313 01-earthquake phenomena

8/6/2019 313 01-Earthquake Phenomena

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ADVANCED REINFORCED CONCRETE EPMAPUA Institute of Technology

Adam C Abinales f.asep, pice

1.1. EARTHQUAKE PHENOMENAEARTHQUAKE PHENOMENA

EARTHQUAKESEARTHQUAKES are one of natures greatest hazards to life; throughout historic timethey have caused the destruction of countless cities and villages on nearly every

continent in the world. They are the least understood of the natural hazards and inearly days were looked upon as supernatural events.

TThe hazards imposed by earthquakes are unique in many respects, andconsequently planning to mitigate earthquake hazards requires uniqueengineering approach. An important distinction of the earthquake problem is thatthe hazard to life is associated almost entirely with man-made structures. Except forearthquake triggered landslides, the only earthquake effects that cause extensiveloss of life are collapses of bridges, buildings, dams and other works of man.

HHowever, it is evident that even a successful prediction cannot eliminate theearthquake hazard; even if all the people are evacuated safely, the structureswhich largely determine the standard of living of the community remain, and their

destruction could be as disastrous loss to the regional economy. This aspect ofearthquake hazard can be countered only by the design and construction ofearthquake resistant structures, and therefore a completely successful earthquakeprediction program could not eliminate the need for effective earthquake

engineering.


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EEarthquake hazard poses a unique engineering design problem in that an intenseearthquake constitutes the most severe loading to which most civil engineering

structures might possibly be subjected, and yet the probability that any givenstructure will ever be affected by a major earthquake is very low. The optimum

engineering approach to this combination of conditions is to design the structureas to avoid collapse in the most severe possible earthquake thus ensuring againstloss of life but accepting the possibility of damage, on the basis that it is lessexpensive to repair or replace the small number of structures which will be hit by amajor earthquake than to build all structures strong enough to avoid damage.

Clearly this design concept presents the structural engineer with a mostchallenging problem: to provide an economical design which is susceptible toearthquake damage, but which is essentially proof against collapse in the greatestpossible earthquake.


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AAnother unique feature of the earthquake excitation provides the key to thesolution of this design problem. In contrast to the other loads considered in

structural design gravity, wind, hydrodynamic, etc. the intensity of theearthquake loading depends on the properties of the structure. Thus adequate

earthquake resistance may be provided either by traditional approach ofincreasing strength, or by the unique seismic design of reducing stiffness andthereby reducing the forces to be resisted. This additional approach to earthquakedesign imposes a greater need for understanding of structural behavior inearthquake engineering than in any other field of civil engineering design.

Seemingly minor changes in the framing system or in design details may have anoverwhelming influence on the seismic performance; and merely adding morematerials though it will directly increase costs will not guarantee satisfactoryperformance.


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Consequences of Earthquake Damage.Consequences of Earthquake Damage.

Two basic results of earthquakesTwo basic results of earthquakes:

Loss and impairment of human life; and

Destruction and damage of the constructed and natural environment.

BBoth financially and technically it is only possible to reduce these hazards for largeearthquakes. The basic design aims are therefore confined to the reduction of lossof life in any earthquake, either through structural collapse or through secondarydamage such as falling debris or fire, and to the reduction of damage and loss ofuse of the constructed environment.


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Consequences of Earthquake Damage.Consequences of Earthquake Damage.

Cost of Earthquake ResistanceCost of Earthquake Resistance

DDuring the briefing and budgeting stages of the design, the cost of providingearthquake resistance will have to be considered, at least implicitly, andsometimes explicitly. The cost will depend on such things as the type of the project,

site conditions, the form of the structure, the seismic activity of the region, andstatutory design requirements.

AA broader economic study of the cost involved in prevention and cure ofearthquake damage may be fruitful. In purely economic terms the cost of an

earthquake may be examined under three categories:

cost of life;

cost of damage; and

losses through a facility being out of service.


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Seismic Activity of a RegionSeismic Activity of a RegionSeismic RiskSeismic Risk

Definition of terms used in seismologyDefinition of terms used in seismology.

The literature relating to earthquakes is afflicted by the lack of precise definitions offundamental seismological terms as follows:

IntensityIntensity is a subjective measure of the effects of an earthquake. It refers to thedegree of shaking at a specified place. Over the years, various scales have beendevised by different people, notably by Mercalli and also by Rossi and Forel. Themost widely adopted is the Modified Mercalli scale (commonly denoted M.M)

which has 12 grades denoted by Roman numerals I-XII.

MagnitudeMagnitude is a quantitative measure of the size of an earthquake, which isdependent of the place of observation. It is calculated from amplitudemeasurements on seismograms, and is on a logarithmic scale expressed inordinary numbers and decimals. The most commonly used magnitude scale is

named after Richter and is denoted by M. It is defined as

(1.1)

where AA is the maximum recorded traced magnitude for a given earthquake at agiven distance as written by a standard instrument, and A0A0 is that for a particularearthquake selected as standard. The greatest magnitude yet recorded is

MM 8.9.

0loglog AAM


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Seismic MeasurementsSeismic Measurements.

Hanging objects swing; vibration similar to passing of

heavy trucks, or sensation of a jolt similar to a heavy ballstriking the walls; standing motor cars rock; windows,dishes, and doors rattle; glasses clink and crockerylashes; in the upper range of IV wooden walls and frames

creak.

4IV

Felt by persons at rest, on upper floors, or favorablyplaced.

2 to 3II

Felt indoors; hanging objects swing, vibration similar topassing of light trucks; duration may be estimated; maynot be recognized as an earthquake.

3 to 4III

Not felt except by a very few under exceptionallyfavorable circumstances.

1 to 2I

Witness ObservationsMagnitudeMercalliIntensity


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Felt by all; many frightened and run outdoors; walkingunsteady; windows, dishes and glassware broken; knick-knacks, books, etc. fall from shelves and pictures fromwalls; furniture moved or overturned; weak plaster and

masonry D cracked; small bells ring (church or school);tress and bushes shaken (visibly, or heard to rustle).

5 to 6VI

Felt outdoors; direction may be estimated; sleeperswakened; liquids disturbed, some spilled; small unstableobjects displaced or upset; doors swing, close or open;shutters and pictures move; pendulum clocks stop, start,

or change rate.

4 to 5V



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General panic; masonry D destroyed; masonry C heavily

damaged, sometimes with complete collapse; masonry Bseriously damaged; general damage to foundations;frame structures if not bolted shifted off foundations;

frames racked; serious damage to reservoirs;underground pipes broken; conspicuous cracks in

grounds; in alluviated areas sand and mud ejected;earthquake fountains and sand craters appear.

7IX

Difficult to stand; noticed by drivers of motor cars;hanging objects quiver; furniture broken; damage tomasonry D, including cracks; weak chimneys broken atroof line; fall of plaster, loose bricks, stones, tiles, cornices(also unbraced parapets and architectural ornaments);

some cracks in masonry C; waves on ponds; water turbidwith mud; small slides and caving in along sand or gravelbanks; large bells ring; concrete irrigation ditchesdamaged.

6VII



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Damage nearly total. Few, if any, structures standing.Bridges destroyed. Wide cracks in ground. Waves seenon ground. Rails bent greatly, underground pipelines

completely out of service.

8XI

Total damage. Waves seen on ground. Large rockmasses displaced; lines of sight and level distorted;objects thrown into the air.

greaterthan 8

XII

Most masonry and frame structures destroyed with theirfoundations; some well-built wooden structures andbridges destroyed; serious damage to dams, dikes andembankments; large landslides; water thrown on banks

and canals, rivers, lakes, etc.; sand and mud shiftedhorizontally on beaches and flat land; rails bend slightly.

7 to 8X



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Description of Classification of Masonry StructuresDescription of Classification of Masonry Structures.

SeismologySeismology may be defined as the science and study of earthquakes, and theircauses, effects and related phenomena.

Masonry of good workmanship and mortar, reinforcedespecially laterally, and bound together by using steel,concrete, designed to resist lateral forces.

A

Masonry of good workmanship and mortar, reinforcedbut not designed in detail to resist lateral forces.

B

Masonry of ordinary workmanship and mortar, noextreme weaknesses like failing to tie in at corners, butneither reinforced nor designed against horizontal forces.

C

Masonry of weak materials such as adobe, poor mortar,

low standard of workmanship, weak horizontally.

D

DescriptionType


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Regional GeologyRegional Geology.

Geological evidence of the seismic activity of a region is a valuable tool in theevaluation of seismic risk. It is helpful in estimating the likely magnitude, locationsand frequency of seismic events. Also, by indicating the type of fault movement

prevalent on a given fault, some of the characteristics of the ground motions in thefault vicinity may be anticipated.

RRegional earthquake geology involves a study of tectonic deformations. The term

tectonictectonic refers to rock structure resulting from deep-seated crustal and sub-crustal

forces in the earth. The object of the study of the tectonic deformations will be todetermine their nature, position, age and movement history.

The main geological features to be studied are

Warping

Tilting

Faulting

Tectonic structure


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TiltingTilting is helpful in determining the amount and recency of crustal movement in aregion, and is measured by the slope of beds which are known to have beenoriginally deposited almost horizontally.

FaultingFaulting. The three main features of faulting relevant to earthquake engineering are

location, activity and type.

Fault LocationFault Location. In most seismically active areas faulting is the main source ofinformation regarding seismic risk. This is partly because faults are relativelyeasy to describe and sensitive to the measurement of movement, and partly

because they provide the focus of energy release in most earthquakes. Faulting ActivityFaulting Activity. Uppermost in the engineers mind is the question Will this

fault move during the lifetime of my project? In some faults there is evidencethat continuous creep movement is taking place, and although this may

mean that no large earthquake will occur on that particular fault while strain-energy is being greatly released, it is clear that few structures should be builtacross the fault. In most cases the best answer the geologist may be able togive is to estimate when the most recent significant movements occurred.For faults which have not been known to move in historical times, this is doneby dating the youngest soil deposit displaying a fault displacement byexamining a section through the fault zone.


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Faults are sometimes classified as active or inactive for engineeringpurposes. Some faults may be unarguably called active, where severalmovements have been recorded in recent times such as on the Valley FaultSystem traversing from Quezon City (northern side) to Laguna.

Types of FaultTypes of Fault. It appears that the characteristics of strong ground motion in thegeneral vicinity of the causative fault can be strongly influenced by the type offaulting. The following four types are considered in the study of destructiveearthquakes:

low-angle, compressive, underthrustunderthrust faultsfaults. These result from tectonic sea-bed plates spreading apart and thrusting under the adjacent continentalplates, a phenomenon common to much of the circum-Pacific earthquakebelt;

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compressive, overthrustoverthrust faultsfaults. Compressive forces cause shearing failureforcing the upper portion upwards;

extensional faultsextensional faults. This is the inverse of the previous type, extensional strainspulling the upper block down the sloping fault plane; and

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strikestrike--slip faultsslip faults. Relative horizontal displacement of the two sides of the faulttakes place along an essential vertically fault plane.

Tectonic StructuresTectonic Structures. Further general information about seismicity may be derivedfrom the relationship of the site to tectonic structure. It has been pointed out thatthe majority of large shallow earthquakes occur in ocean-facing slopes of deep-sea trenches, or in local depressions or troughs or ends of depressions. The

magnitude and frequency of earthquakes in a given area may be derived inbroad terms from the size and strength of the fault blocks. The larger and strongerthe block, the larger is the maximum size of earthquake which can be generatedalong the boundaries of that block. Also the greater the rate of tectonic movement

and the less the competency of the tectonic structures, the more rapid is the build

up of the stress needed for a fault movement, and the more frequent will be theoccurrence of the maximum magnitude of earthquake for that structure.

Adam C Abinales f asep pice


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AppendicesAppendices

Seismic Zone Map of the Philippines (Section 208, NSCP 5th Edition)

Distribution of Active Faults in the Philippines (Section 208, NSCP 5th Edition)

Active Faults in Northern Philippines (Section 208, NSCP 5th Edition)

Active Faults in East Central Philippines (Section 208, NSCP 5th Edition)

Active Faults in West Central Philippines (Section 208, NSCP 5th Edition)Active Faults in Southern Philippines (Section 208, NSCP 5th Edition)

313 01-earthquake phenomena

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