The earth is covered with different variety of soils. It is desirable to systemize or classify the soils into broad groups of similar behavior. it is more convenient to study the behavior of groups than that of individual one. Soil may be classified on the basis of following parameters:
1) Classification based on mode of origin.
2) Classification based on composition.
3) Classification based on particle size.
4) Classification based on particle size and plasticity characteristics of soils.
5) Classification based on shear strength.
Classification based on composition
This classification is mainly based on chemical constituents present in the soil. It includes the following types:
a) Black soil
b) Lateritic soil
c) Pedzol
d) Peat soil
a) Black soil
The clayey soil having colored and formed from the basic igneous rock like. Basalts is a example of black soil. It mainly consists of iron, aluminum , magnesium, calcium oxides and also some organic matter. Its capacity to retain water is high. Also it swells considerably coming in contact with water. It is highly fertile and well known for growing cotton.
b) Lateritic Soil
It is brown colored soil derived from laterite and mainly consists of iron and aluminum hydroxide having a small percentage of manganese hydroxide, titanium oxide, silica, calcium, magnesium etc. it is not much suitable for agricultural works.
c) pedzol
it is a sandy soil of light grey color, with very small percentage of iron and organic content. Being deficient in nutritions components, this soil is not good for agricultural works.
d) Peat soil
This is a dark brownish colured soil formed by the decomposed vegetation matter. Thus, it is rich in organic content with very little sand and clay etc. it is highly porous with good water retaining capacity. For agricultural works it is not very fertile. Being soft and weak it readily subsides under loads.
Classification based on shear strength
On the basis of shear strength, soil can be classified as:
1) Cohesionless soils
2)Purely cohesive soils
3)Cohesive-frictional soils
1)Cohessionless soil
These soils derive the shear strength from the intergranular friction. It has no cohesion i.e.c′= 0. These soil are also called frictional soil. Example: sand and gravels.
2) Purely cohesive soils
These are the soils which exhibit cohesion but the angle of shearing resistance φ =0. These soils are also called φu =0 soil.
Example: saturated clays and silts under undrained conditions.
3)Cohesive-frictional soils
These are composite soils having bothc′andφ′. These are also calledC–φ soil.
Example: clayey sand, silty sand, sandy clay etc.
But, sometimes cohesive-frictional soils are also called cohesive soils. Thus any soils having a value of c′ are called a cohesive soil.
Cracking of concrete is a random process, highly variable and influenced by many factors. Any failure of concrete is the consequence of cracking. It impair the durability of concrete by allowing ingress of aggressive agents. With respect to appearance, cracks are also unacceptable. In addition, cracking may adversely effect the water tightness or sound transmission of structures. Cracking may occur fresh concrete due to plastic shrinkage and plastic settlement. In case of reinforced concrete members, crack generally occur at loads well below service level, and possibly even prior to loading due to restrained shrinkage. Flexural cracking due to loads is not only inevitable, but is actually necessary for the reinforcement to be used effectively.
Influence on Stress-Strain Relationship
It is interesting to note that the two component of concrete, that is, hydrated cement paste and aggregate, when individually subjected to load, exhibit sensibly linear stress-strain relation, although some suggestion about the non-linearity of the stress- strain relation of the hydrated cement paste have been made. The reason for the curved relation in the composite material-concrete- lies in the presence of interfaces between the cement paste and the aggregate and in development of bond microcracks at those interfaces. The progressive development of microcracking was confirmed by neutron radiography.
The development of mocrocracking means that the stored strain energy is transformed into the surface energy of the new crack faces. Because the cracks, develop progressively at interfaces making varying angles with the applied load, and respond the local stress, there is progressive increase in local stress intensity and in the magnitude of the strain. In other words, a consequence of the development of the cracks is a reduction in the effective area resisting the applied load, so that the local stress is larger than the nominal stress based on the total cross-section of the specimen. These changes mean that the strain increases at a faster rate than the nominal applied stress, and so the stress-strain curve continues to bend over, with an apparent pseudo-plastic behavior.
When the applied stress increases beyond approximately 70% of the ultimate strength, mortar cracking (connecting the bond cracks) develops and the stress-strain curve bends over at an increasing rate. The development of a continuous crack system reduces the number of load-carrying paths and, eventually, the ultimate strength of the specimen is reached. This the peak of the stress-strain curve.
In structural design of reinforced concrete, the entire stress-strain curve, often in idealized form, must be considered. For this reason, the behavior of concrete which has a very high strength is of especial interest. Such concrete develops a smaller amount of cracking than normal-strength concrete during all stage of loading; in consequence, the ascending part of the stress-strain curve is steeper and linear up to a very high proportion of the ultimate strength. The descending part of the curve is also very steep so that high strength concrete is more brittle than ordinary concrete, and indeed explosive failure of a local part of specimens of high strength concrete tested in compression has often been encountered. However, the apparent brittleness of high strength concrete is not necessarily reflected in the behavior of reinforced concrete members made with such concrete.
Types of cracking
Non-structural cracks
a) Plastic settlement cracks
Cracking can develop also over obstructions to uniform settlement, e.g., reinforcement or large aggregate particles. This is called plastic settlement.
Details of plastic settlement
Location of appearance
1) In case of over reinforcement member it is generally located in deep section.
2) Where changing in section is inevitable they are seen frequently like waffle slabs.
3) In case of arching they appear at top of column.
Causes
Excess bleeding generally results plastic settlement cracking. Rapid early drying conditions also helps such cracks to be pronounced. Also as stated above over reinforcements and large size aggregate produce differential settlement and finally plastic settlement cracking. Plastic settlement cracking can occur also at normal temperatures but, in hot weather, plastic shrinkage cracking and plastic settlement cracking are sometimes confused with one another.
Arrival time
They are normally appeared within 10 min. to 3 hr of concrete placement.
Remedy
The plastic settlement cracking can be avoided by the use of dry mix, good compaction, and by not allowing too fast build-up of concrete. Reduction of bleeding or revibrating arrest such types of cracking.
b) Plastic shrinkage cracking
When rate of evaporation exceeds the rate at which the bleeding water rise to the surface, plastic shrinkage cracking is likely to occur. Sometimes cracks also form under a layer of water and merely become apparent on drying. The critical evaporation rate is > 1.0 Kg/m2 per hour.
Details of plastic shrinkage cracking
Size of cracks
Plastic shrinkage cracking can be very deep, varying in width between 0.1 and 3 mm, and can be quite short or as long as 1 m.
Encouraging environment
A drop in ambient relative humidity encourage this type of cracking, so that, in fact, the cause of it appear to be rather complex. According to ACI R 305 R-91 the risk of plastic shrinkage cracking is the same at the following combinations of temperature and relative humidity.
410 c (1050 F) and 90 percent.
350 c (950 F) and 70 percent.
240 c (750 F) and 30 percent.
Wind velocity in excess of 4.5 m/s ( 10mph) aggravates the situation.
Location of appearance
The loss of water from by suction by the underlying dry concrete or soil may result cracking. Diagonal cracking is observed in pavements and slabs. Over reinforced section especially steel very near to surface of slab are frequently subjected to such cracking. Random cracking may occur normal reinforced concrete slabs.
Causes
Rapid early drying generally produces this type of cracking. Low rate of bleeding as well can result cracking. As discussed previous, temperature, ambient relative humidity and wind velocity exceeding specified range results crack friendly environment. It should be remembered that evaporation is increased when the temperature of the concrete is much higher than the ambient temperature, under such circumstances, plastic shrinkage can occur even if the relative humidity of the air is high.
Remedy
Complete prevention of evaporation immediately after casting eliminates cracking. The best practice is to protect the concrete from sun and wind, to place and finish fast, and to start curing very soon thereafter. Placing concrete on a dry subgrade should be avoided.
Arrival time
They are generally appeared within 30 min to 6 hr of concrete placement.
c) Corrosion of reinforcement
The corrosion of steel reinforced concrete member by the formation of electro-chemical cell results in cracking (characteristically parallel to the reinforcement), spalling or in delamination of concrete.
This corrosion may occur due to chloride attack and carbonation.
Mechanism of cracking
The corrosion of steel results cracking and further deeper propagation of cracking in two successive steps.
Firstly
The production of corrosion occupies a volume several times larger than the original steel so that their formation results in cracking. This makes it easier for aggressive agents to ingress towards the steel, with a consequent increase in the rate of corrosion.
Secondly
The progress of corrosion at the anode reduce the cross-sectional area of steel, thus reducing its load carrying capacity resulting increase in deflection encouraging cracks to be pronounced.
Location of appearance
These are normally seen in columns and beams where environment is in favor of corrosion.
Cause of cracking
Normally poor quality concrete is subjected to such types of cracking. Inadequate clear cover also makes easy intrusion of aggressive materials like chloride or results carbonation.
Remedy
Good quality concrete adding suitable admixture depending on the environment surroundings of desired concrete member. Providing adequate clear cover also discourage cracking of this type.
Arrival time
These are normally appeared after two years.
d) Cracking due to Alkali-Aggregate reaction
The most common reaction in concrete having aggregate with deleterious chemical is reaction between siliceous minerals in the aggregate and alkaline hydroxides in pore water derived from the alkalis (Na2o and k2o) in cement. This reaction can be disruptive and manifest itself as cracking.
Size of cracks
The crack width can range from 0.1 mm to As much as 10 mm in extreme cases. The cracks are rarely more than 25 mm, or at most 50 mm, deep.
Pattern of surface cracking
The pattern of surface cracking induced by the alkali-silica reaction is irregular. Somewhat reminiscent of a huge spider’s web. However, the pattern is not necessarily distinguishable from that caused by sulfate attack or by freezing and thawing, or even by severe plastic shrinkage. Within the concrete, many of the cracks caused by the reaction can be seen to pass through individual aggregate particles but also through the surrounding hydrated cement paste.
Mechanism of cracking
Consequent of the reaction stated above is formation of alkali-silica gel. These gel takes its position in the planes of weakness or pores in the aggregate(where reactive silica is present) or on the surface of the aggregate particles.In the later case, a characteristic altered surface zone is formed. This may destroy the bond between the aggregate and the surrounding hydrated cement paste.
Causes
Reactive aggregate with high alkali cement produce alkali-silica reaction and consequent cracking.
Source of Alkalis
The most common source of Alkalis in concrete is cement. Alkalis become concentrated in some locations, at the expense of others. Such concentration may be caused by moisture gradients or by alternating wetting and drying. The alkalis may also become concentrated by an electric current passed through the concrete which may occur when cathodic protection is used to prevent corrosion of embedded steel.
The additional sources of Alkalis in concrete include sodium chloride present in unwashed sand dredged from the sea or obtained from the desert. Other internal sources of alkalis are some admixtures, especially superplasticizers, or even the mix water. The alkalis from these sources, and also from fly ash and ground granulated blast furnace slag, should be included in the calculation of the amount of Alkalis present.
Location of Appearance
It is normally appeared in damp location.
Remedy
It has been found that expansion due to the Alkali-silica reaction can be reduced and eliminated by the addition to the mix of reactive silica in a finely powdered form. This finely divided siliceous material added to the coarse reactive particles already present would reduce expansion, although the reaction with the alkalis still takes place. These pozzolanic additions, such as crushed pyrex glass or fly ash, have indeed been found effective in reducing the penetration of the coarse aggregate particles. The fly ash should contain no more than 2 to 3 percent by mass of alkalis.
Pozzolanas in the mix are beneficial also because they reduce the permeability of concrete and therefore reduce the mobility of aggressive agents, both those present with in the concrete and those which may ingress. Furthermore, C-S-H formed by pozzolanic activity incorporates a certain amount of alkalis and thus lowers the value of PH. With the decrease in PH alkali-silica reaction rate also decreased.
Silica fume is particularly effective because the silica reacts preferentially with the alkali. Although the product of reaction is the same as that between the alkalis and the reactive silica in the aggregate, the reaction takes place at the very large surface of the fine particles of silica fume. In consequence, the reaction does not result in expansion.
Ground granulated blastfurnace slag is also effective in mitigating or preventing the deleterious effects of the alkali-silica reaction. It should be noted that the presence of ground granulated blastfurnace slag results in a reduced permeability of concrete.
Lithium salts may inhibit also expansion reactions.
Time of Arrival
These cracks normally appeared after 5 years of construction.
e) Crazing
It is an another type of early cracking like plastic shrinkage cracking and plastic settlement cracking which can occur when the surface zone the concrete has a higher water content than deeper in the interior.
Size and pattern of cracking
The pattern of crazing looks like an irregular network with a spacing of up to about 100 mm (4 in). The cracks are very shallow and may not be noticed until etched by dirt; apart from appearance, they are of little importance.
Location of appearance
Walls and slabs normally subjected to crazing cracks.
Causes
Over trowelling and impermeable formwork are primarily results these cracking. Rich mix associated with poor curing also results these cracking.
Remedy
Properly designed concrete with adequate curing reduces such cracking. Poor finishing avoiding over trowelling reduces crazing to great extent.
Time of arrival
They are normally appeared within 1 to 7 days, sometimes crazing may takes much time.
Most reinforced concrete members are statically indeterminate because they are part of monolithic structure, i.e., in this form of building construction, slabs are cast monolithically with a beam-and –girder floor framing that carries the floor load to columns. A beam is a flexure member. It may be of steel, wood or any other structural materials. Reinforced concrete beams are nonhomogeneous in that they are made of two entirely different materials. A flexural member should suffer a axial compression force less than Ag fc´/10. The member must be at least 10 in wide and its clear span should be least 4 times the effective depth and the width-to-depth ratio should be at least 0.3.
Slab
Slabs are used to furnish a flat and useful surface in reinforced concrete construction. It is broad, flat plate, usually horizontal, with top and bottom surfaces parallel or nearly so. It may be supported by reinforced concrete beams, by masonry or reinforced concrete walls, by structural steel members, by directly by columns or continuously by the ground.
Slabs in which the deflected surface is predominantly cylindrical is called one-way slabs spanning in the direction of curvature. This condition arises when slabs are supported on two opposite sides, and those supported on all four sides with the longer span greater than twice the shorter span and also in cantilever slabs. In many cases, rectangular slabs are of such proportions and are supported in such a way that two-way action results. when loaded, such slabs bend into a dished surface i.e., an any point the slab is curved in both principal directions, and since bending moments are proportional to curvature, moments also in both directions.
Concrete slabs may in some cases be carried directly by columns, without the use of beams or girders. Such slabs are called flat plates. In some cases, to reduce the stresses due to shear and negative bending around columns, a thickened slab region in the vicinity of column and flared column tops are incorporated. Such construction is called flat- slab construction.
Occurrence of Opening
Slabs
In almost all constructions, slab system include openings. These may be of substantial size, as required by stair-ways and elevators shafts, or they may be of smaller dimensions, like those needed to accommodate heating, plumbing, and ventilating risers, floor and roof drains, and access hatches.
Beams
These may occur when opening in slabs pass through the effective flange width of T-beams, for installation of building services. Opening are also frequently provided through beam web.
Code’s Recommendation for Placement of Opening
Slabs
As far as possible, opening in slabs should be located in zones where shear stresses are small and bending moment are below maximum. However, small opening for pipe sleeves etc. can be made anywhere in slab. In case of slabs supported directly by columns, from structural view point, they are best located well away from the columns, preferably in the area common to the slab middle strips. Opening of any size may be provided in slab systems if shown by analysis that the design strength is at least equal to the required strength and that specified limits on deflection meets. For strength the strip method is appropriate.
Code Requirements
13.5.1 Opening of any size are permitted in slabs if analysis shows that both strength and deflection are acceptable.
13.5.2 In lieu of special analysis as required by 13.5.1 opening may be provided in slab systems without beams only in accordance with the following.
13.5.2.1 Opening of any size may be located in the area common to intersecting middle strips.
13.5.2.2 In area common to intersecting column strips, not more than 1/8 the width of column strip in either span shall be interrupted by openings.
13.5.2.3 In the area common to one column strip and one middle strip, opening size is limited to 1/4 the width of column strip in either span.
Beams
In regions of small shear, as near the middle of a beam span, a horizontal pipe sleeve is not serious. Elsewhere, shear strength must be closely watched and in many places bending strength as well.
Strip Method to analyze Opening
The strip method was first developed by Hillerborg, and later Wood and Armer analyzed the method critically and performed test on slabs designed by this method. They found that a design made by the strip method and reinforced exactly according to moments found, was an exact solution.
The strip method is the simplest one for slabs on simple supports, but continuity can be handled on a basis similar to limit design. The most difficult slabs for this method are slabs supported on columns. For such a case, Hillerborg developed the advanced strip method, using a rectangular element carrying load in two directions to a support at one corner of the element.
The equilibrium equation for slabs is
All elements of this equation, except w, follows the Timoshenko’s notation.
i.e., Mx and My = bending moment about respective directions. Mxy = twisting moment. and w = load per unit area on slab. Hillerborg designs the slab to make Mxy unnecessary, that is he assumes Mxy = 0 and than apportions the load to ∂²Mx/∂x² and ∂²My/∂y², usually at a particular spot wholly to one or to the other. This particular apportionment is more of a convenience than a necessity, however, loads in a particular area are assign to particular slab strips and continuity of the resulting moments and shear must be carefully maintained. Apparent discontinuity in torque or deflection may be disregarded, but a discontinuity in moment or shear is not permitted. Both elastic and plastic analysis concepts are permissible in evaluating moments on strips.
The suitability of the method for slabs with opening is a strong point in its favor. This method have been using for many years by designers, designing by their ‘feel’ for the way the load has most apt to be transferred to the supports. As slabs are normally considerably under-reinforced, it is possible to use certain strips near the opening as small beam simply by increasing the local reinforcement. If the opening is so large that even extra slab steel is inadequate to care for the moment, a real beam is needed around one or more sides of the opening, quite probably spanning to the edge beams. The assumed “beam strips” are drawn as dotted around the opening and the point of inflection as suggested by Wood and Hillerborg are added on the slab. Then different moment for different strip is calculated. Depth can be fixed from loads (moment and shear). The reinforcement should be arranged in bends corresponding to the strips used, this calculation is simple. Defection at service load must be considered in checking serviceability. In any actual design the service load is available, and it should be on the safe side to use the strip service load moments with EI based on the cracked section. Treatment for Opening
Larger opening should be framed by beams to restore, as nearly as possible, the continuity of the slab. The beams should be designed to carry a portion of the floor load, in addition to loads directly applied by partition walls, elevator support beams, stair slabs etc. But, in case of flat plate construction, opening near column render difficulties. Effects on Strength and Treatment Slabs The effect of openings on the shear strength as well as flexure action (in case of large opening) of slab must be investigate carefully. The effect becomes severe when the openings are within the column strip areas of slabs or within middle strip areas when the opening are closer than 10 times the slab thickness(10h) from a column, measured radial. To account for reduction of shear strength, it is considered that a portion of the critical section b0 is ineffective which is enclosed by straight lines projecting from the column centroid to the edges of the opening. If two much resisting perimeter is lost, the designer must be sure adequate tow-way bending is really present; otherwise the lower shear permitted in beams become the limiting values for the slab. For slabs with shear reinforcement, the ineffective portion of the perimeter b0 is one-half of that without shear reinforcement. One-half factor is interpreted to apply equally to shearhead reinforcement and bar or wire reinforcement.
Code Recommendation 13.5.2.4 Where opening in slabs are located at a distance less than 10 times the slab thickness from a concentrated load or reaction area or when opening in slabs are located within column strip, the critical sections shall be modified as follows: 1) For slabs without shearhead, that part of the perimeter of the critical section that is enclosed by straight lines projecting from the centroid of the column, concentrated load or reaction area and tangent to the boundaries of the opening shall be considered ineffective. 2) for slabs with shearheads, the ineffective portion of the perimeter shall be one-half that defined in (1) above. Beams
Large opening in beams are particularly weakening. They destroy beam action and force the reduced section to act such as a Vierendeel truss ( a truss without diagonals). In such a truss the average bending moment over the length of the opening is resisted by axial compression one chord and tension in the other, with these two forming a couple in the case of pure flexure. Where shear is present the change in the moment over the length of the opening superimpose a reversed bending resistance in each chord, the total of the four end moments on the chords equaling the external shear times the length of the opening. How the shear and these reverse moments are shared by the chords depends on the relative chord stiffness. Thickening of edge of opening In case of larger openings, the edge of the slab around the opening may be thickened to enable them to behave like trimmer beams for taking up additional moments and forces. If opening reduce a critical design section for moment, must be maintained by providing extra depth to offset the reduced width. Function of Steel
The steels used around opening tie the free ends of the bars which are trimmed to form opening. It also serve as reinforcement to make the opening stable against deformation or any other types of failure. This bars take care of the loss of steel in a slab on account of forming opening. Diagonal bars provided in corner control the cracking that will almost inevitably occur there. Steel Requirement Slabs
With regard to flexural requirements, the total amount of steel required by calculation must be provided, regardless of opening. Any steel interrupted by holes should be matched with an equivalent amount of supplementary reinforcement on either sides, proper lapped to transfer stress by bond. If minor cracking at the corners of an opening is objectionable, it is always advisable to add one or two diagonal bars at each corner, especially at large openings. Bars are always desirable around Window and door opening in concrete wall slabs, because such reinforcement helps to take care of shrinkage stresses. Code Requirement
Slab
13.5.2.1 In case of opening in intersecting middle strips, total amount of reinforcement required for the panel without opening is maintained.
13.5.2.2 In case of opening in intersecting column strips, an amount of reinforcement equivalent to that interrupted by an opening shall be added on the sides of the opening. 13.5.2.3 In case of opening common to one column strip and one middle strip not more than ¼ the reinforcement in either strip shall be interrupted by openings. An amount of reinforcement equivalent to that interrupted by an opening shall be added on the sides of the opening. Beam 11.3.1.3 For members with significant axial tension, requires all shear to be resisted by stirrups (none assign to the concrete).
Steel Arrangement
Reduction in critical design section for moment due to opening, can be overcome providing more closely on each side of the opening to maintain the necessary As. This is possible only when opening is possible to locate where moment is well below the compression capacity of the slab, thereby leaving the arrangement of reinforcement as the only problem. Of course, shear strength must be maintained, but this is rarely a problem except near the columns in flat slab type, as noted previously. The arrangement of bars around any but minor opening can constitute a real problem. Bars running perpendicular to the face of an opening are not fully effective when simply cut0off at the opening. This is acceptable if there is a beam at the opening to act as reaction for the slab. If there is no beam it is better to fan the bars out or splay them to go around the opening. If fanning or splaying leaves too wide an area without steel, extra bars can be placed parallel to the side of the opening, as indicated.
The earth suffers a highly variable intensity of earthquake from time to time. Some of them are catastrophic resulting complete destruction of building and ground, while some can’t be felt, only can sensitive instrument can predict. So, once an earthquake struck, it is important to know about severity of it and impact of it on the community to get prepare for aftershocks and future trembling. In order to have a comparative study of earthquakes and to define areas of known intensities, it is essential to establish a measure or a scale of reference.
Several classification of earthquake intensity have been used. To the
engineer, a classification based on the maximum acceleration of the
ground is of most interest. The acceleration due to gravity is 9.8
m/sec. It should keep in mind that the earthquakes may have different
degree of accelerations as well as that different geological conditions
produce different effects.
Measure of Intensity
To develop a effective scale to measure intensity, the data required are a) the acceleration produced b) the extent of damage caused to buildings and ground surface. Historical Development of Scale
Initially a scale of earthquake intensity having ten divisions was given by Rossi and Forel, which was based entirely on the sensation of people and damage caused. However it has been modified by mercalli and further by Wood and Neumann in 1931, which will be discuss in this article. Modified Mercalli Intensity Scale
Collection of Data It measures the impact of an earthquake by sending out trained observers to look at the damage done to the built environment and the earth (landslides etc.) and at the reaction of people to the event.
Application of Collected Data
Instrumental records of earthquake vibrations are supplemented by information gathered from individuals through such types of questionnaires. Many data that are not brought out by instrumental methods are thus assembled; and information is gathered also from areas where no instruments have been established.
Details of Modified Mercalli Intensity Scale
This scale defines the effects of an earthquake over a limited geographical area. Intensity scales assign whole numbers usually from 1 to 12, expressed in Roman numerals. An intensity I means the earthquake was not felt, while XII means absolute and total destruction. The descriptions used to assign earthquake intensity values on a M.M.I scales are shown in following table : Advantage
The advantage of using the M.M.I scale is that it relies on the observations of people experiencing an earthquake instead of scientific instruments. This allows seismologists to assign earthquake intensities to seismic records, an activity that helps them to estimate seismic risk for earthquake sites today. Disadvantage
The fact that the Mercalli scale relies on peoples observations is a disadvantage because this makes a evaluation subjective and dependent upon the social infrastructural conditions of a country. This scale also not very helpful in an area with little human habitation, since no one would be around to experience the earthquake. Example
In case of intensity value VII on the M.M.I scale not effective or should be rewritten for a country without chimneys and automobiles.
Latest scale of intensity Now a scale , called European Macro seismic Scale (1992) is widely used which has been developed and tested over period of years by a working group of the European seismological commission. The EMS makes the imprecise and subjective nature of assigning intensities more robust and straight forward with regard to earthquake effects on human, objects and buildings. Isoseismal lines
The intensity of earthquake decreases with increase in distance from the centre of disturbance. This decrease is inversely proportional to the square of this distance. In an area, subjected to earthquake, the places suffering same intensity can be determined. A line joining points of same intensity is called an ‘Isoseismal Line’. If the focus of earthquake is a point, the area enclosed by an isoseismal is circular whereas an elongated zone or line results a elliptical area.
An earthquake is the result of a sudden release of energy in the earth’s crust that creates vibration radiating all directions with a decreasing rate from the centre of disturbance. When an earthquake occurs, it is important to know where the seismic event took place, how intense it was, and what its impact was on the built environment. The more we know about earthquakes and how and when they occur, the more we can do to lessen their effects on our communities.
Measure of Energy
Whenever there is an earthquake, it indicates the some of the strain energy held within the body of the earth has been released in the form of rupture. The released energy travels around in the form of earthquake waves, causing displacement of the ground surface. In a low-intensity earthquake this displacement is small, while in a high intensity earthquake it is large. Thus, the displacement is directly a function of the amount of energy released during an earthquake.
If ‘ a’ is the maximum displacement recorded, then magnitude ‘M’ is given by the relationship
M = log10 a +c
According to Richter the relation of magnitude with the energy released ( E in ergs) is given by the equation
log10 (E) =5.8+2.4M.
Details of Scale
In this scale, one can measure size of earthquakes based on recording of ground motion by instruments. The Richter scale value is calculated by measuring the maximum recorded amplitude of a wave. This measurement quantities i.e., the ground motion and the energy released at the source of an earthquake, which is referred to as its magnitude. The Richter scale is open ended and logarithmic (base 10) and accommodates the wide range of ground motions, appeared during arrival of a p-wave or surface waves and measured applying a standard correction for distance to the epicenter, that earthquake can caused. This means that there are no upper and lower limits to the scale and that every time the magnitude goes up by one unit, the amount of energy this represents increase thirty times. Each unit of the Richter Scale represents a 10 times increases in wave amplitude. Richter magnitude is expressed as an Arabic number, which helps to distinguish it from the Marcalli scale.
It is observed that a magnitude less than 5.0 is of no engineering interest as it does not have any adverse effect at all.
Energy in Relation to Magnitude
An earthquake of magnitude 4.0 on Richter scale would release energy equivalent to 6 tons of TNT, or about as much energy as a small atom bomb. A magnitude of 3.0 is equivalent to only 397 pounds of TNT. It should be noticed that 397 pounds is 30 times smaller than 6 tons. In the following table the different magnitude equivalent to TNT energy are stated clearly:
Historical Development of Scale
The first magnitude scale was devised by Charles Richter in 1935. Scales today are based on various aspects of the seismograms as told by the following names:
a) Body-waves magnitude
b) Duration magnitude
c) Moment magnitude
Jamaica uses the duration magnitude and moment magnitude. Moment magnitude is the most true indication of the size of an earthquake because it is based on the amount of movement of fault.
Moment Magnitude
Moment magnitude is the measure of total energy released by an earthquake. It is based on the area of the fault that ruptured in the quake. It is calculated in part by multiplying the area of the fault’s rupture surface by the distance the earth moves along the fault.
Richter Scale in Relation to Mercalli Intensity Scale
The Mecalli scale describe the effect of the energy, while the Richter scale describes the amount of energy associated with an earthquake. So both the scales are used to measure earthquake.
Example:
In 1989 an earthquake struck Loma Prieta which had a magnitude of 7.1 on Richter Scale and VII on the Mercalli Scale. This quake produce moderate loss of life (67 people died) and the built environment suffered relatively little damage. Most of the building affected in the Loma Prieta earthquake had been built with seismic provisions to reduce damage.
In contrast, the 1988 Armenian earthquake measured almost the same magnitude (6.9) on the Richter, but its Mercalli measured intensity was XI. More than 50,000 people lost their lives, and the built environment was almost totally destroyed.
The damage done by the Armenian earthquake was greater than that done by the 1989 Loma Prieta earthquake, although the magnitude was almost the same.
As the quality of the built environment is a big factor in the number of lives lost and the amount of damage done in an earthquake and intensity is a function of loss and destruction. So the magnitude of an earthquake does not always correlated with the intensity or impact. The local codes also keeps focus on the intensity rather than magnitude.
