Tackle Earthquake in Engineering Buildings

Earthquake :Definition A tremor on the surface of the Earth,

sometimes severe and devastating, which results from shock waves generated by the movement of rock masses deep within the Earth, particularly near boundaries of tectonic plates.

Earthquake: CausesOuter Layer of Earth: Made up of layers

called Lithospheric Plates or Tectonic Plates

These Plates are moving (few mm every year) due to the movement of the molten Magma inside the Earth.

These movement causes changes on Earth’s Surface.

• Plate boundaries get interlocked when they slide past each other due to friction.

• The rest of the plates keep moving giving rise to tension in the plates.

• When the force moving the plates overcome the friction, Energy is released causing the earth to shake at that Point (Focus).

• The energy gives rise to Seismic Waves which shake the earth as they move outward through it.

• When they reach the earth’s surface, they shake the ground and anything on it.

Effects of Earthquake

1. Shaking and Ground Rupture

• Ground rupture is a visible breaking and displacement of the Earth's surface along the trace of the fault.

• Ground rupture is a major risk for large engineering structures such as dams, bridges and nuclear power stations .

• Careful mapping of existing faults are required to identify the breakage of the ground surface within the life of the structure.

2. LandslidesEarthquakes, along

with severe storms, volcanic activity, coastal wave attack, and wildfires, can produce slope instability leading to landslides, a major geological hazard.

Landslide During the 2001 El Salvador earthquake

3.Soil LiquefactionSoil liquefaction occurs

when, because of the shaking, water-saturated granular mater-ial (such as sand) temporarily loses its strength and transforms from a solid to a liquid.

Soil liquefaction may cause rigid structures, like buildings and bridges, to tilt or sink into the liquefied deposits.

Sinkage of Structures during the 1964 Alaska Earthquake

4. TsunamiTsunamis are long-

wavelength, long-period sea waves produced by the sudden or abrupt movement of large volumes of water

 Large waves produced by an earthquake or a submarine landslide can overrun nearby coastal areas in a matter of minutes.

All the above effects have a Direct Impact on Engineering Structures

Greatest Challenge Faced By a Civil Engineer

Effect of Earthquake on Buildings When the ground moves, The

building is thrown backward and the top of the building (usually roof) experiences a force called inertia force.

More mass mean higher inertia force, therefore lighter buildings sustain earthquake better.

The inertia force is transferred to the ground via the columns.

Columns tend to come back to their original positions, so an internal force is developed.

So as a result floor slabs, walls, columns and foundations are affected due to the above forces.

Earthquake causes shaking in all directions (i. e. along the horizontal as well as vertical direction.)

All structures are designed to carry the gravity loads (i. e its own weight) downwards.

The vertical acceleration during shocks have little effect.

Horizontal Shaking is much of a concern.

Basic Features OF Earthquake resistant StructuresA building should survive a rare, very severe

earthquake by sustaining significant damage but without globally collapsing.

It should remain operational for more frequent, but less severe seismic events.

ConcreteNot very Earthquake ResistantExtremely strong under Compression but

very weak under TensionThus cracks are caused during EarthquakeConcrete structures are reinforced with steel

rods as steel is strong under tension.Pre stressed concrete is also used.

Characteristics of an Earthquake resistant building

FoundationsFoundations for concrete and masonry

structures should be excavated to the same level throughout the building and should be as far as possible.

The superstructure should be thoroughly tied up to the foundation using reinforcements to offer maximum resistant against sliding at that level.

Isolated base Foundations are much in use nowadays.

BodyThe walls should be as light in weight as possible and

made up of light weight concrete.Stronger walls should be designed with reinforced

rather than plain concrete.Continuity of cross walls should be maintained as far as

possible in such a way that different parts of the building behave as integrals of the same structure.

In masonry walls, keys (bricks and stones) should be inserted in a proper style during each course so that the danger of sliding apart the horizontal joints is minimized.

Strongest mortars preferably cement sand mortars should be used for masonry works in seismic regions.

The RoofFlat RCC roofs give better resistance to

earthquake shocks as compared to sloping roofs.

Even in flat roofs when slates and corrugated sheets are used care should be taken that no lateral stresses are developed.

Projections above or beyond the roof surface such as chimneys, should be avoided or kept to a minimum.

Choice of siteMostly concerned with the Stability of the ground.Stability of slopes: Hill slopes are likely to slide during

shock. Stable Slopes should be chosen.Loose sand Or sensitive clays: During shocks they

would lead to large unequal settlement and damage to the building. If the loose cohesionless soils are saturated with water, they loose their shear resistance altogether during shaking and become liquefied.

Soil particles undergo a lot of compaction during seismic shock thereby causing sinkage.

Structures built on loose soil or sediments will have to withstand a greater risk as compared to those on solid bed rocks(already compacted) on the same region.

Sinkage of Structures

General Plan of buildingSymmetry: The whole

building as a whole or its various blocks should be kept symmetrical about both axes. Asymmetry leads to torsion during shocks. (Fig Alongside)Symmetry is also required in placing and sizing of doors and windows.

• Regularity: Simple Rectangular shapes behave better during shocks than shapes with projections. It is desirable to restrict the length of the block to three times its width as torsional effect is more pronounced in narrow rectangular blocks.

• Separation of blocks: Large buildings are separated into several Blocks s o as to obtain symmetry as well as regularity.

• Simplicity: Large cornices, vertical or horizontal cantilever projections etc. are considered dangerous from seismic point of view. When Ornamentation is insisted it should be reinforced with steel, which should be properly embedded to the main structure.

Enclosed area: A small building enclosure with properly interconnected walls are more resistant to earthquakes.

For larger panels or thinner walls, Framing elements should be introduced.

General ConclusionAll parts of the same building – the

foundations, superstructure and the roof should be firmly tied together so that the entire structure acts as a unit during a shock.

As far as possible uniform height should be given to the structure.

Architectural fancies like domes, cantilevers, arches should be avoided as far as possible. When deemed absolutely essential they should be designed with extra care.