS and T0. In accounting for this variation in the spread between SMS and SD1 and the resulting impact on TS and T0, it is almost impossible to consider all combina-tions. A study of the maps, however, suggests that all variations with high magnitude of contour lines could very well be enveloped by a factor of 1.5. Therefore, T0 is recomputed as follows:

T0 = 0.2 S

D1/(S

DS x 1.5) = (0.2 x 1.24)/(1.90 x 1.5) = 0.09 seconds

(11.0 Hz)

Based on past experience, most electrical equipment exhibits natural frequencies in the range of 5 to 10 Hz. Therefore, they aretested to the peak spectral accelerations required by the code. It is also important to recognize that stopping the peak accelerationshorter than 11 Hz would produce questionable test results due to the elimination of a portion of the spectra that may well contain the natural frequency of the equipment.

Eaton has developed generic seismic requirements that envelop two criteria:

1. The highest possible spectral peak accelerations and ZPA

2. The maximum frequency range required for many different sites

This approach results in a comprehensive and ultra conserva-tive methodology in certifying equipment to the IBC and often exceeds the approach utilized by other manufacturers.

Within the electrical industry, some manufacturers cease the seismic peak spectral acceleration at 7 or 8 Hz. This substantially reduces the amount of energy and frequency content included in the input time history. There are many certifications issued by other manufacturers that claim qualification to 3 or 4g spectral accelera-tion. This raises the question: “What is the true acceleration that was measured at the natural frequencies of the equipment?”

It is very likely that the equipment dominant frequencies were only tested to a fraction of what is claimed. Therefore, the claimed curve should be reduced to the actual spectral acceleration at the domi-nant natural frequencies of the equipment. Eaton accounts for that by testing to peak spectral accelerations even beyond 11.0 Hz.

This completes the ground motion design response spectrum. The spectral accelerations are equal to 0.76g at zero period (ZPA) and increases linearly to a peak acceleration of 1.90g at 0.09 seconds (or 11 Hz) and stays constant to 0.313 seconds (or 3.2 Hz), then gradually decreases to 0.7g at 1 second (or 1.0 Hz). This final curve is shown in Figure 15.

13




Figure 15. Specific Response Spectrum Curve—Ground

This curve indicates the ratio of peak spectral acceleration to maximum input acceleration (ZPA) is 1.90/0.76 and approximately equal to 2.5. This ratio is maintained throughout this document.

The code does not provide formulation for the seismic spectral requirements inside buildings or above grade. Instead, the code provides formulation of the equivalent loads at the center of gravity (C.G.) of the equipment internal to structures or buildings. The purpose is to ensure the structural and mounting integrity of the equipment during and immediately after a seismic event. These requirements will be discussed to determine realistic seismic requirements for equipment mounted anywhere from the ground level to the roof of a particular building.

B. Seismic requirements of equipment installed internal or on top of structures (buildings)

The code provides a formula for computing the seismic require-ments of electrical and mechanical equipment on ground level of a structure or a building. This formula is designed for evaluating the attachment of the equipment to the foundation directly supporting it.

The seismic loads are defined in ASCE 7-05 Section 13.3 as:

Fp = 0.4 a

p S

DS W

p (1+2 Z/h) / (R

p/I

p)

15, 0.09 (11 Hz) (T0)

1.0(1.0 Hz)

0.313 (3.2 Hz) (TS)

(SDS)1.90g

0.7g

0.76g

SD1

Spectral Response Acceleration Sg

Period T (frequency)

Where:

Fp: seismic design force imposed at the component’s C.G. and

distributed relative to component mass distribution

ap: component amplification factor that varies from 1 to 2.50

SDS

: spectral acceleration, short period, as determined in the previous section

Wp: component operating weight

Rp: component response modification factor that varies

from 1.5 to 6.0 (ASCE 7-05 Table 13.6-1)

Ip: component importance factor of either 1.0 or 1.5

Z: highest point of equipment in a structure relative to grade elevation

h: average roof height of structure relative to grade elevation

To produce the maximum required force, the following parameters were chosen:

Z is taken equal to h (equipment on roof)

Ip is taken as a maximum equal to 1.5

ap is taken equal to 2.5 (maximum value allowed by the ASCE code)

Rp is taken equal to 2.5 (lowest value allowed by the ASCE code

for electrical distribution and control equipment).

This combination of ap and R

p provides the most conservative

seismic loading requirements.

SDS

has been computed in the previous section equal to 1.90.

The acceleration at the equipment C.G. when roof mounted is then calculated as:

Acceleration = 0.4 x 2.5 x 1.90g (1+2) / (2.5/1.5) = 3.42g

For equipment on grade, the acceleration at the C.G. is then calculated as:

Acceleration = 0.4 x 2.5 x 1.90g (1+0) / (2.5/1.5) = 1.14g

Based on this criterion, in order to establish the seismic acceptabil-ity of equipment inside a structure or a building, one must impose an equivalent static load at the equipment C.G. and record the results. This approach is very difficult and perhaps impossible to apply.

The C.G. would first have to be located and then physically coupled to a forcing mechanism supported by some type of a fixture and a reaction mass. This approach would provide incomplete data or anal-ysis. Applying a static load will push the entire system as one unit in the force direction without revealing sufficient data about the equip-ment flexibility, the relative motion of internal components to the cabinet structure, or the dynamics and resonance of the equipment.

A more realistic approach with enhanced test results is to expose the equipment to floor motion, causing the inertia forces to occur in the opposite direction when the mass is suddenly accelerated. Bolting the base of a piece of equipment to a shake table, then quickly accelerating it, results in exposing the equipment to inertia loads higher than the source input.

14





As explained previously, many seismic test programs clearly indicate that electrical equipment, which is supported at the base, tends to vibrate in the equipment’s natural dominant frequency, much like a free cantilever beam that is supported at the bottom and free at the top. The seismic response at the middle of the equipment’s C.G. is at least 50% higher than the floor input of the equipment’s natural frequency.

Therefore, the base forces associated with the static loads at the C.G. of the equipment could be computed as 3.42/1.5 = 2.28g. The ZPA associated with this spectral acceleration may be computed per the previous relationships established.

In the context of this discussion, Eaton’s seismic requirements to meet the IBC (Reference 1) are:• For equipment on grade, the base seismic requirements are

shown in Figure 15.• For equipment on the roof of a structure, the base input

acceleration at the equipment natural frequency must demonstrate the ability to withstand levels of 2.28g base acceleration or 3.42gs at the equipment C.G.

C. New Madrid seismic requirements

According to the IBC, the New Madrid fault maximum considered earthquake spectral response acceleration is Ss = 3.69g and S1 = 1.25g. The method to develop the required spectrum and required forces at the C.G. is the same as described above. Based on the exercise in the previous section, and by virtue of the equations being of the first order, the requirements can be directly determined by linearly increasing the complete levels and static force requirements by the ratio of 3.69/2.85 = 1.29. The resultant RSC is shown in Figure 16.

The maximum seismic forces at the C.G. for equipment mounted at the top floor will be equal to 1.29 x 3.42 = 4.41g or 2.94g peak spectral acceleration.

Eaton’s seismic requirements for (equipment on or in proximity to) the New Madrid area is:

1. For equipment on grade, the base seismic requirements are shown in Figure 16.

2. For equipment inside a structure or on top of the roof, the base input acceleration at the equipment natural frequency must exceed the levels of 2.94g base acceleration or 4.41g at the equipment C.G.

Figure 16. Response Spectrum Curve—Ground (New Madrid Area)

Period T

2.46g

0.83g

1.0g

To Ts 1.0

California Building Code (CBC) 2007The 2007 CBC, effective January 1, 2008, adopted the 2006 IBC as CBC-Title 24. The seismic requirements are essentially the same as described in the IBC, with some minor modifications. When considering the maximum seismic requirements, the IBC and CBC are basically identical.

Again, as in the IBC, the RSC starts at 1.24g (Sa) at 1.0 Hz, andincreases to 1.90g (S

DS) at 1.53 Hz (Ts). The peak spectral accelera-

tions then cover a wide band of frequencies up to 7.63 Hz (To) then linearly decrease to 0.76g at the ZPA.

Combined Seismic Requirements for Eaton’s Distributionand Control Equipment

To better compare all levels and determine the final enveloping seismic requirements, the IBC standards are used for California and New Madrid areas, as plotted in Figure 17. All curves are plotted at 5% damping. All curves are determined for equipment mounted ongrade or in the basement of the structure.

An envelopment of the seismic levels in the frequency range of 3.2 Hz to 100 Hz are also shown. This level is taken as Eaton’s generic seismic test requirements for all certifications. These levels are also plotted in Figure 18. The levels are listed below:

Frequency Acceleration

1 0.719

3.2 2.28

11 2.28

33 1.02

100 1.02

Many standards require that seismic levels be increased by 10% to account for differences in commercial hardware. Applying this will bring the spectral peak acceleration to 2.51g and the ZPA to 1.12g.

Frequency Acceleration

1 0.7

3.2 2.51

11 2.51

33 1.12

100 1.12

The vertical levels are taken equal to 2/3 of the horizontal requirements.

In addition, Eaton performs seismic tests on the equipment at approximately 120% of the generic enveloping seismic requirements (see Figure 18). This testing is designed to establish margin in anticipation of future changes in the codes.For seismic certification of equipment located in the New Madrid area, Eaton proceeds as follows:

Complete testing to the generic levels in Figures 17 and 18.

Perform additional tests at approximately 20% higher seismic levels than shown in Figures 18.

During September 2008, Eaton performed experimental seismic testing on electrical equipment levels higher than the combined requirements. Some of the equipment test results are shown in Figures 19, 20, and 21. The levels are provided in the front-to-back, side-to-side, and vertical directions. As indicated, the actual test levels recorded were much higher than current codes require.

19 See discussion under “A. Ground motion” on page 9 for

acceleration at 1 Hz.

15




Figure 17. RRS Comparison

Figure 18. 100% vs. 120%

Figure 19. Test Response Spectrum Curve (Front to Back)

Acc

eler

atio

n (

g)

0.1

1

10

1 10 100 Frequency (Hz)

Eaton Seismic

IBC 2006 New MadridIBC 2006/CBC 2007

Acc

eler

atio

n (

g)

0.1

1

10


Eaton 100% Seismic Envelope

Eaton 120% Seismic Envelope

Acc

eler

atio

n (

g)

0.1

1

10


FB TRS

FB RRS

Figure 20. Test Response Spectrum Curve (Side to Side)

Figure 21. Test Response Spectrum Curve (Vertical)

Acc

eler

atio

n (

g)

0.1

1

10


SS TRS

SS RRS

Acc

eler

atio

n (

g)

0.1

1

100

10


V TRS

V RRS

16





Part IVTest facility and test methodology

Test specimens

Since the inception of Eaton’s test program in 1985, more than 100 specimens have undergone seismic testing. Since it was not feasible to test every single configuration, it was necessary to select a number of test specimens that adequately represent the total product portfolio. Each product line was reviewed and evaluated to determine the number and design configurations of the test specimens. Criteria were established for representation of all equipment in each product line:

1. The test unit structure shall be similar to the major structural configurations being supplied in the product lines. If more than one major structure is being offered, then these configurations shall also be selected for testing.

2. The mounting configuration of the test units to the shake table shall simulate the different mounting conditions for the product line. If several mounting configurations are used, the different product variations are required to be included in the testing program.

3. The major electrical components should be included inthe test specimens. The components shall be mounted at similar locations to their mounting locations in production configurations. The components shall be mounted to the structure using the same mounting hardware used in the typical design.

4. The weight of the test units shall be similar to the typical weight of the equipment being represented. Equal and higher weights than the typical weight shall be acceptable.

5. Other variations, such as the number of structures inproduction assemblies, and indoor and outdoor applications, will also be represented by the test specimens.

Figure 22. Test Specimen

Side-to-SideBase Input

Front-to-BackBase Input

TypicalTest Unit

VerticalBase Input

Shake Table/ Base Motion

Accelerometers

ResponseAccelerometers

ShakeTable

Test facility and test table

Test specimens for current production products are tested on a truly independent triaxial shake table at the locations such as Wyle Seismic Test Laboratory, located in Huntsville, Alabama.Wyle Laboratories is accredited by the American Association for Laboratory Accreditation (A2LA) in the field of vibration testing. The Wyle Laboratories, Huntsville Facility, Quality Management System is registered in compliance with the ISO-9001 International Quality Standard. All instrumentation, measuring, and test equipment used in the performance of test programs is calibrated in accordance with Wyle Laboratories’ Quality Assurance Program, which complies with the requirements of ANSI/NCSL Z540-1, ISO 10012-1, and ISO/IEC 17025. The table and control systems are capable of exciting the test specimens in all three directions simultaneously, using statistically independent and phase incoherent seismic input signals. A sketch of a test unit mounted to the shake table is shown in Figure 22.

Test sequence

The seismic verification testing consisted of the following 10 steps for each specimen:

1. Receipt and inspection

2. Functional operation

3. Hi-pot electrical testing

4. Resonance search testing

5. Seismic test at 50% of the combined seismic requirements

6. Seismic test at 100% of the combined seismic requirements

7. Seismic test at higher than the 100% combined seismic requirements (typically 120%–130%)

8. Functional operation

9. Hi-pot electrical testing

10. Final inspection

Resonance search test

Resonance search (sine sweep) tests are performed on all test specimens. The sine sweep tests are performed in the three prin-cipal axes of the test specimens: front-to-back, side-to-side, and vertical directions. The sine sweep tests are conducted at amplitude of 0.2g. The sine sweep tests are performed from 1 to 50 Hz at a sweep rate of 1 octave per minute.

Seismic test input

The seismic inputs are generated using random signals with a frequency and energy content up to 100 Hz. The test inputs are independent in the three principal directions of the test specimens: front-to-back, side-to-side, and vertical directions. All seismic test inputs are 30 seconds in duration (see Figure 23).

Data acquisition

The test inputs to the shake table are monitored using three accelerometers mounted on the shake table. The accelerometers are oriented in the shake table principal axes, which coincide with the equipment front-to-back, side-to-side, and vertical directions. The seismic response of the test specimens are monitored using several accelerometers mounted on the test specimen and oriented along the three principal axes of each test specimen. The testinput and seismic response of the equipment is recorded on and analyzed using a shock spectra analyzer. The test response spectra are derived at 5% damping (see Figures 23 and 24).

Electrical connection and test specimen monitoring

As stated previously, the acceptability of the test specimen requires that all equipment maintain structural integrity and perform its intended function before and after the seismic test.

17




Test assembly and mounting conditions

At the beginning of each test, the test units are mounted to the shake table (Figure 25) using the specified seismic mounting conditions.

Figure 23. Test Input

Figure 24. Test Data Acquisition

3.0

G

0.0

–2.00.00 30.00

Time (sec) x interval = 2.0000

MIN = – 0.1921E+01 MAX = 0.1588E+01

Min/max x:B in size = 8

Figure 25. Typical Equipment Mounting and Installation of Accelerometers

Test procedure

All test specimens identified in Part VI (Figure 26 See note on pp. 20 regarding list of products.) are subjected to the seismic test requirements specified in Figure 18. Testing is conducted in accordance with IBC (ASCE 7-05), CBC, and ANSI C37-81 test requirements. The test programs are documented in third-party laboratory test reports.

Acceptance criteria

The seismic verification of the test specimens was based on the following acceptance criteria:

1. The test specimens’ structure shall maintain structural integrity with no major structural failure that may impact the electrical performance of the test specimens or impact adjacent equipment.

2. The test specimens shall perform their electrical function immediately after seismic testing.

3. The test specimens shall pass one minute dielectric withstand testing per the associated industry standards after seismic testing.

18





Part VShared responsibilities

The equipment manufacturer, specifier, Authority Having Jurisdiction (AHJ), and installer have a shared responsibility to ensure that the installation will meet the seismic requirements of the code. The equipment manufacturer determines that the equipment will be functional following a seismic event. The equipment specifier and installer must ensure that the equipment is rigidly supported and will not leave its foundation during a seismic event. The AHJ shall confirm that the installation method conforms to the manufacturer’s guidelines.

Previously in this paper, the Eaton interpretation of the various codes and standards, as well as the levels of the test response spectra used in testing, was described. The test results ensure that Eaton equipment will perform the intended function after the seismic event. However, the foundation and the anchorage must also meet the codes and standards for the entire installation to be functional after a seismic event. Equipment poorly anchored or mounted to a flexible foundation will not meet the requirements.

The anchoring of electrical equipment as recommended by the structural or civil engineer is critical. If the equipment is not attached to the building structure in accordance with the minimum standards recommended, the complete equipment installation might become too flexible and may overturn or shear the attachment devices and slide off its foundation. Such movement may damage either the building structure or other components, including items connected to the equipment.

Structural and civil engineers formulate methods of attachment that are applicable to each specific condition based on past experience. They evaluate the equipment, methods, and techniques of attach-ment along with tested anchoring systems. The structural or civil engineers responsible for the structural design review the proposed method of attachment. Based on both established criteria and direct calculation, they verify its performance and the capability of the building’s structural elements to accommodate the seismic forces. In many states, registered professional civil or structural engineers must attest that the design is adequate for the seismic environment and apply their seal.

In evaluating the equipment mounting, the structural or civil engineer performs calculations based on data received from the equipment manufacturer specifying the size, weight, center of gravity, and mounting provisions of the equipment. The embedded concrete anchors, wood, or steel attachments must be adequate to resist the site-specific seismic forces. For either attachment, bolts of the proper grade of steel must be correctly sized and tightened to recommended torque levels. If an embedded anchor is used, engi-neering data for the anchoring hardware will allow the engineer to determine the size required. The mounting depth and the strength of concrete to contain it will also be determined. The embedded anchors must be correctly installed in accordance with the method specified by the anchor manufacturer.

The reliability of electrical connections within the system must also be considered. Electrical equipment is installed as part of a system. Busway or conduits connect individual components of the electrical system to each other. Interface methods that will prevent damage from an earthquake must be specified. For example, bottom entry of conduits is recommended for transformers and switchgear. If top entry is specified, seismic fittings or a flexible interface designed to accommodate the necessary enclosure motion are needed. Transformers are often close coupled to switchgear with a flexible connector to minimize transfer of relative motion. Likewise, a flexible connector can be used to connect generators to the bus duct, and the addition of insulating boots improves the integrity of such installations.

The availability of electrical power following a disaster is often critical. It is certain that earthquakes will occur in the future. It is the responsibility of the engineer to design and specify reliable equipment and systems that will withstand them. The IBC and CBC establish minimum requirements for equipment seismic design and

installation. As required by the IBC and CBC, Eaton has equipment available that has been seismically certified. When specified, such equipment increases the likelihood that the electrical system will function in the aftermath of an earthquake.

Part VITypical Eaton seismic equipment specifications

1.01 The manufacturer of the assembly shall be the manufacturer of the major components within the assembly.

1.02 For the equipment specified herein, the manufacturer shall be ISO 9001 or 9002 certified.

1.03 The manufacturer of this equipment shall have produced similar electrical equipment for a minimum period of five (5) years. When requested by the engineer, an acceptable list of installations with similar equipment shall be provided demonstrating compliance with this requirement.

1.04 Provide seismic qualified equipment as follows:

ote: N To spec writer: To help understand the 2006 IBC/2007 CBC seismic parameters for a specific location, the attached link to the U.S. Geological Society will be extremely helpful: http://earthquake.usgs.gov/research/hazmaps/design/

• Download the file “Java Ground Motion Parameter Calculator—Version 5.0.8 (4.6 MB)” and save it to your hard drive, then run the executable file (.exe) that was downloaded.

• Enter the latitude and longitude of your project location. (To find exact latitude and longitude for a given address, go to http://geocoder.us/)

• The IBC seismic criteria for that location will then be displayed. It is simply a matter of verifying that the criteria shown for your specific building location is less than the equipment certification levels.

1. The equipment and major components shall be suitable for and certified by actual seismic testing to meet all applicable seismic requirements of the 2006 International Building Code (IBC) Site Classification [enter classification from above Web site]. The site coefficients Fa = [enter value from above Web site], and spectral response accelerations of SS = [enter value from above Web site]g, S1 = [enter value from above Web site]g are used. The test response spectrum shall be based upon a 5% damping factor, and a peak (SDS) of at least [enter value from above Web site] g’s (3–12 Hz) applied at the base of the equipment in the horizontal directions. The forces in the vertical direction shall be at least 66% of those in the horizontal direction. The tests shall cover a frequency range from 1 to 100 Hz. Guidelines for the installation consistent with these requirements shall be provided by the equipment manufacturer and based upon testing of repre-sentative equipment. Equipment certification acceptance criteria shall be based upon the ability for the equipment to be returned to service immediately after a seismic event within the above requirements without the need for repairs.

-- OR --

2. The manufacturer shall certify the equipment based upon a dynamic and/or static structural computer analysis of the entire assembly structure and its components, provided it is based upon actual seismic testing from similar equipment. The analysis shall be based upon all applicable seismic requirements of the 2006 International Building Code (IBC) Site Classification [enter classification from above Web site], site coefficient Fa = [enter classification from above Web site], FV = [enter classification from above Web site] and spectral response accelerations of SS = [enter classification from above Web site]g, S1 = [enter classification from above Web site]g. The analysis shall be based upon a 5% damp-ing factor, and a peak (SDS) of at least [enter classification

19




from above Web site]g, S1 (3–12 Hz), applied at the base of the equipment in the horizontal directions. The forces in the vertical direction shall be at least 66% of those in the hori-zontal direction. The analysis shall cover a frequency range from 1 to 100 Hz. Guidelines for the installation consistent with these requirements shall be provided by the equipment manufacturer, and should be based upon testing of repre-sentative equipment. Equipment certification acceptance cri-teria shall be based upon the ability for the equipment to be returned to service immediately after a seismic event within the above requirements without the need for repairs.

A. The following minimum mounting and installation guidelines shall be met, unless specifically modified by the above referenced standards.

1. The contractor shall provide equipment anchorage details, coordinated with the equipment mounting provision, prepared and stamped by a licensed civil engineer in the state. Mounting recommendations shall be provided by the manufacturer, and should be based upon the above criteria to verify the seismic design of the equipment.

a. The equipment manufacturer shall certify that the equipment can withstand, that is, function following the seismic event, including both vertical and lateral required response spectra, as specified in above codes.

b. The equipment manufacturer shall document the require-ments necessary for proper seismic mounting of the equipment. Seismic qualification shall be considered achieved when the capability of the equipment meets or exceeds the specified response spectra.

Eaton’s equipment test levels and ICC-ES-AC156

In December 2006, the ICC-ES issued an “Acceptance Criteria for Seismic Qualification by Shake-Table Testing on Nonstructural Components and Systems.” The criteria was made effective January 1, 2007. Eaton’s methodology for seismic certification of electrical equipment is consistent with the proposed criteria and meets the testing requirements specified. Eaton, however, differs in one impor-tant aspect: Eaton has taken the ratio of the equipment response modification factor (RP) to equipment importance factor (IP) equal to 2.5/1.5. This ratio provides the minimum ratio required by the codes for electrical distribution and control equipment, and also consid-ers that the acceleration is required to be measured at the center of gravity of the equipment. The ICC-ES-AC156 employs a factor of 1.0 to this ratio producing unnecessary and overtesting conditions. One additional difference needs to be mentioned—Eaton’s electrical equipment, with high natural frequencies (per ICC-ES-AC156), are also tested and certified to the same seismic test input as flexible equipment. ICC indicates that this equipment may be tested to 0.4 of the seismic levels developed for flexible equipment. Eaton’s test program is more conservative by testing all equipment types to the highest levels.

In addition, an important note should be made regarding the mount-ing configurations of the test units. Eaton’s equipment is mounted to the shake table in their most conservative and common mounting configurations to establish the lower bound of the equipment seismic capabilities. For example, Eaton seismic certification curves for motor control centers (MCCs) are based on a test unit mounted at the base as a free cantilever item, free at the top and supported only at the bottom. This test configuration encompasses all other mounting configurations because of its conservative nature. The test capability in Eaton’s certificates, therefore, covers all other applications.

When the MCC test units were tested supported at the bottom and with the top attached to a lateral wall, the seismic capacity of the test units were found to be much higher than their seismic capacity when supported at the bottom only. The seismic capac-ity of equipment presented in some papers appears to be based on testing of equipment with both top and bottom supports. It is important, therefore, to recognize that those published curves only apply to equipment mounted using top and bottom supports. Other mounting arrangements without top lateral supports will need to be re-established based on new testing programs.

3RD PARTY TEST ENGINEER IN CHARGE

S E I S M I C Q U A L I F I E D

TEST CERTIFICATE OF SEISMIC WITHSTAND CAPABILITYEaton’s Cutler-Hammer equipment identifi ed below was tested for seismic withstand capability and tested in accordance with the combined requirements specifi ed in the International Building Code, California Building Code and the Uniform Building Code. As required by the codes, the equipment demonstrated its ability to function after the seismic tests. The seismic capability of the equipment exceeds the worst-case required levels, as illustrated in the fi gure below.

For interpretation of testing datarefer to Eaton

Publication SA12501SE

The frequency sweep tests revealed that the lowest equipment natural frequency is:

4.0

3.5

3.0

2.5

2.0

1.5

1.0

.5

0

Frequency (Hz)

Zer

o P

erio

d A

ccel

erat

ion

Period (seconds)

Damping = 5%

ResponseAcceleration

(g)

3.2 4 5 6.4 8 10 13 17 20 26 32

.31 .25 .20 .16 .13 .10 .08 .06 .05 .04 .03 0

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Part VII

References

http://www.math.montana.edu/~nmp/materials/ess/geosphere/expert/activities/earthquakes/index.html

http://rchrd.com/weblog/archives/archive_2004-m09.php

http://w3.salemstate.edu/~lhanson/gls100/gls100_plate_tec.htm

Beer, Ferdinand P. and Johnston, E. Russell, Jr., (1962),Vector Mechanics for Engineers Statics and DynamicsMcGraw Hill Book CompanyNew York, NY, 1962

California Building Standards Commission,California Building Code (2007)428 J. Street, Suite 450Sacramento, CA 95814

International Code Council,International Building Code (2006)Suite 708,Falls Church, VA 22031-3401

International Conference of Building Officials, (1997),Uniform Building Code5360 Workman Mill RoadWhittier, CA 90601

Institute of Electrical and Electronics Engineers, Inc.,ANSI/IEEE C37.81 (1987)Guide for Seismic Qualification of Class 1E Metal Enclosed Power Switchgear AssembliesEast 47th StreetNew York, NY 10017-2394

Institute of Electrical and Electronics Engineers, Inc.,ANSI/IEEE 344 (1987)Recommended Practice for Seismic Qualification of Class 1E Equipment for Nuclear Power Generating StationsEast 47th StreetNew York, NY 10017-2394

Newmark, Nathan M. and Rosenblueth, Emilio, (1971),Fundamentals of Earthquake EngineeringPrentice-Hall, Inc.Englewood Cliffs, NJ 01971

The Building Officials and Code Administrators International, Inc.W Flossmoor RdCountry Club Hills, Il 60478-5795

Wiegel, Robert L., (editor),Earthquake EngineeringPrentice-Hall, Inc.Englewood Cliffs, NJ 01971

Figure 26. Seismic Test Units

Low Voltage Metal-Enclosed Switchgear

Panelboards

Switchboards

MCC

IT.

Low Voltage Busway

�

Dry Type Transformers

Transfer Switches

Uninterruptible Power Supplies (UPS)

Enclosed Control

Safety Switches

Medium Voltage Switchgear

MV Bus

Network Protectors

Medium Voltage Control

Substation Transformers

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Eaton Corporation

Electrical Sector1111 Superior Ave.Cleveland, OH 44114United States877-ETN-CARE (877-386-2273)Eaton.com

© 2009 Eaton CorporationAll Rights ReservedPrinted in USAPublication No. SA12501SEE / Z8772August 2009

PowerChain Management is a registered trademark of Eaton Corporation.

All other trademarks are property of their respective owners.




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