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Design of Earthquake and Cyclone Proof House for Poor

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Introduction

Background

The population of the world is constantly increasing; it currently lies at 6.7 billion people and is predicted to increase to 9.2 billion in the next forty years. Majority of this growth will occur in urban areas and it is predicted that by the year 2050 urban areas alone will contain 6.4 billion people (United Nations, 2008). This continuous growth of urban areas is known as urbanization and is mainly occurring in developing countries, in particular in the peri-urban regions (the outer fringes of larger towns/cities also known as slums, shanty towns or favelas depending on the region). Much of these peri-urban areas however are already highly populated with inadequate living conditions, therefore any increase in population is a major problem and in turn means an increase in poor housing, health and services (Mara, 2008). This report is going to specifically look at the peri-urban regions and housing of Latin America.

Latin America is generally defined as those countries in the Americas where Spanish or Portuguese is spoken. This includes Mexico and the countries of Central America, South America and the Caribbean (Bumgarner, 2008), as shown in figure one. It currently has a combined population of approximately 590 million people, 470 million of this total are found in urban areas (United Nations, 2008). South America is the region of the world with the largest proportion of its population living in slums at 26% and these numbers continue to increase (SASI Group and Newman, 2006). Many of its countries are frequently subjected to natural disasters such as earthquakes, volcanic eruptions, hurricanes and flooding. This is particularly due to the diverse topography of the region; oceans, mountains, rainforests, volcanoes and fault lines can all be found throughout the area (Bumgarner, 2008).

“In addition, the twenty largest cities of Latin America are in areas with steep slopes, swamps, floodable land or seismic activity. As a result many of the regions worst disasters have hit cities” (World Bank, 2005).

In 1985 Mexico City was hit by a major earthquake, killing approximately 9,500 people and thousands more were injured and left homeless. In 1970 an earthquake hit Peru that destroyed many areas in particular affecting cities such as Lima, Casma and Chimbote. In total 20,000 people died and major damage to the cities occurred, according to preliminary reports building collapses caused most of the fatalities. The worlds largest recorded earthquake hit Chile in 1960, thousands were killed or injured and over 2 million people were left homeless with $550 million of damage caused in Southern Chile alone (USGS, 2009). Other cities such as Rio de Janeiro and Caracas have seen major destruction through landslides (World Bank, 2005) and areas in Venezuela (such as Caracas) and Southern Brazil have been affected by cyclones. Hurricane Mitch tore across Central America and Southern Mexico in 1998 and left a path of destruction killing over 10,000 people and leaving millions more either homeless, missing or severely affected.

The poor are put at particular risk from natural disasters because of the hazardous locations and poor quality of their dwellings (World Bank, 2005). As previously mentioned the living conditions of much of the urban population, in particular in the peri-urban regions is less than satisfactory, usually densely populated and often unfit for human habitation. Figures 1.2 and 1.3 below show images of peri-urban areas in Latin America, as can be seen the shelters are poorly made and very densely spaced.

The social, physical and mental health of an individual is majorly influenced by the environment in which they live (Tinker, 2008) poor housing results in poor health and this is particularly evident in the peri-urban regions of Latin America for example the Neza Chalco Itza barrio of Mexico City and slums of Peru, Brazil and Chile. Many of the low-cost settlements are overcrowded and lack basic but vital amenities such as clean water, sanitation, access to work and shelter. This in turn leads to a high rate of disease and low life expectancies with many people dying at a young age. A major problem is poor sanitation and contaminated water supply resulting in faeco-oral diseases such as salmonellosis, viral diarrhoea (rotavirus) and cholera. Diarrhoea alone is a major problem in developing countries especially in children; killing 1.3 million children aged under five, globally, per year (Mara, 2008).

Housing related diseases are also often of major concern, the poorly constructed shelters and overcrowding leads to many insect and rodent related diseases, such as plague and Chagas' disease both of which often result in death.

Aims And Objectives

“Gaining access to housing that provides adequate shelter and physical safety is one of the greatest challenges confronting the urban poor. Most poor people live in informal housing, often located in marginal areas that are vulnerable to natural disasters and poorly served by public services or utilities.” (World Bank, 2005)

This quote taken from the book “The Urban Poor in Latin America” published by the World Bank, perfectly describes the issues confronting the urban poor of Latin America. It highlights the main problems they face and summarizes the key objectives of this report.

The initial brief of this report is to design a suitable house for the peri-urban poor of Latin America. It needs to be able to resist earthquake and cyclone forces but also be low cost and feasible for the local area. Listed below are the key aims of this report and these will help to ensure the final solution to the brief is met successfully.

  • Gain an understanding of earthquakes and cyclones and their effects.
  • Gain an understanding of existing earthquake and cyclone resistant designs.
  • Ensure the final design is both earthquake and cyclone resistant.
  • The design must be of low-cost and suitable for peri-urban regions.
  • The design needs to provide adequate shelter which in turn will help to reduce housing related diseases.
  • The design needs to provide a water source and adequate sanitation which in turn will help to reduce diseases.

Throughout the world housing construction is increasing, including areas affected by natural hazards, such as cyclones and earthquakes. This increase in population increases the risks of structural damage and loss of life when natural disasters strike. Therefore to ensure that the number of fatalities and damage caused, in areas subject to hazards, are minimal, special precautions and design standards must be adopted (United Nations, 1975).

This report will follow a specific structure in order to obtain an understanding of these precautions and design standards to ensure that the final design meets all the objectives. It will begin by analysing the title in more depth and collecting information that will help to establish the necessary details for designing a low cost earthquake and cyclone resistant house.

Earthquakes

“An earthquake is a spasm of ground shaking caused by a sudden release of energy in the earths lithosphere (i.e. the crust plus part of the upper mantle)” (Dowrick, 1987) “They are among the most destructive natural events [on the planet]” (BBC News, 2005).

Causes, Type And Strength

Earthquakes can vary significantly in their strength, way they are caused and effects they have on the surrounding landscape. They may originate from natural processes such as tectonic activity or human processes such as mining or bomb detonation. Some are very powerful causing large scale damage, injury and/or death whilst others are much weaker.

As suggested by Bolt (2004) there are a number of different types of earthquake and it is useful to classify them in their mode of generation. Each type varies in their strength, how often they occur and level of hazard they pose.

Earthquakes Generated Through Human Processes

Collapse Earthquakes

These relatively small earthquakes involve the collapse of underground mines or caverns. They may be generated through two different processes, either the roof collapses or mine bursting occurs. Mine burst is a process in which the stresses around the cavern or mine cause large pieces of rock to explosively fly off the underground rock face. Both processes induce seismic waves and thus ground shaking.

Explosion Earthquakes

When chemicals or nuclear devices are detonated they can cause the surrounding ground to shake significantly. When nuclear devices are detonated in boreholes beneath the ground enormous nuclear energy is released. This energy then vaporizes the surrounding rock and induces seismic waves and so can generate relatively significant earthquakes.

Impact Earthquakes

Although not so common these earthquakes are generated from the impact of meteorites on the Earth's surface. They strike with such a powerful force that they can generate seismic waves, which travel great distances, such as the 1908 meteorite impact in Siberia that caused a moderately large earthquake.

Earthquakes Generated Through Natural Processes

Land Sliding Earthquakes

Massive landslides can produce substantial earthquakes. For example in Peru, 1974, a large landslide triggered seismic waves comparable to a moderate earthquake. As the soil and rock falls with significant speed the movement is converted to seismic waves and thus an earthquake is generated.

Volcanic Earthquakes

These are simply earthquakes that occur in conjunction with volcanic activity. Earthquakes and volcanoes often accompany each other and both originate through tectonic forces. Sometimes however they do occur individually.

Tectonic Earthquakes

These are the most common type of earthquake. They are produced through various geological processes and are of great social significance because they pose the greatest hazard.

The Earth is made up of a number of layers, the inner and outer core, mantle and the crust that ‘floats' on top. The crust and upper mantle form a strong layer known as the lithosphere and this is broken up into a number of different plates that are moved in different directions through convection currents (BBC News, 2005).

Convection currents are caused due to the heating of rock in the lower part of the mantle. As the temperature of the rock increases it becomes less dense and so begins to rise to the outer region of the mantle, the cooler higher density rock above sinks due to gravity. The cooler rock is then heated as it gets closer to the core of the earth and the rising hot rock cools as it moves further away. The process then continues in the same cycle over millions of years gradually moving the tectonic plates around on the surface. Figure 2.1 shows a diagram of the layers making up the earth and the convection currents and heat loss present.

Subdivisions of the Earth's interior and heat loss via convection in the mantle and outer core.

The plates that make up the Earth's surface are all interconnected much like a jigsaw, as shown by figure 2.1. As they are moved in different directions they are forced into or away from one another at their boundaries. It is at these plate boundaries that most earthquakes occur.

Tectonic Plate Boundaries

There are three main types of plate boundary each with different characteristics, (Platetectonics.com, 2005).

1. Convergent Boundaries: At these boundaries the two plates collide with one another. They are also known as destructive boundaries because the crust is destroyed as one plate is forced beneath the other, forming a subduction zone. There are three types of convergent boundary Oceanic-Oceanic, Continental-Oceanic and Continental-Continental.

Oceanic-Oceanic: This involves two oceanic plates converging (e.g. The Pacific and Mariana Plates). A deep oceanic trench is then formed due to one of the plates sinking beneath the other. Often with this type of convergence volcanoes are formed below the ocean surface and over millions of years of eruptions they build up eventually to be exposed above the surface as volcanic islands usually in chains called island arcs. Figure 2.3 shows a diagram of oceanic-oceanic convergence.

Oceanic-Continental: This involves an oceanic and continental plate colliding, the older and heavy oceanic plate then sinks below the continental forming a trench. An example of this is the Peru-Chile Trench (also known as the Atacama Trench) this is formed due to the oceanic Nazca Plate being subducted beneath the South American Plate. Often deep in the subduction zone the oceanic plate breaks up into smaller pieces and these pieces are locked in place for long periods of time then may suddenly move forming large earthquakes. Figure 2.4 shows a diagram of oceanic-continental convergence.

Continental-Continental: This involves two continental plates, when the two plates collide neither is subducted because they both resist the downward motion. Instead they buckle upwards forming extensive mountain ranges such as the Himalaya's, which continue to grow throughout millions of years of convergence. Figure 2.5 shows a diagram of continental-continental convergence.

2. Divergent Boundaries: At these boundaries the tectonic plates are pushed apart as convection currents move them in different directions. This process then leads to a large separation between the plates and new crust is formed as molten rock rises up from the Earth's core, for this reason they are also known as constructive boundaries. The process can separate whole landmasses over millions of years, into two singular landmasses. This is currently happening throughout Iceland as the Eurasian and North American Plates diverge.

3. Transform-Fault Boundaries: This type of boundary also known as conservative plate boundaries involve two plates sliding past one another. For example the San Andreas Fault between the Pacific and North American Plates. As the plates move in different directions they grind against each other and the friction between them can build up and be released suddenly generating an earthquake.

It is through the geological processes of convergence and divergence that earthquakes are generated. As the plates move elastic strain builds up in the crustal rock and when a fault ruptures the energy stored in the rocks is released, partly as heat, partly in cracking underground rocks, and partly as elastic waves. These waves are the earthquake (Bolt, 2003). This is the theory of elastic rebound; the elastic strain in a block of the Earth's crust over a long period of time can suddenly be released by the movement along a fault, causing an earthquake (Eiby, 1967).

Latin America lies upon five tectonic plates, the Cocos, Caribbean, Nazca, South American and Scotia plates. Together these plates converge and diverge generating many earthquakes throughout Latin America.

Seismic Waves

Although there is a number of ways that earthquakes may be generated the same kind of seismic waves are present in each quake.

An earthquake emits its power as two main types of waves of energy these are body waves and surface waves. Both have different characteristics in the way they travel throughout the earth and damage they cause.

Body Waves

These waves travel through the inners layers of the earth, they arrive before the surface waves and are of a high frequency. There are two types of body wave, primary and secondary.

Primary waves also known as P waves or compressional waves are the fastest type of wave they are able to travel through solid and fluid masses. This means they are the first to be felt during an earthquake, they cause particles to move backwards and forwards in a push and pull motion.

Secondary waves or S waves are slower than primary and can only travel through solid masses. They are the second to be felt during an earthquake and cause particles to move in a side-to-side or up and down motion.

Surface Waves

These waves are only able to travel through the Earth's outer crust. They have a lower frequency than body waves and arrive after. Although they are slower, nearly all damage caused from an earthquake is due to the surface waves. Like body waves there is two types of surface wave, Love and Rayleigh.

Love waves named after A.E.H Love who predicted their existence in 1911 are the fastest type of surface wave and move particles in a side-to-side motion.

Rayleigh waves named after Lord Rayleigh who predicted their existence in 1885 roll across the ground much like a wave in an ocean. They cause particles to move in a side-to-side or up and down motion. Majority of the shaking felt during an earthquake is from the Rayleigh waves (Michigan Tech, 2007).

Strength

When an earthquake occurs both types of wave are emitted as previously discussed, the strength of these waves however varies significantly with each earthquake and so the damage and effects each event has on the surrounding areas can be very different.

The strength of an earthquake is defined in two ways, the intensity of the earthquake (i.e. the strength of shaking at any given place) and the magnitude of the earthquake (i.e. the actual size or total strength of the event). For each type of measurement a scale has been devised, these can then be used to determine the actual specifics of each earthquake.

Intensity measures the severity of the seismic ground motion at a specific point (Dowrick, 1987). This is determined by the Modified Mercalli (MM) Scale, which is the most widely used scale for this type of measurement. It is composed of twelve increasing levels of intensity and at each level a type of response is listed for example damage to windows, people awakening or at higher levels, structures totally destroyed. Appendix A gives a detailed description of the Modified Mercalli Intensity Scale.

Magnitude measures the size of an earthquake at a specific point. It is established using seismographs, which record the various amplitude changes of the ground oscillations below. They record a zig-zag trace and this is then used to determine the magnitude which is found from the logarithm of the amplitude of waves recorded. The data recorded by a seismograph can be used to establish the time, location and magnitude of an earthquake (USGS, 2009).

The Richter scale ranges from 3.5 and below up to 8 and above, the lower the value indicates a weaker earthquake and so higher indicates a much stronger one. The magnitude of the earthquake does not indicate damage however (the Mercalli scale is used for this) because a high magnitude earthquake may occur in a remote region therefore little damaged is caused, on the other hand a weaker event may occur in a densely populated region and thus the damage is greatly increased. Appendix B gives a detailed description of the Richter scale.

Understanding the strength, causes and types of earthquake helps to determine appropriate designs for specific areas of the world. Latin America is in a region that is subjected to earthquakes of varying strengths from frequent occurring events of small magnitudes to much larger events of greater magnitudes and intensity. For example, more recently in Peru (2007) an earthquake of magnitude 8.0 occurred and in 1960 the largest earthquake to be recorded in the world to date, with a magnitude of 9.5, hit Chile. Therefore structures need to be designed to be able to resist forces of varying levels.

Effects Of Earthquakes

“Although a great deal is known about where earthquakes are likely, there is currently no reliable way to predict the days or months when an event will occur in any specific location” (Ludwin, 2004).

Likewise the actual magnitude and intensity of an earthquake cannot be predicted and are only established once the event has taken place. For this reason it is important to know the effects of earthquakes on buildings and thus appropriate methods can be adopted during their design to ensure damage is minimized.

When an earthquake occurs the ground is subjected to various types of seismic waves (as previously mentioned), these waves cause the ground to move in all directions. The most damaging effects on structures are from the horizontal movements of the ground because the majority of structures are designed to withstand vertical loads. Therefore when designing structures to resist earthquake forces the main effect of an earthquake is considered in terms of horizontal forces, similar to wind forces (Ambrose J. & Vergun D, 1995).

Each time a major earthquake occurs an advance in design technology can be made. This is because when an event occurs that results in major structural damage, the effects on the buildings in that area can be investigated. Buildings that have withstood the earthquake forces can be established and the design methods used for these particular buildings used again in the future. Other structures that have failed to withstand the earthquake forces can be investigated and the reason for their failure can be determined, improvements on their design can then be made.

There are a number of hazards that arise from earthquakes and each has different damaging effects (Dowrick, 1987)

  • Direct Movement of Structures - This is due to the ground shaking beneath the structure, it can cause general destabilization of the building and various levels of damage.
  • Ground Displacement Along a Fault - As the ground moves, displacement along a fault may be caused. This in turn can lead to cracking of the ground, settlement of an area, land/mud slides and avalanches.
  • Flooding, Fires, Gas Leaks - When the ground moves various services and structures may be damaged, such as dams, underground piping, river levees and so on, this in turn can cause various types of disaster.
  • Tsunamis - The energy released during an earthquake can cause large tidal waves, which in turn can have devastating effects when they reach the mainland.
  • Liquefaction - When an earthquake is generated it may compact the soils beneath a building, this in turn causes an increase in pore water pressure and causes a loss in shear strength. The soil changes to a liquefied state, this process can have disastrous effects when it occurs below a building.

These hazards in turn have two main physical consequences, death and injury to human beings and damage to the constructed and natural environments. The area is then affected socially and economically because of these physical effects. This can include, cost of damage, losses to businesses and cost of healthcare and aid. Financially and technically it is only possible to reduce these consequences (Dowrick, 1987) and design considerations (Section 4) must be made to ensure that they are reduced.

Although there are a number of effects caused by earthquakes this report is specifically going to look at the effects on structures and how they influence the design.

Tropical Cyclones

Tropical cyclone is the generic name given to warm core, low pressure storm systems that develop over tropical or sub-tropical waters and have organized circulation (NWS JetStream, 2008). The warm central core makes them differ from mid-latitude cyclones and because of this warm-core structure the strongest winds occur at ground levels therefore having the potential to cause significant amounts of damage (Gray, 2003). These rapidly revolving winds can reach speeds of over 160mph and unleash 9 trillion litres of rain a day. They begin as tropical disturbances in warm ocean waters and their wind speeds increase as they are fed from the warm ocean waters. At wind speeds of 38mph they become known as tropical depressions, at 39mph and above they become known as tropical storms and are assigned a name (National Geographic, 2009). Once the system reaches wind speeds of 74mph and above they become classified as hurricanes, typhoons or cyclones depending on the region of the world they occur and can sustain these conditions for several days. In the Eastern Pacific and Atlantic they are known as hurricanes, Western Pacific as typhoons and Indian Ocean as cyclones. Therefore in Latin America they are referred to as hurricanes, during this report however the generic term tropical cyclones will be used (Tinker, 2008).

Every year approximately 80 tropical cyclones occur, two thirds of which attain hurricane intensity and one eighth of this global total occur in the Atlantic alone (to the east of Latin America). Tropical cyclones have a significant effect on the globe. The World Meteorological Organization (WMO) estimates from 1963-1992 tropical cyclones caused almost three times as much damage globally compared to earthquakes and influenced the lives of almost five times as many people. They also account for approximately 50% more deaths than earthquakes (Gray, 2003).

Due to the significant impact that tropical cyclones have on the globe socially, economically and physically it is vital that their formation, characteristics and effects are clearly understood. This in turn can help to ensure structures are correctly designed to resist the forces that they may encounter during a cyclone.

Cause, Structure And Strength

Cause And Structure

The conditions must be just right for a tropical cyclone to form, there are various trigger mechanisms required to transform more frequent storms and tropical depressions into significant tropical cyclones. Cyclones derive their energy from warm moist air, as warm water evaporates from tropical seas energy is transferred into the storm system. The energy is stored within the water vapour of the moist air, as it ascends and condenses the energy is released and causes large cumulus clouds and rain.

As previously mentioned tropical cyclones begin as tropical disturbances (clusters of thunderstorms) over tropical waters, with a minimum temperature of 26°C, they then begin to grow as energy is drawn from the ocean. Warm ocean waters heat the air above their surface, which in turn rises as a current of warm moist air, leaving an area of low pressure at the ocean surface. This low pressure causes trade winds to rush in and these along with the rotation of the Earth cause the storm to begin spinning around a cylinder of relatively still air known as the eye, (spinning clockwise in the Southern Hemisphere and anti-clockwise in the Northern, due to the rotation of the Earth). The rotating winds begin to ascend and release heat and moisture energy before beginning to descend. As heat and moisture energy is released the pressure begins to drop further and at higher altitudes, air then begins to rise faster to fill the area of low pressure and so the amount of warm air drawn from the sea increases. Therefore the storm begins to increase in size and speed developing into a much higher intensity (wind speeds of 74mph and above) (BBC, 2009).

Once a tropical cyclone has formed there are three main parts to the storm, the eye, eye wall and rain or feeder bands. Figure 3.1 and 3.2 show the structure of a tropical cyclone and the three sections present, each section has its own properties and effects on the storm and surrounding areas.

  • The Eye - this is located at the centre of the storm it is the calmest part with a low pressure and light winds no more than 15mph. Air descends in the eye clearing the skies of clouds and produces relatively calm conditions. It can range from 20-30 miles in diameter and usually develops when maximum sustained winds exceed 74mph.
  • The Eye Wall - is a complete or partial ring shaped wall of high velocity winds which surrounds the central eye. It consists of tall thunderstorms that produce the fastest and strongest winds and intense rains, making it the most destructive part of the storm.
  • Feeder/Rain Bands - these are the found at the outer regions of the storm they include bands of gusty winds and rain and indicate the first signs of a storm. They can spread over very large surrounding areas and so can increase the diameter of the storm to distances of 340 miles.

Another feature associated with tropical cyclones is a storm surge. They are caused by the high speed winds and low pressures of a tropical cyclone, as the storm travels across the ocean the winds push water towards the shore. This surge of water then combines with the natural tide to increase the mean sea levels up to 18 feet or more. In turn this has a tremendous impact on coastal areas as large scale flooding occurs. It is the storm surge that causes the greatest loss of life (NOAA, 2007).

Strength

Tropical cyclones can vary significantly in size and strength, some may cause little structural damage or injury whilst others cause major destruction and death, such as Hurricane Mitch in 1998. It is therefore particularly important to be able to measure the scale of cyclones for both prediction purposes and prevention of loss of life and structural damage.

The most widely used and recognised method of measurement for the intensity of tropical cyclones is the Saffir/Simpson scale. This scale was originally developed by Herbert S. Saffir in 1969 to measure the structural effects of tropical cyclones at different wind speeds ranging from 74mph to more than 155mph. It was then added to during the early 1970s by Robert Simpson the then-director of the National Hurricane Centre who also applied storm surge levels and central pressures to the scale (Saffir, 2003). The scale consists of five levels of intensity based on the wind speeds, structural damage and storm surge levels of a cyclone.

  • Wind Speeds are sustained values of one-minute duration at elevations of 10m above the surface.
  • Storm Surge values measured from mean sea level.

Expected Structural Damage (NOAA, 2007)

Category 1 - No real damage to buildings. Damage to unanchored mobile homes and some damage to poorly constructed signs. Also, some coastal flooding and minor pier damage.

Category 2 - Some damage to doors, windows and roofing materials of buildings. Considerable damage to mobile homes. Flooding and damage to piers, small crafts in unprotected anchorages may break their moorings. Some trees blown down.

Category 3 - Some structural damage to small residences and utility buildings. Large trees blown down. Mobile homes and poorly built signs destroyed. Flooding near the coast destroys smaller structures with larger structures damaged by floating debris. Terrain may be flooded well inland.

Category 4 - All trees, shrubs and signs blown down. More extensive curtain wall failures with some complete roof structure failure on small residences. Major erosion of beach areas and terrain may be flooded well inland.

Category 5 - Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. Flooding causes major damage to lower floors of all structures near the shoreline. Massive evacuation of residential areas may be required.

The scale shows the level of destruction cyclones are capable of and properties that they attain at different levels of intensity. Latin America has been subjected to storms of varying levels throughout history, from tropical storms and hurricanes of category 1 or 2 to much stronger and destructive hurricanes such as Hurricane Dean in 2007 and Hurricane Gilbert in 1988 both reaching a level of category 5. Therefore appropriate design methods need to be considered to ensure that the low-cost structure will be able to resist the forces associated with intensities of these levels.

Although tropical cyclones can be predicted and an idea of their strength and location established, they can not be stopped, therefore it is vital to ensure that structures are capable of resisting their effects.

Effects Of Tropical Cyclones

There are a number of hazards associated with tropical cyclones and each has its own effects on the population, environment and structures. The destruction caused from tropical cyclones is often on a very large scale and is the result of the high velocity winds, storm surge or flooding due to heavy rains. The effects each hazard has on structures in particular needs to be considered, this in turn will help during building design.

Storm surges and flooding can have devastating effects on an area they can cause beach erosion, destroy housing, roads and railways and cause major losses of life.

There are several effects that the high velocity winds from a storm have on buildings and this is one of the main aspects of a cyclone to consider throughout building design, the higher the intensity of the storm the faster the wind speeds and higher the levels of damage.

  • Direct Positive Pressure - large forces are produced on the surfaces of a building perpendicular to the wind direction. They receive a direct impact during a storm which can severely damage the structure.
  • Negative Pressure - this occurs on surfaces on the leeward side of the building (sheltered from the impact of the wind). The location of the surfaces however, results in a suction effect being produced.
  • Aerodynamic Drag - surfaces of the building parallel to the direction of the wind may receive positive or negative pressures but also forces of another nature. As the wind hits the structure it flows around it and this produces a drag effect in the direction of the wind.

The above three effects together produce a combined force on the building causing potential uplift and general deformation in the direction of the wind which in turn can damage and move the structure. Figure 3.3 gives diagrams showing these forces.

Figure 3.3 - Forces Generated on a Structure During a Tropical Cyclone

There are other potential effects that can occur to a structure due to the forces of a cyclone, for example, harmonic effects, rocking effects and clean-off effect. Harmonic effects occur due to vibration of the building, this often will occur if the structure has loose connections or weaker, flexible materials. Rocking effects involve rocking of the structure, if the building is highly flexible then the swirling winds are able to rock the structure. Clean-off effects occur when objects protrude from a building, such as lights or chimneys, friction from the wind and drag effects can damage and remove objects of this kind (Ambrose J. & Vergun D, 1995).

The damage and intensity of the effects caused from the wind forces of a cyclone can differ dramatically. There are various factors that influence the scale of the damage, for example shape and location of the structure, topography of the area and wind speed and direction, therefore these will all need to be considered for the design.

Design Considerations

There are many aspects to consider when designing an earthquake and cyclone resistant building and a number of different methods that can be used to ensure its resistance. The main forces created by an earthquake are horizontal and therefore the main hazards from these types of forces are that the walls fall and consequently the roof collapses. The main objective during earthquake resistant design is to ensure the walls are unable to fall and that the roof is securely fixed in place. Likewise the wind forces during a tropical cyclone are applied horizontally, the main objective during design is to ensure forces are reduced and so effects such as drag, uplift and positive or negative pressures are minimised thus reducing the risk of failure and collapse.

Location

The topography of the ground that the building stands on can play a large role in its resistance. It can effect the hazard intensity and so increase or decrease the forces being applied to the structure. Therefore certain types of ground and landscape should be avoided.

Earthquake Resistance

If the land is sloped or uneven then caution must be adopted in the placing of the building. As suggested by Minke (2001) the building should not be cut into the slope (see figure 4.1) this is because the horizontal forces will cause the wall that's cut into the slope to collapse. If cutting is necessary then a retaining wall should be used and the building placed a safe distance from it, see figure 4.2.

The building must not be simply placed on the slope because it may slip down, see figure 4.3. It should not be located too close to the edge of steep slopes or at the bottom of steep slopes (see figure 4.4) because falling rocks and landslides can cause collapse.

The best possible method for sloping landscape is to form a platform in the slope and ensure the building is located at a safe distance from either edge of the platform.

Cyclone Resistance

The effects from a tropical cyclone can be dramatically increased in particular environments. Areas such as the tops of hills, steep slopes, the bottom of open ended valleys or vast open areas all cause an increase in wind velocity, therefore these should be avoided so the effects caused from higher wind speeds are minimised. Coastal regions where possible should be avoided, especially in areas of low lying land as this will reduce the chance of damage from storm surges or flooding.

Sheltering the structure can help to reduce the force levels it endures during a storm. By placing buildings in groups of random placing the wind speeds are reduced. Another method is to use enclosed valleys or natural vegetation, if vegetation is used however, the distance between tree and building should be 1.5 times the height of the tree, in case it falls, which could damage the building (Agarwal A, 2007; Tinker, 2008).

Shape

The shape of the building can have an important influence on its stability and therefore how it will cope when put under seismic or wind forces.

Earthquake Resistance

The building should be simple and symmetrical both in plan and elevation; this ensures ease of construction but also prevents any torsional effects occurring, that may be produced due to lack of symmetry. These torsional effects if produced can be very destructive on the building.

The longer the plan the greater the variation in ground movement and soil conditions over the length of the building. Differences in ground type and movement can affect the building during seismic forces dramatically. For example if one area of the building is located on rocky ground and another is on soft sand then the forces transferred will differ greatly throughout the building which in turn would have very destructive effects. The different soil types will react to seismic forces differently and therefore settlement may occur in one area and not another. Therefore the more compact the plan the better the stability. The best shape to ensure stability is a circular plan as this is very compact; a square plan is also ideal. A rectangular plan may be used however the length should be kept minimal thus keeping the building compact and reducing differences in ground movement and type throughout its length (Dowrick, 1987).

Buildings of H, L, T and Y shape in plan should also be avoided. These shapes are often severely damaged during earthquakes; the orientation of the wings causes them to vibrate differently thus causing high stresses at the junctions and in turn leading to collapse. If these types of plan are used then the building should be separated into simpler rectangular and square shapes with crumple zones joining them. The crumple zones (for example made of chicken wire and plaster) then fail and allow the separated sections to move freely and prevent further damage (Tinker, 2008). Seismic separation joints can also be used which allow some degree of independent movement between the different parts of the structure (Dowrick, 1987; Minke, 2001).

The elevation of the building must also be considered; tall and slender buildings can prove to be very unstable. As the ground moves back and forward during an earthquake the forces transferred cause different sections of the structure to move in opposite directions at the same time, causing a whiplash effect, (Ambrose J. & Vergun D, 1995).

To ensure the problems associated with multi-storeyed buildings do not occur, a single storey building can be used. Not only will this help in the buildings stability but it will also ease construction and lower costs. This will also be more appropriate for a low-income house in a peri-urban area.

Cyclone Resistance

The shape of the building is one of the most important factors in determining cyclone resistance it is also a factor that the designer has control over. As with earthquake resistance the best types of shape are symmetrical, compact and simple in design. Unsymmetrical buildings may result in torsional effects through twisting of the structure, due to the wind forces, this in turn will add to the forces on individual elements.

Circular buildings have the best resistance, they are stable and have good aerodynamic properties therefore reducing the effects such as drag, uplift and positive or negative pressures. Buildings square in plan also have good resistance and their compact shape allows high winds to flow around them. Rectangular structures can be used however they should be kept as compact as possible, the length should not be any greater than 3 times the width. More complex shapes such as the ‘L' shape should be avoided, these give more surface area and recesses for the wind to impact and therefore increase the wind forces and overall negative effects of the storm.

The elevation of the building, shape of roofing and location of openings also influence the effects from the cyclonic storms. Structures should be kept close to the ground to minimise wind resistance, dome like structures further decrease the wind resistance of a structure therefore reducing the cyclonic effects. Tall slender buildings are more likely to overturn and in general have greater horizontal deflections therefore should be avoided.

The location and size of openings plays a major role in the pressure levels present in a structure during a storm. Buildings with large openings or recesses on the windward side of the structure cause a build up in internal pressures. The wind catches in the recesses or flows into the structure, this in turn increases internal pressures and can cause the roof to blow off and walls to effectively explode. Openings should be covered up during a storm to help reduce any increase in pressure, they should also be symmetrically placed and this helps to minimise pressure build up if the covers fail or are not present. Figure 4.7 gives a diagram showing the forces present on a structure due to wind direction and location of openings (Tinker, 2008).

Damage to roofing is very common during tropical cyclones, lightweight roofing is easily blown off through uplifting forces, therefore it needs to be securely fixed to stop this occurring. Flat roofing should be avoided, the roof should be designed with a pitch of no less than 22°, 30° - 40° is best. Hipped roofing gives the best resistance to uplifting forces and ventilation at the ridge of the roof helps to minimise internal pressure build up, therefore where possible should be included in the design. Finally, overhangs must be kept small as pressure can build up beneath them adding to the uplifting and overturning forces on the structure. Figure 4.8 below gives an example of hipped roofing. (Agarwal A, 2007; Ambrose J. and Vergun D, 1995; Tinker, 2008)

Materials

The resistance of a building is greatly influenced by the types of materials used throughout the structure.

“It is seldom possible to use the ideal materials for all elements, as the choice may be dictated by local availability or local construction skill, cost constraints, or political decisions.” (Dowrick, 1987)

This has to be considered particularly for this project because it is aimed specifically for a low-income area of Latin America, therefore this will dictate which materials will be available for the design.

Throughout Latin America there is many resources available for construction, one of the most widely used and key materials is timber, the availability and good properties of this material makes it particularly appealing for construction. Similarly bamboo is a very good material to use in construction, it has a high tensile strength and very good strength to weight ratio and is widely available in Latin America (BMTPC, 2009).

“Bamboo's rapid growth, ease of caring and wide distribution make these plants an ideal renewable resource for the development of local economies in Latin America.” (Londoño,1998)

The high strength to weight ratio also make it particularly resilient against forces created by high winds and earthquakes (BMTPC, 2009)

Bamboo can be used for many sections of a structure including reinforcement, the frame/matrix for walls or water tanks (covered with an earth or cement mortar) and also for roofing. It can be made into mat board flat and corrugated which provides a strong and durable material for sections such as doors, shutters and doorways (BMTPC, 2009).

Adobe Blocks

One of the most widely used building materials throughout the world and in particular regions of Latin America are adobe mud blocks. They are a low cost and readily available form of construction, which can easily be produced by the local community. Adobe blocks are produced from a mix of soil (comprising of clay and sand) and water, the mixture is then placed in a mould (usually a wooden frame) to produce the block shape, the mould is then removed and the block is allowed to dry. Generally adobe structures are self-made because of the ease of construction and often the blocks are made from local soil in close proximity to the building location (Minke G, 2001). Figures 4.9 and 4.10 below show adobe construction being carried out in Peru.

Another method of making adobe blocks is with the use of a CINVA-Ram or CETA-Ram

These are manually operated presses that aid in the making of the blocks, the soil mixture is placed in the mould of the press and then with the pull of a lever the mixture is firmly compacted producing a well defined block. The CETA-Ram works the same as a CINVA-Ram however it has the modification of two cylinders which slide into the block producing two central holes, these are then used to insert reinforcement of bamboo or steel. A disadvantage of these presses is that the overall production in a day is often less than the formwork blocks and the mixture often requires the addition of 4 - 8% cement to ensure stability (Minke, 2001). give images showing the Cinva-Ram Ceta-Rams.

The use of adobe as a building material has many advantages, it is very low-cost, it has good thermal and acoustic properties and it involves simple construction methods. Unfortunately when exposed to seismic forces it does not respond well, the structures are low in strength, brittle and very heavy. Likewise if the structures are exposed to cyclonic effects such as high winds and in particular rain or flooding they do not perform well. These properties therefore often result in severe structural damage and high death levels when hazards occur. In 2001 earthquakes in Peru caused the damage or complete collapse of 61,000 adobe structures, killing 81 people (USAID Peru, 2001; cited in EERI, 2003). Also in 2001, 1,100 people were killed due to the destruction of 200,000 buildings in El Salvador (USAID El Salvador, 2001; cited in EERI, 2003) these figures indicate the importance of the resistance of buildings to natural hazards.

To improve the resistance of adobe structures reinforcement of bamboo cane or steel can be added to the structure which aids in securing the walls together and therefore increasing stability. The inclusion of a ring beam is particularly important as this ensures all walls and roofing are tied together, it may be made from timber or concrete. When producing the blocks themselves additives may be used to strengthen the mixture for example coarse sand and straw help to ensure the mix binds adequately. If the blocks are left to rest for a couple of days this helps to ensure the water distribution and therefore binding of particles is effective throughout the block. Finally simple methods such as allowing the blocks to dry in the shade (preventing cracking) and removal of all foreign particles can be very effective in improving the resistance of the adobe (EERI, 2003).

Quincha (Wattle And Daub)

Quincha is the term used in Latin America for the building system of wattle and daub. It involves a woven timber or bamboo matrix with an earth infill and has been used for many years throughout regions of Latin America. The process begins with vertical elements usually round timber poles being set in the ground, these are then interwoven horizontally with thinner bamboo or reed elements to form a grid. The grid is plastered with a mix of soil and water (much like the adobe mixture) to fill the spacing in the timber matrix.

Although this process has been used for many years, it does have its disadvantages. The coating of earth is often thin and brittle and the timber expands and shrinks causing cracks, insects living in the walls also produce further damage. The Quincha system therefore requires a lot of maintenance and it often results in damage during natural hazards such as earthquakes and tropical cyclones. Seismic forces and high winds can easily damage the brittle walls and due to the earth infill they do not respond well to rain or flooding (Minke, 2001).

In May 1990 an earthquake struck a region of Peru destroying 3,000 and damaging a further 5,000 quincha houses. This in turn influenced the design and construction of a number of improved quincha buildings better at resisting earthquakes, by a development charity called Practical Action. Then in April 1991 another earthquake struck damaging 9,600 homes the buildings that had been constructed using the improved quincha system were able to successfully resist the earthquake forces and therefore a further 4,000 buildings were constructed using this method (Practical Action, 1997).

There are a number of reasons that the improved quincha system is so successful in resisting hazards compared to the traditional quincha method.

  • Concrete is used for the foundations and wall bases to give extra support and stability to the structure and help protect the timber elements in the walls from humidity.
  • Any timber used in the structure is treated with tar or pitch to protect against humidity.
  • The bamboo wall elements are interwoven vertically rather than horizontally for extra stability.
  • The timber columns are concreted into the ground to ensure stability, nails are also embedded into the timber to help with anchorage and prevent uplift during strong winds.
  • All joints are carefully connected and beams and columns are well tied to give extra stability and protection against strong earth movements or winds.
  • Roof elements are nailed to roof beams and are lightweight to reduce potential danger during collapse.
  • Two coats of plaster are used for the walls, the first of which is deeply scored to reduce cracking during shrinkage upon drying this also helps to give a good grip for the second coat and ensure a strong binding to the wall elements (Practical Action, 1997).

These additions, the use of locally available materials and the simple construction methods make the improved quincha system of construction very appealing and widely used in area of Latin America.

Rammed Earth

Rammed earth wall elements involve the compaction of moist earth, formwork is constructed with spacers between. Layers of around 10-15cm of moist earth is then poured into the formwork and compacted using tampers. This may be done manually or using pneumatic tampers, if pneumatic tampers are used however then stronger formwork is required.

Once the earth has dried the formwork is removed leaving a very stable solid wall element, more so than a wall made up of adobe blocks. Although these type of walls are generally very stable, to be able to withstand seismic and wind forces successfully they need to be around 60-100cm thick. The labour intensity and expenses required to produce elements of this size however make them no longer feasible and so adaptations have been made.

There are two methods to adapting and improving rammed wall elements, the first is constructing angular elements and second by including vertical reinforcement.

Angular elements can stabilise thinner rammed walls by giving better stability against lateral forces, the best shapes are L, T, U, X, Y or Z shapes. The corners should also be angled off to help improve strength as shown in figure 4.13 below. Structures should then be made up of these elements as shown in figure 4.14.

Reinforcement of the wall elements involves the addition of bamboo or rods, these are then fixed in the foundations and ring beam above and give extra stability to the structure. Horizontal reinforcement is avoided because this often weakens the walls and causes cracking, it is also difficult to compact the earth effectively.

A good method is to include angular blocks with reinforcement, a method developed in Guatemala, 1978, was to use T shaped elements with four vertical bamboo canes. The elements were placed side by side in between a ring beam and plinth which the bamboo is fixed into. When the earth dries it shrinks and leaves spaces between each of the elements, these form crumple zones which can easily be filled with earth. If the structure is subjected to seismic forces then the elements can deform individually and the crumple zones fail instead, these can then be easily filled afterwards. The roof is not attached to the walls but stands on posts itself, this then allows it to sway separate from the walls and thus causing less damage (Minke, 2001).

Water And Sanitation

Water is a key requirement for healthy living it is required for washing, drinking, cooking, rearing livestock and growing crops. Lack of clean, safe water can put peoples life at risk and cause illness and death with significant issues arising such as diseases and starvation. Sanitation is also a major issue and the correct disposal of excreta is vital, without proper methods in place water and food sources may become contaminated putting people at high risk of excreta-related diseases such as cholera, dysentery, jaundice, diarrhoea, polio, typhoid and worms (Practical Action, 2009).

These are common problems in peri-urban regions and although they have major impacts on the areas, the use of simple systems and resources can help in greatly reducing the issue. The low income areas of Latin America do not have the integrated networking of water and sewerage, therefore other methods need to be considered and installed to ensure that adequate facilities are provided.

Water

There are several methods in obtaining and transporting water to a house, one of the simplest and cheapest are manually operated pumps for groundwater extraction. There are a variety of ways of manual extraction.

The simplest method is open wells, these are vertical open shafts lined with concrete or stone, that can go to depths of 100m, the water can then be extracted using a rope and bucket. If the groundwater is very deep or needs to be accessed via a borehole then pumping may be required.

Shallow-well pumps extract water by sucking up the water through changes in atmospheric pressure, they are suitable for depths up to 7m. There are three main types of shallow-well pump.

  • Shallow-Well Piston Pump - these are widely used as household pumps to supply clean water, they are easy to use and install. They can be adapted to deliver water under pressure to village water mains or to deliver water to areas of higher elevation such as water tanks.
  • Treadle Pump - this type of pump can be produced from locally available materials and thus makes it relatively cheap and simple to construct. Instead of being powered by the arms it is powered by the feet making the extraction easier.
  • Rower Pump - The rower pump is a simplified version of the piston pump it draws up water through a rowing action, it is set at 30° into the ground and can easily be installed and maintained using local skills and materials.
  • Deep-well pumps have the ability to extract water from depths greater than 7m and deep as 150m. There are five main types of deep-well pump.
  • Deep-Well Piston Pump - this kind of pump is much like the shallow version however its main system is below the ground, they are capable of withdrawing from depths up to 100m. Similarly to the shallow piston pump they may be adapted to deliver water at force.
  • Diaphragm Pump - these pumps extract water through contraction and expansion of a flexible diaphragm at the bottom of the well. Unfortunately they are need regular maintenance which in turn makes them relatively expensive.
  • Rope Pump - these involve rotation of a wheel which in turn pulls a rope system of washers through a pipe. The washers are an exact fit to the pipe and so pull water up to the surface, the rope then continues back into the well and up again in a continuous cycle
  • Helical Rotor Pump - these are capable of extracting water from depths up to 100m, rotation of one or two handles at the surface sucks the water to the surface. Maintenance does involve complete removal of the pump using specialist equipment, thus making it relatively expensive and unfeasible for village maintenance.
  • Direct Action Pump - these involve the displacement of water through a pipe upon lowering a handle. They are capable of extracting water from up to 12m and are relatively cheap to maintain and install.

Other systems such as hydraulic ram pumps which extract water using hydropower are available however these can prove to be expensive which is not feasible for low-income peri-urban areas (Practical Action, 2009).

Water storage tanks are a good way of providing a structure with a readily available source of water, these can be fed from mains water sources or used to collect rainwater.

Sanitation

The sanitation of an area involves the removal of excreta and basic hygiene and cleaning facilities. Water plays a major role in sanitation and can ensure a family has the required resources to keep clean and help to remove waste. As previously discussed there are a several ways to provide a house with a water supply. Likewise there are a number of systems to appropriately dispose of excreta products.

A very simple and common type of low-cost toilet is the pit latrine, these are simple and cheap to construct and easy to maintain and use. They consist of a pit covered by a platform with a hole in, the user can then excrete into the pit, this then sits until the latrine is emptied or the pit is covered and latrine relocated. This method of sanitation also does not require a water source which in turn can be an advantage but also lead to poor hygiene and transportation of bacteria to other sources. Furthermore because the pit is located in the ground, contamination of ground water may occur if the pit is not sufficiently lined.

The pit latrine can be adapted to incorporate a pour-flush system, this is slightly more expensive to manufacture and involves a plastic u-bend which creates a water seal, a small amount of water is then poured into the system to flush it. This system reduces flies and odours and also helps to improve general cleanliness. It does however still have some of the disadvantages of the simple pit latrine (Practical Action, 2009).

Another more developed system is the compost toilet this involves a system of two chambers, with defecation hole at the top. The user opens the seal and defecates into the chamber they also urinate into a separate urination funnel. The user then adds some ash, lime or sawdust to the chamber and washes the system down which is then resealed. The urine and wash water runs off to an evaporation plant bed outside, the faeces in the chamber is then allowed to compost. The two chambers allow alternate use and composting of each chamber.

The compost toilet is a very clean and effective low cost system, it may be built beside or as part of a house in peri-urban areas. It does require specific methods of usage and therefore education on use and maintenance is required upon installation (Practical Action, 2009).

A system which is useful for incorporating several buildings is simplified sewerage this is a very effective method and ensures correct disposal of waste.

“Simplified sewerage is an important sanitation option in peri-urban areas of developing countries, especially as it is often the only technically feasible solution in these high-density areas. It is a sanitation technology widely known in Latin America.” (Mara D et al. 2000)

As suggested by Mara et al. this type of system is would be very appropriate for peri-urban areas and is already used throughout regions of Latin America, it will therefore be strongly considered for the final design.

Simplified sewerage involves a system of pipes laid at shallow depths often throughout private land such as front or back yards, the shallow depths allow easy maintenance and access through small access chambers. There are a number of factors that influence the design of the simplified sewerage system, for example:

  • Average household size
  • Average daily water consumption
  • Peak wastewater flow factor
  • Groundwater infiltration
  • Allowance for storm water

Each of these should be considered during the design process, some values that can be considered are the minimum diameter and gradient values. The minimum diameter of the piping should be 100mm and gradient at 1:200, this gradient value is sufficient to allow the waste to flow throughout the pipe and in low-income areas this has not resulted in any significant operational problems (Luduvice, 2000 - cited in Mara et al. 2000). The piping network should follow the contouring of the local area and respect its topography.

Further Aspects Of Consideration

There are many other areas of design that will need to be considered, in particular the actual structural elements making up the building. This includes the roofing, foundations, beams and columns, ring beam, plinth course and rendering. For each of the sections the materials, dimensions and connections used will all need to be considered and appropriately selected to ensure structural resistance against earthquake and cyclone forces.

Design Of Low-Cost Earthquake And Cyclone Resistant House

As mentioned throughout chapter four there are many design aspects to consider, these have all been looked at and the most appropriate methods and systems adopted. This should ensure that the design of the low-cost building is successful and completes all the objectives of this report. Figures 5.1 - 5.13 show the details of the structure giving dimensions and an idea of scale, shape and materials, which are discussed in more detail in section 6. (Practical Action, 1997; City University London, 2009).

Discussion

Discussion Of Final Design

Shape And Layout

As previously mentioned the types of shape most resistant to seismic and wind forces are compact, symmetrical and simple in design. Although a circular shaped structure would have been more ideal for resisting these forces a square shape has been selected. This will still give good stability and will help to make the construction process easier. It will also be ideal for a peri-urban region and the shape will allow several of these structures


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