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Friday, 2 February 2018


An earthquake is a sudden, rapid shaking of the earth surface caused by the breaking and shifting of rocks beneath. During earthquake, ground motion occurs in a random fashion in all directions radiating from a point within earth crust, called epicentre. It causes vibrations of structures and induce inertia forces on them. As a result structure may collapse resulting into loss of property and lives. Earthquakes do not kill people, vulnerable buildings do so. Hence there is need of designing earthquake resistant buildings, especially in the earthquake prone areas.
Depending upon the possible causes, the earthquakes may be classified as:
1. Natural earthquake
2. Earthquakes due to induced activities.

Natural Earthquakes
Natural earthquakes may be due to
(i) active faults (ii) movement of tectonic plates or
(iii) due to volcanic eruptions.
In earth’s crust there are some faults which are not yet settled. The displacement of rocks along
faults cause earthquake.
Tectonic means large scale process affecting the structure of the earthcrust. This process causes
gradual movement of material within the crust of earth. Sometimes it shakes the earth crust.
Volcano is a mountain or hill having a crater through which lava, rock fragments, hot vapour and
gas are or have been erupted from the earths crust. Occasionally the volcanoes become active and create earthquake near the mountain crater.

Earthquakes due to Induced Activities
These are caused by vibrations induced by atomic explosions and collapse of ground due to
faulty mining.

1. Focus: The point on the fault where slip starts is the focus. It is also known as hypocentre
[Ref. Fig. 20.1].
2. Epicenter: The point vertically above the focus on the surface of the earth is the epicentre.
3. Focal Depth: The depth of focus from the epicentre is called the focal depth.
4. Epicentral Distance: Distance from epicentre to any point of interest on the surface of earth is
called epicentral distance.
Magnitude is a quantitative measure of the actual size of the earthquake. Professor Charles Richter
proposed the scale of magnitude that goes from 0 to 9. It is a geometric scale. Now this scale is known as Richter scale. It is obtained from the seismograph. It depends on waveform amplitude on epicentral distance. It is denoted by letter M followed by the number. An increase in magnitude by 1 implies 10 times higher waveform amplitude and about 31 times higher energy released. Thus energy released in M6 and M5 earthquake have the ratio 31, and M8 to M5 have the ratio 31 × 31 × 31. There are other magnitude scales, like the Body Wave Magnitude, Surface Wave Magnitude and Wave Energy Magnitude. Intensity is a qualitative measure of the actual shaking at a location during an earthquake. Hence for the same earthquake, it has different values at different places, highest value being at epicentre. This is a linear scale. It is assigned as Roman Capital Numbers from I to XII. Intensity depends upon
1. Amount of source energy released
2. Distance between the source and the place of interest
3. Geographical features of the media of travel and importantly on the type of structure.
Modified Mercalli Intensity (MMI) scale is commonly used to express the intensity. MMI scale
is as given below:
I. Very slight, felt only by instruments
II. Felt by people resting
III. Felt by passing traffic
IV. Furnitures and windows rattle
V. Can be felt outdoors, clocks stop, doors swing
VI. Furnitures move about, cracks appear in walls
VII. People knocked over, masonry cracks and falls
VIII. Chimneys and monuments fall, buildings move on foundations
IX. Heavy damage to buildings, large cracks open on ground
X. Most buildings destroyed, landslides occur, water thrown out of lakes
XI. Catastrophic, railway lines badly bent
XII. Utter catastrophic, no building is left standing.
Koyna, Great Assam, Bihar and Shillong, Kashmir earthquakes had magnitude 8. Uttarakhand,
Chamoli, Jabalpur, Latur, Gujarat approached levels 5.5 to 6.5.

After observing Indian earthquakes for several years Bureau of Indian Standard has divided the country into five zones depending upon the severity of earthquake. IS 1893-1984 shows the various zones. The following IS codes will be of great importance for the design engineers:
IS 1893–2002: Criteria for Earthquake Resistant Design of Structures (5th revision).
IS 4928–1993: Code of practice for Earthquake Resistant Design and Construction of Buildings.
(2nd revision).
IS 13827–1992: Guidelines for Improving Earthquake Resistance of Low Strength Masonary
IS: 13920–1997: Code of practice for Ductile Detailing of Reinforced Concrete Structures
Subjected to Seismic Forces.
IS: 13935–1993: Guidelines for Repair and Seismic Strengthening of Buildings.

The earthquake resistance of small buildings may be increased by taking some precautions and measures in site selections, building planning and constructions as explained below:
1. Site Selection: The building constructions should be avoided on
(a) Near unstable embankments
(b) On sloping ground with columns of different heights
(c) Flood affected areas
(d) On subsoil with marked discontinuity like rock in some portion and soil in some portion.
2. Building Planning: Symmetric plans are safer compared to unsymmetric. Hence go for
square or rectangular plans rather than L, E, H, T shaped. Rectangular plans should not have
length more than twice the width.
3. Foundations: Width of foundation should not be less than 750 mm for single storey building
and not less than 900 mm for storeyed buildings. Depth of foundation should not be less than
1.0 m for soft soil and 0.45 m for rocky ground. Before foundation is laid remove all loose
materials including water from the trench and compact the bottom. After foundation is laid
back-fill the foundation properly and compact.
4. Masonry: In case of stone masonry:
·         Place each stone flat on its broadest face.
·         Place length of stones into the thickness of wall to ensure interlocking inside and outside
·         faces of the wall.
·         Fill the voids using small chips of the stones with minimum possible mortar.
·         Break the stone to make it angular so that it has no rounded face.
·         At every 600 to 750 mm distance use through stones.
v  In case of brick masonry:
·         Use properly burnt bricks only.
·         Place bricks with its groove mark facing up to ensure better bond with next course.
v  In case of concrete blocks:
·         Place rough faces towards top and bottom to get good bond.
·         Blocks should be strong.
·         Brush the top and bottom faces before laying.
      In general walls of more than 450 mm should be avoided. Length of wall should be
restricted to 6 m. Cross walls make the masonry stronger. It is better to build partition
walls along main walls interlinking the two.
5. Doors and Window Openings:
·         Walls with too many doors and windows close to each other collapse early.
·         Windows should be kept at same level.
·         The total width of all openings in wall should not exceed
·         Doors should not be placed at the end of the wall. They should be at least at 500 mm from the cross wall.
·         Clear width between two openings should not be less than 600 mm.
6. Roof:
·         In sloping roofs with span greater than 6 m use trusses instead of rafters.
·         Building with 4 sided sloping roof is stronger than the one with two sided sloping, since
·         gable walls collapse early.
7. Chejjas:
·         Restrict chejja or balcony projections to 0.9 m. For larger projections use beams and
·         columns.
8. Parapet: Masonry parapet wall can collapse easily. It is better to build parapet with bricks
·         up to 300 mm followed by iron railings.
9. Concrete and Mortar: Use river sand for making mortar and concrete. It should be sieved
·         to remove pebbles. Silt should be removed by holding it against wind. Coarse aggregates of
·         size more than 30 mm should not be used. Aggregates should be well graded and angular.
·         Before adding water cement and aggregates should be dry mixed thoroughly.
10. Bands: The following R.C. bands should be provided
(a) Plinth band
(b) Lintel band
(c) Roof band
(d) Gable band.
For making R.C. bands minimum thickness is 75 mm and at least two bars of 8 mm diameters
are required. They should be tied with steel limbs of 6 mm diameter at 150 mm spacing.
If wall size is large, diagonal and vertical bands also may be provided.
11. Retrofitting: Retrofitting means preparing a structure in a scientific manner so that all element  of a building act as an integral unit.
              It is generally the most economical and fastest way to achieve safety of the building. The following
             are some of the methods in retrofitting:
·         Anchor roof truss to walls with brackets.
·         Provide bracings at the level of purlins and bottom chord members of trusses.
·         Strengthen gable wall by inserting sloping belt on gable wall.
·         Strengthen corners with seismic belts.
·         Anchor floor joists to walls with brackets.
·         Improve storey connections by providing vertical reinforcement.
·         Induce tensile strength against vertical bending of walls by providing vertical reinforcement at all inside and outside corners.

·         Encase wall openings with reinforcements.

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