Factor Affecting the Durability of the Reinforced Concrete in Seawater

Disintegration of concrete and corrosion of reinforcement is the sign of deterioration of marine concrete structure. Many researchers have identified, after laboratory investigation and extensive studies on existing marine structures, a number of factors affecting the deterioration of reinforced concrete in such an aggressive environment. Some of these important parameters are stated below:

1) Aggregate Type

The properties and proportion of constituent materials of concrete have a large influence on the durability of concrete. As the aggregates occupy up to 80% by volume of concrete, the resistance surface properties of the aggregates are important parameters affecting durability of concrete. To ensure adequate durability of marine structure, the aggregates material should be dense, non shrinking and alkali resistant.

2) Cement Content

The cement content also has a marked influence on the durability of reinforced concrete. Several researchers and authorities have been given recommendation for the minimum cement content of concrete exposed to different zone of marine environment.

3) Water-Cement (W/C) Ratio

The water-cement ratio influences both the strength and durability of concrete. According to Abram’s Law, the strength of concrete at a given age and normal temperature decrease with increasing the water-cement ratio assuming full compaction of concrete have been done. Permeability of concrete to water depends mainly on the W/C ratio, which determines the size, volume and continuity of the capillary voids. It is clear that even a small increase of W/C ratio can increase the concrete permeability to a great extent. Again, permeability is the most important Characteristics determining the long-term durability of reinforced concrete exposed to seawater as it controls the diffusion of aggressive salt-ions into the concrete. ACI 318-83 requires that normal –weight concrete subjected to freezing and thawing in a moist condition should have a maximum W/C ratio of:

- 0.45 in case of curbs, gutter, guard rail or their sections and
- 0.50 for other elements.

4) Cement Type

The resistance of concrete against the action of various aggressive agencies depends to a great extent on the type and proportion of cement. Ordinary Portland and pozzolana and sulfate resisting cement are the various types mainly used in marine concrete construction. Various researchers assessed the performance of these cements individually by either exposing the mortar and concrete specimens in the seawater or in its constituent salt solution, to study their strength and durability characteristics.

5) Air Entrainment

Harden concrete containing entrained air is more uniform, has less absorption and permeability and is more resistant to the action of freezing and thawing. Normally, about 4-6% of air by volume of concrete is entrained which is dispersed throughout, the concrete in the form of minute, disconnect bubble. It has been reported that the amount of entrained air necessary for imparting the highest resistance to concrete to frost action in the seawater is in the range of 10-20% which is more then twice as large as concrete with 3-6% air entrained when exposed to plain water in similar environment. However, the amount of air entrained 10-20% reduces the compressive strength of concrete to about one-half of the strength without air entrainment.

6) Carbonation Process

The hydrated concrete has a tendency of combining with carbon dioxide, CO2 present in the atmosphere and forming carbonates, which partly neutralizes the alkaline nature of concrete. This process is known as carbonation. When carbonation depth exceeds depth of cover to the reinforcement, the salt ions find a suitable environment leading to greater corrosion.

7) Quality of Mixing Water

Sea water contains a total 3.5% salinity of which

- 78% is NaCl ( i.e. 2.7% of total salinity)
- 15% is MgCl2 and MgSO4

According to ACI 318-83 the mixing water should be potable and free from salts. Giving no specific reference to sea water, it specifies that mortar cubes made with computable mixing water shall 7 days and 28 days strength equal to at least 90% of strength of similar specimen made with potable water.


7) Influence of Crack

Reinforced concrete structure, either reinforced develop unavoidable cracks during their service life. Cracking may stem from various causes; construction cracks as a result of initial and drying shrinkage, settlement, and heat of hydration; load cracking during normal service as a result of flexure, stress reversal, torsion, shear etc. However cracks offer a path to the interior of concrete for the deactivating and corroding agents.

8) Depth of Cover to the Reinforcement

The thickness of the concrete cover to the steel is an important factor regarding rebar corrosion in an aggressive environment. It affects the time taken for the salts to penetrate to the steel, and the subsequent rate of arrival of oxygen at the steel surface as in the case of permeability. Lesser the cover, shorter the time required to deactivate the embedded steel. Also moisture content of the cover, defines permeability to surrounding salts and gases.

9) Age of Immersion

The age of immersion also referred to, as period of procuring is the time duration of concrete commencing immediately after casting up to the formal curing. In a marine environment, procuring of concrete can be done with either sea water of fresh water. The dissolved salts start reaching with the concrete affecting its rate of gain in strength when procuring is done with seawater.

10) Diffusion of Salts Under Pressure

The harmful salt ions enter to the body of concrete at various depths under hydrostatic pressure and also to the embedded steel. This result disintegration of concrete which in turn increase its permeability and provides greater accesses to the chloride ions for coming in contact with steel. But due to limited availability of oxygen, the corrosion process is often ineffective.

11) Wetting and Drying Cycles

In marine environment, the structural concrete in tidal zone undergoes alternate wetting and drying process due to tidal action.

12) Freezing and Thawing Cycle

Concrete is greatly affected freeze-thaw cycles. The change in physical state of water ( liquid-solid) inside the mass of concrete results in an increase of volume 9%. The volume change in a cyclic fashion (freeze-thaw cycle) causes disruption of concrete by dilution process.

Component Comprising Biogas System

Biogas production is most suitable for farms that handle large amount of manure as liquid, slurry, or semi-solid with little or no bedding added. To produce a cost effective system, it should be designed by an experienced animal waste digester designer. A typical biogas system consists of the following components:

A) Manure Collection
B) Anaerobic Digester
C) Effluent Storage
D) Gas Handling
E) Gas Use

All of these components are discussed briefly:

A) Manure Collection:

A manure management system is developed in livestock farms to consider environmental, sanitary and farm operational facilities which includes collection and storage of manure. This can be collected and stored as liquid, slurries, semi-solids, or solids.

a) Raw Manure

The solid content of 8-25% is excreted as raw manure. It can be diluted by various processes.

b) Liquid Manure

Such manure is diluted to a solid content of less than 5%.

c) Slurry Manure

Manure handled as slurry is diluted to a solids contents of about 5-10%.

d) Semi Solid Manure

Manure handled as a semi-solid has a solid content of 10-20%. Water is not added to such manure and it is typically stored until it is spread on local fields.

e) Solid Manure

Manure having a solids content of greater than 20% is handled as a solid by a scoop leader.

B) Anaerobic Digester

Naturally occurring anaerobic bacteria is decomposed in a digester and manure is also treated in it during the process of biogas generation. An air-tight impermeable cover is used to trap the gas for on –farm energy use.the manure handling system defines which type of digester can be used.

Different types of digester are listed below:

a) Cover Lagoon Digester

It is used to treat and produce biogas from liquid manure( solids <>b) Complete Mix Digester

It is used to treat slurry manure with a solid content of 3-10%.


c) Plug Flow Digester

It can treat scraped dairy manure having a solid concentration of 11-13%.

d) Fixed Film Digester

It is best suited for dilute waste streams typically associated with flush manure handling.

C) Effluent Storage

The anaerobic digestion of manure produces biogas and effluent. The effluent is a organic solution having a quality to be used as fertilizer and other potential uses.
The size of storage facility and storage period depends on farm requirements during non-growing season. The longer storage facility provides flexible management of the waste to account for weather changes, equipment availability or break down or other operational management.

D) Gas Handling

The expected biogas is received from the digester and transport to the final use i.e., an engine or a plant through as a gas or a plant through a gas handling system which includes: piping, gas pump or blower, gas meter, pressure regulator, and condensate drain.

The trapped gas under an air-tight cover over the digester, is collected by pulling a slight vacuum on the collection pipe using a gas pump or blower to the end of pipe. A gas meter is provided to monitor gas flow rate. Sometimes a gas scrubber is used to purify the gas to remove corrosive compounds ( e.g., hydrogen sulfide). When warm gas is cooled during traveling through pipe system and water vapor in the gas condenses. The condensate product is removed using a condensate drain(s).

E) Gas Use

Methane content in biogas is about 60-80% which have a heating value of approximately 600-800 Btu/ft3. Such gas can generate electricity, can be used as fuel for a boiler, space heater, or refrigeration equipment, or it may be directly combusted as a cooking and lighting fuel.

Checklist for Supplies and Equipment for Earthquake

Earthquake can happen at any time, anywhere. When a earthquake hits, one have only few seconds between realization that this is an earthquake and the time when the shaking stops. This is the time; advanced planning becomes effective to make one or one’s family safer. If one knows what to expect , what to do and what supplies and equipments required to survive during quake after quake, he can make right decision and right action that may mean the difference between injury, life or death.

The necessary supplies are as below:

Ø Working gloves
Ø Ax/ maul ( minimum 6 lb)
Ø Shovel ( flat head and pointed)
Ø Broom
Ø Hammer and nail
Ø Screwdrivers
Ø Crowbar and claw tool (36” or longer)
Ø Plastic sheeting roll (4mm,10’x25’)
Ø Plastic garbage bags (heavy duty, 30 gal. or larger)
Ø Small or larger plastic bags
Ø Coil of rope 1/4”,1/2”,3/4” (25’-50’)
Ø Coil of wire
Ø Tent (family or tube type)
Ø Tarp (PVC or canvas, minimum two, 8’x10’)
Ø Sleeping bags, blanket or space blanket
Ø Cheese cloth ( to strain particles from water)
Ø Cash money (small denominations and coins )
Ø Dry food
Ø Clothing
Ø Walking shoes and shocks
Ø Local road map
Ø Fire extinguisher ( preferably a dry chemical type with a minimum size with an earthquake restraining strap, a hose-type nozzle and a metal head.
Ø Compass
Ø Flash light w/batteries or chemical light sticks, matches ( in water proof container)
Ø Small radio (battery-powered portable)
Ø Entertainment pack –family photos, note books, literature and genes

Sanitation Supplies

Ø Plastic bags ( heavy-duty garbage can size and small zipper types)
Ø Powdered chlorine lime ( proper storage is required, it is an oxidizer and is corrosive )
Ø Portable camp toilet with chemical
Ø Toilet paper
Ø Handi-wipes, wet-n-drys etc. for water-free cleanup
Ø Toilet supplies (toiletries, shampoo, toothpaste, deodorant, sanitary napkins etc.)
Ø Insect, fly, mosquito and ant sprays.

Storage Location

When organizing supplies for an earthquake, remember that you need to get to them after an earthquake has turned your house into a mess. Store supplies in an easy-to-find location that has a minimal chance of being buried under falling objects. If you are short on space, a large trash can makes an excellent storage container. If live in an apartment, the can be hidden under decorative tablecloth. Food supplies should be rotated within at least six month.

Mechanism involved drifting of continents

Diastrophism

Various stresses operating within the body of the earth result in regional crustal deformations or movements are known as diastrophic movements. This phenomenon is known as diastrophism.

Isostasy

Isostasy is used to explain various movements in the crust. The principle of isostasy is that the different masses of the earth’s crust are standing in equilibrium, tending to reach and maintain a hydrostatic balance. At certain depth these crustal blocks exert equal pressure

and stand equilibrium.

Holme’s Hypothesis of Convection Current:

Holme’s believed that neither tidal friction nor any force from outside can drift the continents and are related to mountain building. He explained the whole process with the help of convection current cycle which is operating within the earth.

Assumption

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This convection current theory assumes the formation of convection currents within the substratum which travel from the hotter parts near the core towards cooler parts under the crust.

Formation of Oceanic Deeps

When two ascending currents diverge under a continental block, they exert a tensional force. As the convective system gain strength the tensional force also increases, resulting in rupturing and splitting of the continental blocks leaving a gap in between. The broken fragments left in between sink into gap, reaching the ocean floor.

Drifting Continents

The currents flowing horizontally beneath the continental blocks drift them. The underlying basaltic layer which provides obstruction to the continental drift, move down into the substratum under the dragging effect of the currents directed downwards.

Mountain Building

When two currents coming from opposite sides meet and are directed downwards, there is deepening and narrowing effect. The continental mass, dragged downwards and compressed, gives rise to mountains and their and their roots. The underlying basaltic layer which also gets pushed downwards undergoes metamorphism, changing into a highly compressed type of rock called Eclogite.

Restoring the Basaltic Layer

The heavy matter (Eclogite) will descend further, merging with the substratum and further on heating, changing into magma which being a lighter material will again tend to move upwards. Most of this magma or basaltic material will rise up along with ascending currents, reaching the gaps left between the torn outstretched continents, restoring the basaltic layer, also forming the basaltic plateaus and causing volcanic activity. The basaltic layer in front of the continental mass when dragged down-wards, results in the formation of oceanic deeps.

Termination of system

The convective system as explained above, results in complete circulation of the basaltic matter beneath the crust. When the convection currents begin to wane out, the system loses its power. As a result the dragging and drifting action also diminishes and finally ceases. The lighter mountain roots which were being pulled downward continuously are left to rise up.

Holme’s Hypothesis and Isostasy

This convection current hypothesis provides appropriate explanation for the continental drift and the associated and related phenomenon of mountain building. Holme’s established this system depending on Isostasy which explains very successfully

Ø The circulation and recycling of basaltic materials

Ø Formation of ocean deep

Ø Presence of sima ( viscous substratum) sometimes with patches of sial at the ocean floors.

Ø Mountain building

Ø The lighter material of the mountains and their roots.

Improved Design Strategies of Soft Storey

Reinforced concrete (RC) frame buildings are very common in the world. In such types of structure for functional requirements of parking space under the buildings no masonry in fill are provided resulting a construction with stilts.

Design Approaches

Open ground storey building is inherently poor systems with sudden drop in stiffness and strength in the ground storey. In the current design practice, stiff masonry walls are neglected and only bare frames are considered in design calculations. Thus, the inverted pendulum effect is not captured in design.

Safeguard Against Failure

The failure can be avoided following two considerations in structural proportioning:

a) To avoid soft storey

b) When soft storey cannot be avoided, providing special design provision in designing such structure.


How to Avoid Soft Storey

Architects and structural designers can use the following conceptual design strategies to avoid undesirable performance of open ground storey buildings in earthquake:

Ø Provide some shear walls at the open ground story level : this should be possible even when the open ground story is being provided to offer car parking

Ø Select an alternative structural system (e.g., RC shear walls) to provide earthquake resistance: when the number of panels in the ground storey level that can be filled with masonry walls is insufficient to offer adequate lateral stiffness and resistance in the ground storey level, a ductile frame in not an adequate choice. In such cases an alternative system, like a RC shear wall, is required to provide earthquake resistance.Some remedial measures to counter the bad performance are shown in fig:

 




Special Design Provision

To safeguard the soft first storey from damage and collapse code provides two alternative design approaches:

1) The dynamic analysis of the building is to be carried out which should include the strength and stiffness effects of infills as well as the inelastic deformations under the design earthquake force disregarding the reduction factor R.

2) The building is analyzed as a bare frame neglecting the effect of infills and, the dynamic forces so determined in columns and beams of the soft (stilt) storey are to be designed for 2.5 times the storey shear and moments: or the shear walls are introduced in the stilt storey in both directions of the building which should be designed for 1.5 times the calculated storey shear forces.

Economic Section

Normal beam and column sections used in construction are rectangular or circular in reinforced concrete frames. But, it is noticed that the fibers near the neutral axis are understressed compared with those at top or bottom i.e., extreme end of section from neutral axis. The fact that a large portion of the cross section is thus understressed making it an inefficient for resisting flexure.

The flexure stress is derived by

α = (MC) / I

Where, I = moment of inertia of the desire section about a reference axis (neutral axis)

C = distance from the neutral axis to remotest fibers

M = resisting bending moment


α = flexural stress at remotest fiber.


If the area of a beam of rectangular section [fig-1(a)] is arranged so as to keep the same overall depth but have the shape shown in [fig-1(b)], the moment of inertia would be greatly increased, resulting in a greater moment resisting capacity. Actually, the increase in resisting moment is due to more fibers being located at a greater distance from the neutral axis. These fibers carry a greater stress and have a larger moment arm about the neutral axis to resist the applied bending moment. But, the section in [fig-1(b)] is not practical; the two parts of it would collapse together. It is necessary to use some of the area to fix these parts in place relative to each other, [fig-1(c)]. The web area transmits practically all the vertical shear.

[fig-1(c)] represents a wide-flange beam (referred to as a W shape). This is one of the most efficient structural shapes manufactured because it not only provides great flexural strength with minimum weight of material but is highly when used as a column. Another structural shape is the I beam (referred to as an S shape) in [fig-1(d)], it preceded the wide flange and because it is not so efficient has been largely replaced by the wide-flange beam.

Structural section should be such that resisting moment Mr = (αI)/c = αS must be equal to or greater the applied bending moment M.

That is

S ≥ M/ α

Where, S = section modulus of the selected section

The above equation indicates that a beam must be selected whose section modulus is equal to or greater than the ratio of bending moment to allowable stress.

The compression flanges of beams tend to buckle horizontally side sway if the beam is too long. This buckling is a column effect. When this lateral deflection is prevented by the floor system or by bracing the compression flanges at proper intervals, the full allowable stresses may be used. Otherwise, the stresses should be reduced.

Simulation of Tsunami

Physical Process


Tsunamis generate through quite distant but three overlapping physical process:

i) Generation by any force that disturbs the water column.

ii) Propagation from deeper water near the source to shallow coastal areas.

iii) Finally, inundation of dry land and consequent destruction over coastal areas.


Generation is the process by which a seafloor disturbance reshapes the sea surface into a tsunami. The disturbance may be a movement along a fault.



Assumption

Modelers assume that an ocean-surface displacement is identical to that of the ocean bottom, but direct measurements of ocean-floor motion have never been available and it may not possible ever. Instead researchers use an idealized model of the quake; they assume that the crustal plates slip past one another along a simple, rectangular plane inside the earth.



Difficulties to Simulation


Even after these assumption, predicting the tsunamis initial height requires at least 10 descriptive parameters, which includes
→ The amount of slip on each side of the imaginary plane.

→ The length of crustal plate.

→ The width of crustal plate.


But, from the seismic data only


→ The orientation of the assumed fault plane

→ The quake location and depth

→ The quake magnitude

Can be interpreted.

Tsunami, Tides and Wind-Generated Waves

The terms tsunami comes from the Japanese meaning harbor (“tsu”) and waves (“nami”). Approximately 190 event of tsunami have struck coast of Japan. According to ordinary English practice an s can be added to represent it in plural form.

The Greek historian Thucydides was the first to relate tsunami to submarine quakes, but understanding the nature of tsunami remained slim until the 20th century and sometimes it merged with tides and wind generated waves. Therefore it is very important to precisely distinguish them depending on their characteristics.


Wave Length


Everyday wind waves has a wave length (from crest to crest) of about 1000m (300ft). But, the tsunami have a wavelength of about 200km and sometimes 750km 750 km in the open ocean.

Amplitude


Wind generated waves have an amplitude of about 2m (7 ft) high. But, the tsunami have only about 1m(3ft).


Sea surface slope


The generated waves produce a steep slope of sea surface as it has relatively high amplitude and much small wave length. But, the tsunami waves generate a gentle slope due to its great wave length and small amplitude. This makes tsunamis difficult to detect over deep water.


Speed of approach

Tsunami travels at a speed of 700 kilometers per hour in deep ocean and easily pace with a Boeing 747 which can not be compared with other.

Depth of Disturbance

Breezes blowing across the ocean crinkle the surface into relatively short waves and create currents restricted to a shallow layer. Strong gales can whip up waves 30 meters or higher in the open ocean, but even these do not more deep water. Tides, which sweep around the globe twice a day, do produce current that reach the ocean bottom just as tsunami do.



Cause of Occurrence

Cyclones, hurricanes are particular form of meteorological storm which is generated by deep depression, result a storm surge which can be several meters above normal tide levels. The centre of depression has low atmospheric pressure. This surge when reaches ashore, it can inundate vast areas of land. Such a storm surge inundates Burma (Myanmar) in May 2008.

The tides are generated by the gravitational pull of the Moon or Sun.

The tsunamis are produced impulsively by an undersea earthquake or, much less frequently, by Volcanic eruptions, meteorite impact or underwater landslides.



Inland Movement of Water

Tsunamis, tides or strong storm all produce waves of water that move inland, but in case of tsunami the inland movement of water is much greater and lasts for long period, giving the impression of an incredibly high tide.



Special Feature of Tsunami

When tsunami becomes nearer to coast and travel over shallow depth, the wave is compressed due to wave shoaling and its forward travel slows below 80 Kmph (50 mph). it wave length diminishes to less than 20 Km (12miles) and its amplitude grows enormously, producing a distinctly visible wave. As the wave length still remains in the order of several km (a few miles), the tsunami may take minutes to reach its full height. Open Bays and coastlines adjacent to very deep water may shape the tsunami further into a step-like wave with steep break point.

Tsunami

Definition: Tsunami is ocean waves produced by earthquake or under water land slides. The word tsunami means "harbor waves". Tsunamis are often incorrectly refferred to as tidal waves, but a tsunami is actually a series of waves.


Properties of waves

a) Speed

i) Open ocean

The waves propagate across the deep ocean at jetliner speeds. The speed is of the order of 450~600 miles per hour in open ocean.


ii) Near coast

The wave slows down to highway speeds as it enters shallow water, and it sometimes runs ashore as a tide like flood.



b) Wave Length

Tsunami would not be felt by ships. This is due to large wave length of its propagation. This length may be hundreds of miles long.


c) Amplitude

i) Open ocean

The amplitude of waves in open ocean is only few feet. As the length of waves is large a sea surface slope of gentle value occurs.

ii) Near coast

As the waves approach the coast, their speed decrease and their amplitude increase. These unusual wave heights have been known to be over 100feet height.

Plan for Tsunami

Development of tsunami disaster plan is very important. As people do not respond appropriately, after warning is sounded, the most reliable warning becomes ineffective. Tsunami specific planning should include the following:

1) Contact with local disaster and emergency management office and learn about tsunami risk in your community. Know height of your street, hone or the place you may frequently visit as well as distance of these from the coast or other high risk waters as evacuation orders may be based on these parameters.

2) If you are not in your community and visiting an area having risk from tsunamis, contact with the hotel, motel or campground operators for tsunami warning and evacuation information and how one would be warned. Designated escape routes should be known before a warning is issued.

3) Plan an evacuation route from the place where you may present during tsunami risk. If possible, select an area 100ft above sea level or go up to two miles inland, away from the coastline. If such place is not available, go as high as you can. As every foot upwards or inland make you safer. The safe location should be such that within 15 minutes you can reach their on foot. Be prepared to further evacuation by foot if necessary as after a disaster, roads may become impassable or blocked.

4) Follow footpaths as it normally lead to uphill and inland. But many roads may parallel to coastlines. Local emergency management officials can also advice you to learn best route to take safe shelter.

5) Practicing the evacuation routes, may familiar you with the routes and can help saving your life. Be able to follow the route at night and during stormy and very cold weather. These help you to take a response quickly without requiring less thinking during actual emergency situation.

6) Listen weather report which will warn you of potential danger.

7) Talk to insurance agent. Homeowners’ policies do not cover flooding from a tsunami. Ask about National Flood Insurance Program.

8) Review flood safety and preparedness measures with your family. Tsunami is nothing but a large amount of water that crush onto the coastline, creating floods.

9) Discuss with your family what to do when all family members are not together. These will help reducing fear and anxiety during actual situation.

Protection for properties

Protection for properties can be made by taking following steps:

a) Avoid building or living in building within several hundred feet of the coastline. These areas are more likely to suffer damage from tsunamis, strong winds, or coastal storms.

b) Make a list of items to bring inside in the event of a tsunami. This list help you remember anything missing that can be swept away by tsunami waters.

c) Most tsunami waves are less than 10 feet. So elevating house will help reduce damage to your property from most tsunamis.

d) Consult with a professional for advice to derive waves away from your property.

e) Take help from engineers to check your home and make it more resistant to tsunami water. Improperly built walls may make situation worse.

Soft and Weak Stories

Open Ground Storey 

An open ground storey building has both columns and masonry infill walls in the upper stories but only columns in the ground storey. It is the most common type of vertical irregularity occurs in building which is left open for the purpose of parking, i.e., columns in the ground storey do not have any partition walls (of either masonry or RC) between them.

Characteristics of Open Ground Storey

The infilled brick walls in the upper stories increase the lateral stiffness of the frame by a factor of three to four times than that of lower weak storey. The following two features are characteristics of open ground storey buildings:

a) Relatively flexible ground storey in comparison to the stories above, i.e., the relative horizontal movement at the ground storey level is much larger than the stories above. This flexible ground storey is called soft storey.



b) Relatively weak ground storey in comparison to the stories above, i.e., the total horizontal earthquake force (load) resisted at the ground storey level is significantly less than the stories above resulting a weak storey.

Earthquake Behavior
The much stiffer upper stories behaves like a rigid block, and most of the horizontal displacement of the building occurs locally in the soft storey alone. This makes the building behave like an inverted pendulum, with the ground storey columns acting as the pendulum rod while the rest of the building act as a rigid pendulum mass which swings back-and-forth during earthquake shaking, and the columns in the open ground storey are severely stressed.

In such building the dynamic ductility demand during the probable earthquake gets concentrated in the soft storey and the upper storeys tend to remain elastic. Hence whereas the ‘soft’ storey is severely strained causing its total collapse, much smaller damages occurs in the upper storeys, if it all.

Soft Storeys at Mid Height

Generally the soft or weak storey usually exists at the ground storey level, but it could be at any other storey level too. Sometimes a soft storey is created some where at mid-height of the multi-storey building to provide gathering place or restaurant. Such buildings also behave likes open ground building and also collapse in different earthquake over the world.

Continental Drift

Definition

It is believed that there was a single great continent mass in the beginning which broke up into smaller units with the passage of time. The broken continental blocks started separating out and drifting apart over underlying elastic and viscous substratum under the influence of gravitational forces.


Nature of Continents and Sea Floor
The continents are the slabs of sial (crust) underlain by the basaltic layer. The ocean floors are essentially the basaltic. However, there are thin patches of sial present along the sea floors. Also the outer boundaries of continental blocks of each group are marked by orogenic belts (mountain ranges).


Background of this concept

The geological studies on different continents indicate that the continent have been experiencing changing climates in the geological past. Places having warm climates now were at one time cold region so. This raises the question that, was it the climate which was changing or the continents shifted their positions. The presence of similar plant and animal fossils and the identical geological structures on these widely separate continents also needs explanation that whether the in-between block have sunk or the continents were initially united and separated later on. The mountain building process also demand explanation for the horizontal compression and is related to the continental drift. The outer margins of the continents have mountain ranges marking their boundaries.

Wegener’s hypothesis of continental drift:
Wegener observed that the shorelines of some continents looked like they could fit into the shorelines of others, almost like a Jigsaw puzzle. Wegener also realized that there were geological elements common to both shorelines. Rocks of around 200 millions year of age (200 Ma) were similar on both sides and they contained identical land-based beasties (e.g. dinosaurs). He hypothesized that in the past (200 Ma), Africa and South America were united together. Not only that, he was able to put most of the continents together into a supercontinent, he called Pangaea. He published a map shown in fig-1 in 1912 and his idea soon became known as Continental Drift.


Hypothesis
He believed in the existence of single great continental mass, a compressed combination of Laurasia and Gondwanaland, to which he gave the name Pangaea. Where northern group of continents comprises Laurasia and southern group comprises Gondwanaland. This great continental mass was surrounded by a vast ocean called Panthalassa.

With the passage of time this single continental block, Pangaea, broke into pieces which started drifting apart, taking the present set-up. Along with continental drift, he also believed in the wandering of the poles. According to him the poles have been migrating from place to place.


Drifting Directions

According to Wegener the drift was equator-ward and west-ward. He ascribed the equator-ward movement of continents to the gravitational attraction caused by the earth’s equatorial bulge; and west-ward movement to the force of attraction caused by the sun and the moon. The equator-ward drift is stated to be responsible for the formation of the Alpine-Himalayan mountain ranges; and the west-ward drift responsible for the formation of Rockies and Andes mountain ranges. The gaps left in between the continents made the oceans.

Wegner’s hypothesis of continental drift very nicely explains the past climate variations , similarities in the geological structures and fossil content of the separated parts of the earth’s crust (continents) and mountain building.

Causes of Tsunami

In 1950s it was hypothesized that larger tsunamis than had previously believed possible may be caused by landslides, explosive volcanic action and impact events when they contact water.


These phenomenons rapidly displace large volume of water, as energy from falling debris or expansion is transferred to the water into which the debris falls at a rate faster than the ocean water can absorbed it. These have been named by people as “Mega Tsunami”. But such 

Figure. (Top) Location and parameters of a Norfolk Canyon Slide. Thirty-seven, 10 x10 km simple slides comprise this model. (Panels 2, 3 and 4) Tsunami after 1/2, 1, and 2 hours. Note how long it takes for the waves to traverse the shallow continental shelf.

mechanism may produce a tsunami which is unlike trans-oceanic tsunami caused by some earthquakes due to dissipation of energy and rarely affect coastlines at a large distant due small sea area impact. Sometimes this mechanism can give rise to much larger local shock waves (solitons), like the land slide at Litya Bay 1958, which produce a wave with an initial surge estimated at 524 m. However, an extremely gravitational landslide might generate a so called “Mega Tsunami” which can have the ability to travel oceanic distances.


The most common and strong cause of tsunami is earthquake. It is not possible that it can generate from movement of Divergent (Constructive) or Conservative Plate Boundaries. This is due to vertical displacement of the water column is impossible on such movements. Tsunami can be generated when Convergent or Destructive Plate Boundaries abruptly move and vertical displacement of overlying water takes place. Earthquake related to subduction zone generate the majority of all tsunami.

Examples

Convergent Boundaries

The example includes Storegga during the Neolithic era, Grand Banks 1929, Papua New Guinea 1998.


Subduction Zone

On April 1, 1946 a magnitude 7.8 (Richter scale) earthquake occurred near the Aleutian Islands, Alaska. It generated a tsunami which inundated Hilo on the island of Hawai’i with a 14 m high surge. The area where the earthquake occurred is where the Pacific Ocean floor is subducting under Alaska.

Some earthquakes cause sediments to become unstable and subsequently fail. These slumped and as they flowed down slope a tsunami was generated. Another causes of disturbance of sediments may include as a release of gas hydrates ( Methane etc.)



Sometimes Mega thrust earthquakes generates tsunami which can cross the oceans.

Examples

1) The Great Chilean Earthquake (19:11 hrs UTC) May 22, 1960 (9.5 Mw)

2) The Good Friday Earthquake, March 27, 1964, Alaska (9.2 Mw)


3) The Great Sumatra-Andaman Earthquake (00:58:53 UTC) December 26, 2004 (9.2 Mw)

But, smaller ( 4.2 Mw) earthquakes in japan can trigger tsunami that can devastate nearby coasts within 15 minutes or less.

After Tsunami Measures

The following measures are suggested to be taken after tsunami:

1) The tsunami may have damaged roads, bridges, or other places that may be unsafe to use and keep listening to weather reports, Coast Guard emergency frequency station.

2) Look for help. If possible help injured or trapped persons. In appropriate case give first aid but do not move seriously injured people as it may cause deteriorating their conditions.

3) Telephone lines are frequently overwhelmed in disaster situations. They need to be clear for emergency calls to get through requiring to make calls on telephone for emergency calls only.

4) Tsunami waters, as by flood water, can undermine foundation resulting foundation to sink fallowed by collapse of walls and cracking in floor. So, come out of building if water surrounds it.

5) Flood water driven by tsunami may have damaged structures. So extreme caution have to take before entering any building.

6) To avoid fire hazard use battery powered lanterns of flash lights to examine your residence.

7) Wear safe shoes to avoid injury associated with cut feet which is common case after such disaster.

8) Look for broken or leaking gas lines, flooded electrical circuits, or submerged furnaces or electrical appliances.

9) Flammable or explosive materials may come from upstream resulting most frequent hazard and keep aware of this.

10) Open a window and leave quickly the building and turn off the gas using the outside main valve, if possible, and call Gas Company. If you smell gas or hear a blowing and hissing noise, without a professional the valve should not open.

11) Electrical equipment should be checked and dried before being returned to service.

12) Inspect electrical system damage. Any spark or broken or frayed wires or smell of burning insulation is a sign of electrical hazard. The main fuse box or circuit breaker should be turned off at once. But, if, to reach their, you have to step in water call an electrician first for advice.

13) Tsunami flood waters flush snakes and animals out of their homes and may have come into buildings with the water. Use a stick to search in debris.

14) Take pictures of the damage, both of the buildings and its contents, for insurance claims.

15) Clear mud with shovel from walls and floors to dry up them.

16) Any food that became in contact with flood waters may be contaminated and should be thrown out.

17) If sewage and water line have damaged, avoid using the toilets and call a plumber.

19) Safe water can be derived from undamaged water heater or by melting ice cubes. Use tap water if local health officials advise it is safe.

20) Examines walls, floors, doors, staircases and windows to become sure that the structure is not in danger of collapsing.

21) Inspect foundations for cracks or other damages to avoid possible collapsing hazard.

Tsunami Warning System

Predicting where a Tsunami may strike helps to save lives and properties only if coastal inhabitants recognize the threat and respond appropriately. So scientists are trying to develop accurate and effective Tsunami warning system for many years.


Different organization like FEMA, NOAA and USGS are trying to tackle the threat of both local and remote source Tsunamis. As tsunami cannot be restricted, they are trying to establish an appropriate and effective warning system. Some of these attempts are stated below:


a) Tsunami Inundation Maps

The threat to specific coastal areas can be assessed by means of tsunami inundation maps. It delineates areas susceptible to Tsunami flooding, Earthquake-shaking Intensity, Liquefaction and Landslides. These maps provide critical guidance to local emergency planners, charged with identifying evacuation routes.


Coastal tide gauges have modified specially to measure tsunamis and seismic network have upgraded to have more rapid and more complete reports on the nature of the earthquake. But, as the tidal gauges spot tsunamis close to shore, they cannot measure tsunami energy propagating toward a distant coastline. As a result, an unacceptable 75% false-alarm rate has observed. These incidents are expensive, undermine the credibility of the warning system, and place citizens at risk during the evacuation.


b) Deep-Ocean Assessment and Reporting of Tsunami (Dart) System


Seismometers staked out around the ocean can almost instantly pinpoint a quakes location. In the next moment, complex computer programs can predict how long a triggered tsunami would take to reach coastlines, even though there have no yet a evidence of wave exists. After some minutes, tide gauges scattered along coastline may detect tsunami. But the only way to be sure whether a dangerous wave is headed toward a distant coast is to place tsunami detectors in its path and track it across the open ocean. This tsunami detectors is the DART system.

The DART system depends on bottom pressure recorders. As the crest of a tsunami wave passes by, the bottom recorder detects the increased pressure from the additional volume of overlying water. Even from 6,000 meters depth, the sensitive instrument can detect a tsunami no higher than a single centimeter. Ship and storm waves are not detected, because their length is short and, as with currents, changes in pressure are not transmitted all the way to the ocean bottom. When the bottom recorders detect a tsunami, acoustic chirps will transmit the measurements to a car size buoy at the ocean surface, which will than relay the information to a ground station via satellite.



More buoys would reduce the possibility that tsunami waves might sneak between them. Combined with the buoy measurements, the simulations of tsunami in computer will provide more accurate predictions to guide officials who may have only a few minutes to decide whether to sound an alarm.



c) Zoological Hypothesis:



Some zoologists hypothesise that animals may have an ability to sense subsonic Rayleigh waves from an earthquake or a tsunami. Some animals seem to have the ability to detect natural phenomena and if correct, careful observation and monitoring could possibly provide advance warning of earthquakes, tsunami etc. However, the evidence is controversial and has not been proven scientifically. There are some unsubstantiated claims that animals before the Lisbon quake were restless and moved away from low lying areas to higher ground. Yet many other animals in the same areas drowned. The phenomenon was also noted by media sources in Sri Lanka in the 2004 Indian Ocean earthquake.