Buddhi S. SharmaManager,ACECOMS, AIT

About the Author

Introduction

Some of the most frequently asked questions bythe general public since the recent ‘Asia Earthquakeand Tsunami’ are

• Knowing that the Earthquake scale/magnitude at Sumatra, Indonesia measured9 on the Ritcher scale (RS), what is the scaleof that earthquake in my city?

• What Ritcher scale earthquake can myhouse/building withstand?

• Can you design a house/building to resistan earthquake measuring 8 or 9 on theRitcher scale?

• What is the extra cost to design a buildingfor earthquake?

• How often will such earthquake occur?• Can humans not do something to control

earthquakes?In addition to all the above questions, there are aseveral other issues that the general public wouldlike to know and be aware of. The main objective ofthis article is not to address the hard core technicalconcepts, theories, methods, procedures, etc. relatedto the questions faced by an engineer but to sharesome general ideas and knowledge that an engineercan find helpful while attempting to answer thesequestions from general public.

The situation of an engineer when somebody askshim the above questions is analogous to that of amedical doctor when a patient asks him thefollowing questions:

• How long will I live?

• How often will I fall sick?• What is the maximum fever that my body

can sustain?• Can you give me something in advance that

helps to fight against various diseases?The above analogy makes it clear that though thequestions sound quite simple, obvious and straightforward, the answers are not instant. A doctor maybe in a position to answer some of those questions(deterministically or probabilistically) after detailedexamination, tests, judgments, etc. and with somereservations and within certain limits. A similar isthe situation of an engineer while attempting toanswer the questions related to earthquakes.

Who is Qualified to Answer?For human body, which is of very complex nature,we need so many specialists to provide health/medical services. No single doctor can answer all thequestions related to the functioning of the humanbody or its problems. Similar is the case withengineers. Earth itself, being a complex object, hasseveral fields of studies to understand it fromdifferent perspective and applications. Ideally aperson who can be considered as ‘fully qualified’ toanswers all the earthquake related questions, facedby engineers, is the one who has in depth knowledgein geology, seismology, meteorology, geotechnical,structural engineering, etc. However, can we findsomeone like this? In the present day of super-specialization, probably not. Therefore, it is not asingle person who can answer these questions, buta team of experts from all these areas ofspecializations. Practically, structural engineers are inthe front line facing the public and in turn have to

Recent Large Earthquakes:

- 9.5 in 1960 in Chile

- 9.2 in 1964 in Alaska

- 9.1 in 1957 in Alaska

- 9.0 in 1952 in Russia

-9.0 in 2004 in Indonesia

On average every 10 Year!

What Richter Scale Earthquakecan my Building Withstand?

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acquire the information fromvarious fields of studies,assemble it and apply it tosolve the public safety relatedproblems. Another reason isthat while all other disciplinesprovide information on howearthquakes may effect a

structure it is the structural engineer who designsthe structures to counter the effects of such groundshakings. Let us now look at some of thebackground concepts and their relationship to thetask of the structural engineer.

Measuring EarthquakesThere are two ways in which we can quantify the sizeof earthquakes: magnitude and intensity.Magnitude is a measure of the amount of energyreleased during an earthquake. Almost everytime,when an earthquake occurs, we hear news reportson the magnitude of the earthquake using theRichter scale. Magnitude of an earthquake is a singlenumber regardless of where it originates and whereit’s felt but intensity will vary from place to place.

Using a simple analogy to illustrate the intensityand magnitude, is dropping a stone into a waterpond, the difference between magnitude andintensity is similar to the difference between theheight of the splash exactly where the stone hit thewater and the height of the waves over the pool.Therefore in general, the intensity is greater near theepicenter than at large distances from the epicenter.This decrease in intensity with distance is known asattenuation. Earthquakes are measured byseismographs, some of which are shown in Figure 1.

How a seismograph works?To give an overall idea about how a typicalseismograph works, let’s consider the simple deviceshown in Figure 2.

• • • • • A heavy weight is fastened to a horizontalrod as shown in the diagram.

• This rod hangs from a pole and is free toswing from side to side when the groundshakes.

• At the other end of the rod (away from thepole) is an ink pen and directly underneaththe pen is a piece of paper rolled around acylinder.

• This cylinder rotates so that the pencontinuously draws an ink line along themoving paper.

If the ground does not move, the rod does notswing, and the pen stays in place, so the ink line issmooth and straight. If the ground shakes,however, the rod swings and therefore the pen willdraw a zigzag as the paper turns. The stronger theshaking, the sharper the zigzags. This zigzag picturemade on the paper roll is called a seismogram. Asample of such a seismogram is shown in Figure 3.

What is Richter Scale?The Richter scale was invented in the 1930s by Dr.Charles Ritcher, a seismologist at the CaliforniaInstitute of Technology. It is a measure of the largestseismic wave recorded on a particularkind of seismograph located 100 kmfrom the epicenter. The wave heightis measured by seismograph (type ofpendulum with an ink pen or digitalrecorder) and the output is calledseismograms.

As the measurement is based onlogarithmic scale, a wave 1 mm (1000microns) high on a seismogram hasmagnitude of 3 (because log(1000)

Magnitude of anearthquake is asingle numberregardless ofwhere it originatesand where it's felt,but the intensitywill vary fromplace to place.

Figure 1: Digital seismographs

Figure 2: Conceptual seismographs

Magnitude 7e a r t h q u a k eproduces waves10 x 10 = 100times as high andrelease energy 32x 32 = 1024 timesas great as amagnitude 5earthquake.

Figure 3: Sample seismogram

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L = Fault Length (km)M = Earthquake MagnitudeVd = Long-term Slip Rate (mm/year)Tr = Recurrence Time (year)

(Kakimi Model Used for Tokyo City)

Figure 4: Relationship between L, M, Vdand Tr (Kakimi Model)

Figure 5: Expected pick accelerations forBangkok (Warnitchai and Lisantone, 1996)

Figure 6: Common way to present theperformance of a building (Source: ATC-40)

=3). Similarly, a wave 10 millimeters high would havea magnitude of 4. For reasons that a factor of 10change in the wave height corresponds to a factor of32 change in the amount of energy released during theearthquake, magnitude 7 earthquake produces waves10 x 10 = 100 times as high and release energy 32 x 32= 1024 times as great as a magnitude 5 earthquake.Therefore in general sense, an earthquake of 8 RS isnot twice as strong as the 4 RS earthquake but manythousand times stronger than 4.

Why Seismic Map not based onRichter Scale?Originally Ritcher scale was developed and usedprimarily to compare various earthquakes at differentlocations. It is primarily related to wave height andenergy. Relative to engineering design perspective,structures are designed to withstand the forces anddeformations. Newton’s well known second lawof motion, states that force is related to mass andacceleration. This means that if we can estimate theacceleration of a structure due to an earthquake wecan calculate the force applied on the structure.The mass of the building can be easily calculatedfrom the geometry and material property of thebuilding.

Engineers and researchers have done a lot of workto understand and correlate the important factorsthat determine the effects of earthquakes onstructures such as the length of active faults, sliprate, the distance from source, magnitude ofearthquakes, frequency of occurrences, and groundacceleration (to find force) at the building site. Anexample of such correlation is shown in Figure 4.Knowing that the ground movement of a certain

fault and its distance from thepoint under consideration, onecan find the earthquakesmagnitude and its recurrenceperiod.

Earthquake and BuildingPerformanceStructural responses of abuilding to an earthquakedepends upon various factors;primarily the origin (epicenter),magnitude and frequency ofground motion, geological andsoil conditions, and dynamiccharacteristic of a building.Methods, codes, software tools,etc. in current practice for theseismic design of buildings arebased on the followingphilosophy.

Buildings should be able to resist:

• Minor earthquakes without any damage(occupy immediately after earthquake)

• Moderate earthquakes with negligiblestructural damage and some nonstructuraldamage (may need some repair)

• Major earthquakes with predictable majordamages (building remains standing butonly barely).

• A properly designed building must respondto strong earthquake without collapse.

In the past, buildings were designed to resist someestimated earthquake forces. This kept the dooropen for questions from clients, “What will happento my building if earthquake of higher magnitudehits?” To answer these “What If ” type of questions,there is a promising trend towards using the‘Performance Based Design”. This approachattempts to answers the questions in qualitative ifnot in quantitative sense. With the currentknowledge and computing technology, it is possibleto predict the performance in terms of level ofdamages, safety, etc. of the buildings for variouslevels of earthquakes. Pushover Analysis is one ofthe computational methods for such predictions.Performance-based design together with PushoverAnalysis provides engineers with valuable insightto actual performance of a building during anearthquake.

For Bangkok, PRA and Return Period:0.025 g for 50 year0.050 g for 500 year0.075 g for 2500 year

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RS and Acceleration:The currentlyr e c o m m e n d e dmaximum design EQlevel for Bangkok(PRA = 0.075g) isbased 7.5 RS EQ atKanchaburi and 8.0RS on Andaman sea.

Figure 7: Sample design spectrumfor Bangkok (Warnitchai 2004)

Consideration of Designing New BuildingNew buildings can be designed to resist theanticipated level of an earthquake during the servicelife of the building. A typical procedure of designfor a new building for earthquake load is as follows:

• Carry out the seismic hazard analysis of thesite where the building will be located or usethe information available from reliablesource for that area to find the designaccelerations

• Perform the preliminary design of thebuilding based on experience or quickcalculations

• Compute the primary dynamic properties ofthe building (natural frequencies, modeshapes, damping, etc) using appropriatecomputer tools

• Use code specified methods to compute theforces due to earthquake

- Equivalent Static Methods (oftenconsidered highly simplified)

- Response Spectrum Methods(considered intermediate method)

- Time History Methods (generallyconsidered rigorous method)

• Most of the commercially available software(such as ETABS, SAP2000, etc) providefacilities to use any or all of these methods.

• Alternatively the better approach (butrequiring more manual calculations) couldbe to compute the forces using modal mass,displacement demand, force andsuperposition approach. This method hasseveral advantages over the other methods.The main one is that the analyst can studythe effect of individual modes and theircontribution.

The most common practice is to use the codespecified response spectrum to compute the designforces due to earthquake, combine with other deadload, live load, etc. and design the system/component for the combined loads.

From the above discussion, we can see that thoughthe buildings are not directly design for given RitcherScale of earthquake, there exist some correlationsbetween RS and design spectrum. These correlationsform the basis in the development of the designspectrums or acceleration maps which we can findeasily be based on design codes.

For example, the current recommended maximumdesign earthquake level for Bangkok is based 7.5 RSearthquake at Kanchaburi and 8.0 RS in the AndamanSea. This means, that if we design a building for thePRA=0.075 g, this can be understood as that thebuilding has been designed for 8.0 RS in Adaman Sea.

Evaluation ofExisting BuildingThis topic will be discussed in detail with a realproject case study in the next issue of this magazine.

Improving the Seismic Performance ofExisting BuildingThis topic also has been discussed in detail in article“ Swinging the Towers” In the September 2003 toMarch 2004 issue of this magazine. The main ideais to identify the weak points in the structure bydetailed inelastic analysis (such as pushover) andprovide the engineering solutions with an objectiveto achieve a target performance level.

Additional CostFrom the author’s experience and several otherreferences indicate that the typical range of costdifference between earthquake and withoutearthquake loads for buildings is 3-5% of the total

project cost and 10-15% of the structural cost.

Concluding RemarksEarthquakes are a natural phenomenon. Althoughhumans are unable to control the earthquake itselfbut we are able to control or/and minimize the lossof life and properties. This is the main philosophybehind the earthquake resistant design in currentuse and in the foreseeable future. With state-of-artknowledge in earthquake and related fields, mostof the questions from engineers and general publiccan be addressed in a probabilistic way if not in adeterministic way.

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