earthquake Resistant Structures | Engineering TipsEarthquakes are a major geological phenomena. Man has been terrified of this phenomena for ages, as little has been known about the causes of earthquakes, but it leaves behind a trail of destruction. There are hundreds of small earthquakes around the world everyday. Some of them are so minor that humans cannot feel them, but seismographs and other sensitive machines can record them. Earthquakes occur when tectonic plates move and rub against each other. Sometimes, due to this movement, they snap and rebound to their original position. This might cause a large earthquakes as the tectonic plates try to settle down. This is known as theElastic Rebound Theory.

Haiti Earthquake 2010Every year, earthquakes take the lives of thousands of people , and destroy property worth billions. The2010 Haiti Earthquakekilled over 1,50,000 people and destroyed entire cities and villages. DesigningEarthquake Resistant Structuresis indispensable. It is imperative that structures are designed to resist earthquake forces, in order to reduce the loss of life. The science of Earthquake Engineering and Structural Design has improved tremendously, and thus, today, we can design safe structures which can safely withstand earthquakes of reasonable magnitude.Natural CalamitiesNatural calamities are the phenomenon which cant be prevented, but we can take precautions to minimize their effects. Calamities such as Floods, Cyclones, Volcanic eruptions, Tsunamis and Earthquakes can cause a lot of damage to life and property, and cause disturbance to our day-to-day life.

Natural DisastersWhat is an Earthquake?An earthquake is a sudden, rapid shaking of the Earth caused by the breaking and shifting of rock beneath the Earths surface. For hundreds of millions of years, the forces of plate tectonics have shaped the Earth as the huge plates that form the Earths surface move slowly over, under, and past each other. Sometimes the movement is gradual. At other times, the plates are locked together, unable to release the accumulating energy. When the accumulated energy grows strong enough, the plates break free causing the ground to shake. Most earthquakes occur at the boundaries where the plates meet; however, some earthquakes occur in the middle of plates.

Types of Seismic WavesDuring fault ruptures which cause earthquakes, the sudden breakage and movement along the fault can release tremendous amount of energy. Some of this energy is used up in cracking and pulverizing the rock as the two blocks of rock separated by the fault grind past each other. Part of the energy, however, speeds through the rock as seismic waves. This waves can travel for and cause damage at great distances. Once they start, these waves continue through the earth until their energy is used up.There are two basic types of seismic waves, and they travel at different speeds through earth. The faster p waves and the slower s waves.Primary or push waves or P waves

Primary WavesThese are longitudinal in nature like sound waves. The velocity of P waves is highest about 5.4 km/s and depends on the density of the rock and resistance to compression. P waves can pass through liquids also.Hazardous Effects of EarthquakesEarthquakes cause massive vibrations in the Earths crust. This can cause a number of problems in the ground, which in turn becomes a hazard to all life and property. The effect depends on the geology of soil and topography of the land.

1964 Niigata earthquakeGround MotionThe most destructive of all earthquake hazards is caused by seismic waves reaching the ground surface at places where human-built structures, such as buildings and bridges, are located. When seismic waves reach the surface of the earth at such places, they give rise to what is known as strong ground motion. Strong ground motions causes buildings and other structures to move and shake in a variety of complex ways. Many buildings cannot withstand this movement and suffer damages of various kinds and degrees.

Effect of Earthquakes on StructuresViolent Ground Motion During EarthquakesThe seismic waves travel for great distances before finally losing most of their energy. At some time after their generation, these seismic waves will reach the earths surface, and set it in motion, which we surprisingly refer to as earthquake ground motion. When this earthquake ground motion occurs beneath a building and when it is strong enough, it sets the building in motion, starting with the buildings foundation, and transfers the motion throughout the rest of building in a very complex way. These motions in turn induce forces which can produce damage.

Haiti Earthquake Damage 2010Real earthquake ground motion at a particular building site is vastly more complicated than the simple wave form. Here its useful to compare the surface of ground under an earthquake to the surface of a small body of water, like a pond. You can set the surface of a pond in motion by throwing stones into it. The first few stones create a series of circular waves, which soon being to collide with one another. After a while, the collisions, which we term interference patterns, are being to predominate over the pattern of circular waves. Soon the entire surface of water is covered by ripples, and you can no longer make out the original wave forms. During an earthquake, the ground vibrates in a similar manner, as waves of different frequencies and amplitude interact with one another.

Disaster-Proof Architecture: 13 Super-Strong StructuresByStephinArt & Design

High-profile earthquakes, tsunamis, floods, hurricanes and other natural disasters have made it more clear than ever that in the face of climate change, stronger buildings able to withstand such events are not just advisable but necessary. These 13 designs range from fantastical concepts for entire floating cities to real homes that have already proven themselves disaster-proof, and from large-scale billion-dollar projects to low-cost housing solutions for the poor.Earthquake-Proof Coral Reef Island for Haiti

(images via:vincent callebaut)After 2009s massive earthquake wiped out much of Haitis infrastructure, the nation is still struggling to rebuild, and imaginative architect Vincent Callebaut has a suggestion: disaster-proof floating housinginspired by coral reefs. The Coral Reef Project consists of 1000 modular residences in dual wavy stacks, supported on an artificial pier built on seismic piles in the Caribbean. With energy harvested from the waves, hydro-turbines and sea thermal energy conversion, the structure improves the standard of living, providing green terraces for each plug-in pod and simplifying delivery of supplies.Soccer Ball-Shaped Floating Houses

(images via:treehugger)From a doghouse to a 540-square-foot family dwelling, the Barier is an earthquake-proof home shaped like a soccer ball that becomes a floating rescue ship in the event of a natural disaster. The 32-sided urethane-walled surface of the house distributes force, and the base acts as a ballast, ensuring that it stays upright if swept away in a tsunami.Noahs Ark A Floating Hotel

(images via:yellowpelow)In the event of an earthquake or flood, this hotel would be one of the safest buildings in town. The concept, designed by Remistudio, is structured to resist seismic impact and has an entirely transparent facade to create a biosphere that could allow food production if necessary. Solar panels and rainwater collection would provide inhabitants with energy and water and the bottom half of the hotel rests in a depression in the ground, allowing it to come loose and float.Earthquake-Proof Solar-Powered Volcano Towers

(images via:ofis)Looking like a strange sort of man-made volcano, the All-Seasons Tent Tower by OFIS Architecture is a multi-function cylindrical tower powered with solar energy and covered in a mesh skin that filters sunlight for temperature regulation. A system of concrete cores protects the interior filled with apartments, shops, restaurants, offices and recreational space from the impact of earthquakes.Harvest City: Floating Concept for Haiti

(images via:yanko design)Yet another natural disaster-proof concept takes Haiti from the land to the sea, creating an offshore haven complete with agriculture and industry. Harvest City by E. Keven Schopfer is a complex of floating modules measuring 2 miles in diameter, with four zones connected by a linear system of canals. Cables secure the whole complex, which includes a harbor city center, to the sea bed. The design even makes use of debris from the 2009 earthquake, putting concrete rubble to work as breakwater filler.Sticky Rice Mortar in China

(image via:physorg)Ancient Chinese construction workers found a secret recipe for mortar that has helped their buildings survive for centuries: its made with sticky rice.Chemists determinedin 2010 that a complex carbohydrate in the sticky rice soup which was mixed with lime and used to fill in gaps between stones over 1500 years ago is largely responsible for the strength of the structures, which have withstood multiple earthquakes and even bulldozers.Analytical study shows that the ancient masonry mortar is a kind of special organic-inorganic composite material, the scientists explained. The inorganic component is calcium carbonate, and the organic component is amylopectin, which comes from the sticky rice soup added to the mortar. Moreover, we found that amylopectin in the mortar acted as an inhibitor: The growth of the calcium carbonate crystal was controlled, and a compact microstructure was produced, which should be the cause of the good performance of this kind of organic-organic mortar.Floating Shipping Container Houses for Pakistan

(images via:inhabitat)Millions of people remain homeless in Pakistan after disastrous 2010 floods could low-cost, eco-friendly shipping container houses be the solution? The Amphibious Container concept by Richard Moreta is made with reused shipping crates and pallets, resting on a foundation of truck inner tubes which serve as a flotation device in the event of high waters. It can handle a maximum water level of 7.5 feet.Lilypad Floating City Concept

(images via:vincent callebaut)Floating mega-cities are Vincent Callebauts specialty, and theLilypad Floating Ecopolisis an especially beautiful example of imagination run wild. Designed for ecological refugees in the year 2100, the Lilypad is an amphibious self-sufficient city able to accommodate 50,000 people along with enough plants and animals to sustain them. The lower portion includes a submerged lagoon which filters rainwater.Low-Income Disaster-Proof Bamboo Housing

(images via:inhabitat)What if we could keep all of a homes key elements in a disaster-proof core, surrounded by a bamboo structure that would be inexpensive to replace if a natural disaster destroyed it? Thats the idea behind this low-income housing concept by a group of Indian architects, a design that won the Design Against the Elements competition to create disaster-proof housing. Each three-story apartment complex contains an earthquake, wind and water-resistant core holding water and power lines, bathrooms, kitchens and stairways and an escape hatch to the roof. This provides a safe haven for a low cost, raising survival rates among the most vulnerable populations.Hurricane-Proof Dome House in Florida

(images via:cyber sharp)There are lots of cool concepts, but what about disaster-proof homes that have already been built and proven effective? This unusual-looking dome house in Pensacola Beach, Florida has survived four hurricanes including the devastating Katrina, Dennis and Ivan. Owners Mark and Valerie Sigler came up with this $7 million design after Hurricane Opal destroyed their previous house in 1995, leaving them without a residence for 14 months. During Hurricane Dennis in 2004, an NBC News crew stayed in the house and had this to say about it: You have a one-piece concrete house with five miles of steel in it. The house did exactly what its supposed to do.Raised Home Escapes Hurricanes, Brush Fires & Floods

(images via:inhabitat)The owners of this raised house, located on an island off the coast of South Carolina, were determined that their home be able to survive brush fires, hurricanes and floods. The resulting off-grid pre-fabricated house made of recycled steel and SIP panels is engineered to FEMA flood zone requirements and built on helical foundations to withstand 140-mile-per-hour winds. All that space under the house isnt wasted in fair weather, it functions as a screened-in shade porch.The Citadel: Floating Apartment Complex in the Netherlands

(images via:citadelhetnieuwewater.nl)Not content to simply talk about the dangers of rising sea levels (like much of the rest of the world), the Dutch have begun taking matters into their own hands with architecture that can withstand dramatic changes in the canals that are such an integral part of the Netherlands. As part of a new development called New Water, Koen Oltuls of Waterstudio designed The Citadel, Europes first floating apartment complex. 60 luxury apartments, a car park, a floating road and boat docks will work with the changing water levels rather than against themFoundation (9 Boxes): Absurdist Architecture by Luke OSullivan

(images via:luke osullivan)Technically, this isnt an architecture concept; its a work of art screenprint on wood by Luke OSullivan. But Foundation (9 Boxes) still offers an absurdist take on solutions to flood-proof housing, and one that makes a very simple point: build higher.Says the artist, Foundation (9 Boxes) was inspired by dystopian films, absurd architectural concepts, and natural disaster prevention. It was around the time when the housing market crashed, and I was thinking a lot about modular housing units, and faades.

The taller a building, the longer its natural period tends to be. But the height of a building is also related to another important structural characteristic: the building flexibility. Taller buildings tend to be more flexible than short buildings. (Only consider a thin metal rod. If it is very short, it is difficulty to bend it in your hand. If the rod is somewhat longer, and of the same diameter, it becomes much easier to bend. Buildings behave similarly) we say that a short building is stiff, while a taller building is flexible. (Obviously, flexibility and stiffness are really just the two sides of the same coin. If something is stiff, it isnt flexible and vice-versa).

Displacement of Building according to their Height & StiffnessDuctility is the ability to undergo distortion or deformation without resulting in complete breakage or failure. To see how ductility can improve a buildings performance during an earthquake, see the above figure. In response to the ground motion, the rod bends but does not break. (of course, metals in general are more ductile than materials such as stone, brick and concrete) The ductility of a structure is in fact one of the most important factors affecting its earthquake performance. One of the primary tasks of an engineer designing a building to be earthquake resistant is to ensure that the building will possess enough ductility to withstand the size and types of earthquakes it is likely to experience during its lifetime.

IntroductionThe conventional approach to earthquake resistant design of buildings depends upon providing the building with strength, stiffness and inelastic deformation capacity which are great enough to withstand a given level of earthquakegenerated force. This is generally accomplished through the selection of an appropriate structural configuration and the careful detailing of structural members, such as beams and columns, and the connections between them.

(fig. 1).

In contrast, we can say that the basic approach underlying more advanced techniques for earthquake resistance is not to strengthen the building, but to reduce the earthquakegenerated forces acting upon it. Among the most important advanced techniques of earthquake resistant design and construction are base isolation and energy dissipation devices.Base IsolationIt is easiest to see this principle at work by referring directly to the most widely used of these advanced techniques, which is known as base isolation. A base isolated structure is supported by a series of bearing pads which are placed between the building and the building's foundation.(See Figure 1) A variety of different types of base isolation bearing pads have now been developed. For our example, we'll discuss leadrubber bearings. These are among the frequentlyused types of base isolation bearings. (See Figure 2) A leadrubber bearing is made from layers of rubber sandwiched together with layers of steel. In the middle of the bearing is a solid lead "plug." On top and bottom, the bearing is fitted with steel plates which are used to attach the bearing to the building and foundation. The bearing is very stiff and strong in the vertical direction, but flexible in the horizontal direction.Earthquake Generated Forces

(fig. 2)To get a basic idea of how base isolation works, first examine Figure 3. This shows an earthquake acting on both a base isolated building and a conventional, fixedbase, building. As a result of an earthquake, the ground beneath each building begins to move. In Figure 3, it is shown moving to the left.Each building responds with movement which tends toward the right. We say that the building undergoes displacement towards the right. The building's displacement in the direction opposite the ground motion is actually due to inertia. The inertial forces acting on a building are the most important of all those generated during an earthquake.It is important to know that the inertial forces which the building undergoes are proportional to the building's acceleration during ground motion. It is also important to realize that buildings don't actually shift in only one direction.Because of the complex nature of earthquake ground motion, the building actually tends to vibrate back and forth in varying directions. So, Figure 3 is really a kind of "snapshot" of the building at only one particular point of its earthquake response.

(fig. 3)In addition to displacing toward the right, the unisolated building is also shown to be changing its shape from a rectangle to a parallelogram. We say that the building is deforming. The primary cause of earthquake damage to buildings is the deformation which the building undergoes as a result of the inertial forces acting upon it.The different types of damage which buildings can suffer are quite varied and depend upon a large number of complicated factors. But to take one simple example, one can easily imagine what happens to two pieces of wood joined at a right angle by a few nails, when the very heavy building containing them suddenly starts to move very quickly the nails pull out and the connection fails.Response of Base Isolated BuildingBy contrast, even though it too is displacing, the baseisolated building retains its original, rectangular shape. It is the leadrubber bearings supporting the building that are deformed. The baseisolated building itself escapes the deformation and damagewhich implies that the inertial forces acting on the baseisolated building have been reduced.Experiments and observations of baseisolated buildings in earthquakes have been shown to reduce building accelerations to as little as 1/4 of the acceleration of comparable fixedbase buildings, which each building undergoes as a percentage of gravity. As we noted above, inertial forces increase, and decrease, proportionally as acceleration increases or decreases.Acceleration is decreased because the base isolation system lengthens a building's period of vibration, the time it takes for the building to rock back and forth and then back again. And in general, structures with longer periods of vibration tend to reduce acceleration, while those with shorter periods tend to increase or amplify acceleration.Finally, since they are highly elastic, the rubber isolation bearings don't suffer any damage. But what about that lead plug in the middle of our example bearing? It experiences the same deformation as the rubber. However, it also generates heat as it does so.In other words, the lead plug reduces, or dissipates, the energy of motioni.e., kinetic energyby converting that energy into heat. And by reducing the energy entering the building, it helps to slow and eventually stop the building's vibrations sooner than would otherwise be the case in other words, it damps the building's vibrations. (Damping is the fundamental property of all vibrating bodies which tends to absorb the body's energy of motion, and thus reduce the amplitude of vibrations until the body's motion eventually ceases.)Spherical Sliding Isolation SystemsAs we said earlier, leadrubber bearings are just one of a number of different types of base isolation bearings which have now been developed. Spherical Sliding Isolation Systems are another type of base isolation. The building is supported by bearing pads that have a curved surface and low friction.

(fig. 4)During an earthquake, the building is free to slide on the bearings. Since the bearings have a curved surface, the building slides both horizontally and vertically (See Figure 4.) The force needed to move the building upwards limits the horizontal or lateral forces which would otherwise cause building deformations. Also, by adjusting the radius of the bearing's curved surface, this property can be used to design bearings that also lengthen the building's period of vibration.For more information read this article titledProtective Systems for Buildings: Application of Spherical Sliding Isolation Systemsas it describes one particular type of spherical sliding isolation system, and its successful use in making some structures more earthquake resistant.Energy Dissipation DevicesThe second of the major new techniques for improving the earthquake resistance of buildings also relies upon damping and energy dissipation, but it greatly extends the damping and energy dissipation provided by leadrubber bearings.As we've said, a certain amount of vibration energy is transferred to the building by earthquake ground motion. Buildings themselves do possess an inherent ability to dissipate, or damp, this energy. However, the capacity of buildings to dissipate energy before they begin to suffer deformation and damage is quite limited.The building will dissipate energy either by undergoing large scale movement or sustaining increased internal strains in elements such as the building's columns and beams. Both of these eventually result in varying degrees of damage. So, by equipping a building with additional devices which have high damping capacity, we can greatly decrease the seismic energy entering the building, and thus decrease building damage.Accordingly, a wide range of energy dissipation devices have been developed and are now being installed in real buildings. Energy dissipation devices are also often called damping devices. The large number of damping devices that have been developed can be grouped into three broad categories: Friction Dampers these utilize frictional forces to dissipate energy Metallic Dampers utilize the deformation of metal elements within the damper Viscoelastic Dampers utilize the controlled shearing of solids Viscous Dampers utilized the forced movement (orificing) of fluids within the damperFluid Viscous DampersOnce again, to try to illustrate some of the general principles of damping devices, we'll look more closely at one particular type of damping device, the Fluid Viscous Damper, which is one variety of viscous damper that has been widely utilized and has proven to be very effective in a wide range of applications.The article, titledApplication of Fluid Viscous Dampers to Earthquake Resistant Design, describes the basic characteristics of fluid viscous dampers, the process of developing and testing them, and the installation of fluid viscous dampers in an actual building to make it more earthquake resistant.Damping Devices and Bracing Systems

(fig. 5)Damping devices are usually installed as part of bracing systems. Figure 5 shows one type of damperbrace arrangement, with one end attached to a column and one end attached to a floor beam. Primarily, this arrangement provides the column with additional support.Most earthquake ground motion is in a horizontal direction; so, it is a building's columns which normally undergo the most displacement relative to the motion of the ground. Figure 5 also shows the damping device installed as part of the bracing system and gives some idea of its action.Resource