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.
“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
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.
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.
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.
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.
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.
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.
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.
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).
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 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).
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 establis