Haiti Earthquake 2010

Every year, earthquakes take the lives of thousands of people , and destroy property worth billions. The 2010 Haiti Earthquake killed over 1,50,000 people and destroyed entire cities and villages. Designing Earthquake Resistant Structures is 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.

Index of all posts on Earthquake Resistant Structures

1. Design Earthquake Resistant Buildings | Engineering Tips
2. Earthquakes and Natural Calamities
3. Types of Seismic Waves
4. Hazardous Effects of Earthquakes
5. Effect of Earthquakes on Structures
6. Building Stiffness and Flexibility | Earthquake Engineering
7. Inertial Forces in a Structure
8. Effects of Deformations in Structures
9. Horizontal and Vertical Shaking of a Structure
10. Flow of Inertia Forces to Foundations
11. How Earthquakes affect Reinforced Concrete Buildings
12. Building Planning | Earthquake Resistant Buildings
13. Earthquake Resistant Structures by Planning and Design Approach
14. Design Philosophy of Earthquake Resistant Designs
15. Building Construction Materials for Earthquake Resistance
16. Concept of Earthquake Resistant Engineering
17. Seismic Base Isolation Technique for Building Earthquake Resistance
18. Energy Dissipation Devices for Earthquake Resistant Building Design
19. Active Control Devices for Earthquake Resistance

Natural calamities are the phenomenon which can’t 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 Disasters

What is an Earthquake?

An earthquake is a sudden, rapid shaking of the Earth caused by the breaking and shifting of rock beneath the Earth’s surface. For hundreds of millions of years, the forces of plate tectonics have shaped the Earth as the huge plates that form the Earth’s 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.

Ground shaking from earthquakes can collapse buildings and bridges; disrupt gas, electric, and phone services; and sometimes trigger landslides, avalanches, flash floods, fires, and huge, destructive ocean waves (tsunamis). Buildings with foundations resting on unconsolidated landfill and other unstable soil, and trailers and homes not tied to their foundations are at risk because they can be shaken off their mountings during an earthquake. When an earthquake occurs in a populated area, it may cause deaths and injuries and extensive property damage.

Hence the saying,

“Earthquake don’t kill people, buildings do.”

The dynamic response of building to earthquake ground motion is the most important cause of earthquake-induced damage to buildings. The damage that a building suffers primarily depends not upon its displacement, but upon acceleration. Whereas displacement is the actual distance the ground and building may move during an earthquake, acceleration is a measure of how quickly they change speed as they move. The conventional approach to earthquake resistant design of buildings depends upon providing the building with strength, stiffness and inelastic deformation capacity which are great to withstand a given level of earthquake-generated force. This is generally accomplished through the selection of an appropriate structural configuration and the carefully detailing of structural members, such as beams and columns, and the connections between them.

In contrast, we can say that the basic approach underlying more advanced techniques for earthquake resistance is not to strength the building, but to reduce the earthquake-generated forces acting upon it. By de-coupling the structure from seismic ground motion it is possible to reduce the earthquake-induced forces in it. This can be done in a number of ways. Some popular techniques are

1. Increase natural period of structure by “Base Isolation Techniques”.
2. Increase damping of the system by “Energy Dissipation Devices”.

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 Waves

These 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.

Secondary or shake waves or S waves

Secondary Waves

These are transverse in nature like light waves. The velocity of S waves is about 3.3 km/s. The velocity of S waves depends upon density of the rock and resistance to distortion. The S waves cannot pass through liquids.

Surface waves

When the P-waves and S-waves travel on the surface, they are known as surface waves

L waves and Rayleig Waves

These are also transverse in nature like S waves. The velocity of S waves is about 3.0 km/s. L waves are formed due to dashing of P and S waves against the solid crust of the earth. These are the waves, which we feel in the form of earthquake. These waves are responsible for the destruction of the life and property. Height of L waves is about 30cm and distance between two successive crests is about 10 m. and increase in their amplitude beyond only 1/16 of an inch is capable of causing lot of destruction. Due to high velocity of these waves, the civil engineering structures vibrate and a typical sound due to passage of energy is heard.

Earthquakes cause massive vibrations in the Earth’s 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 earthquake

Ground Motion

The 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 cause’s 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.

Most deaths, injuries, damages and economic losses caused by earthquake result from ground motion acting on buildings and other manmade structures not capable of withstanding such movement.

Ground Failure

Strong ground motion is also the primary cause of damages to the ground and soil upon which, or in which, people must build. These damages to the soil and ground can take a variety of forms: cracking and fissuring and weakening, sinking, settlement and surface fault displacement.

One of the most important types of ground failure is known as liquefaction. Liquefaction takes place when loosely packed, water-logged sediments at or near the ground surface lose their strength in response to strong ground shaking. Liquefaction occurring beneath buildings and other structures can cause major damage during earthquakes.

Ground Sliding

Strong ground motion is also the primary cause of damages to the ground and soil upon which, or in which, people must build. These damages to the soil and ground can take a variety of forms: cracking and fissuring and weakening, sinking, settlement and surface fault displacement.

Ground Tilting

Sometimes, due to earthquake, there is tilting action in the ground. This causes plain land to tilt, causing excessive stresses on buildings, resulting in damage to buildings.

Differential Settlement

If a structure is built upon soil which is not homogeneous, then there is differential settlement, with some part of the structure sinking more than other. This induces excessive stresses and causes cracking.

Soil Liquefaction

During an earthquake, significant damage can result due to instability of the soil in the area affected by internal seismic waves.  The soil response depends on the mechanical characteristics of the soil layers, the depth of the water table and the intensities and duration of the ground shaking.  If the soil consists of deposits of loose granular materials it may be compacted by the ground vibrations induced by the earthquake, resulting in large settlement and differential settlements of the ground surface. This compaction of the soil may result in the development of excess hydrostatic pore water pressures of sufficient magnitude to cause liquefaction of the soil, resulting in settlement, tilting and rupture of structures

Violent Ground Motion During Earthquakes

The 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 earth’s 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 2010

Real earthquake ground motion at a particular building site is vastly more complicated than the simple wave form. Here it’s 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.

Building Frequency and Period

The characteristics of earthquake ground motions which have the greatest importance for buildings are the duration, amplitude (of displacement, velocity and acceleration) and frequency of ground motion.

Frequency

Frequency is defined as the number of complete cycles of vibration made by the wave per second.

Here we can consider a complete vibration to be the same as the distance between one crest of the wave and the next, in other words one full wavelength. Surface ground motion at the building site, then, is actually a complex superposition of vibration of different frequencies. We should also mention that at any given site some frequencies usually predominate.

The response of building to the ground motion is as complex as the ground motion itself, yet typically quite different. It also begins to vibrate in a complex manner, and because it is now a vibratory system, it also posses a frequency content. However, the buildings vibrations tend to center around one particular frequency, which is known as its natural or fundamental frequency. In general…

The shorter a building is, the higher its natural frequency. The taller the building is, the lower its natural frequency

Period

The natural period is the time it takes for the building to make one complete vibration.

The relationship between frequency F and period T is thus given as

T = 1 / F

This means that a short building with a high natural frequency also has a short natural period. Conversely, a very tall building with a low frequency has a long period.

Displacement of Building according to their Height & Stiffness

Ductility is the ability to undergo distortion or deformation without resulting in complete breakage or failure.  To see how ductility can improve a building’s 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.

Inertial Forces in a Structure

This is much like the situation that you are faced with when the bus you are standing in suddenly starts, your feet move with the bus, but your upper body tends to stay back making you fall backwards!

This tendency to continue to remain in the previous position is known as inertia. In the building since the walls or columns are flexible, the motion of roof is different from that of ground.

Consider a building, whose roof is supported on columns. Coming back to the analogy of yourself on the bus; when the bus suddenly starts, you are thrown backwards as if someone has applied a force on the upper body. Similarly, when the ground moves, even the building is thrown backwards, and the roof experiences a force, called inertia force. If the roof has the mass M and experiences an acceleration a, then from Newton’s second law of motion, the inertia force F1 is mass M times acceleration a, and its direction is opposite to that of the acceleration.

“More mass means higher inertia force”

Deformation in a Structure

In the figure above, this movement is shown as quantity u between the roof and the ground. But, given a free option, columns would like to come back to the straight vertical position, i.e. columns resist deformations. In the straight vertical position, the columns carry no horizontal earthquake force through them. But, when forced to bend, they develop internal forces. The larger is the horizontal displacement u between the top and bottom of the column, the larger this internal force in columns. Also, the stiffer the columns are (i.e. bigger is the column size), larger is the force. For this reason, these internal forces in the columns are called stiffness forces. In fact, the stiffness force in the columns is the column stiffness times the relative displacement between its ends

Horizontal and Vertical Shaking

All structures are primarily designed to carry the gravity loads, i.e. they are designed for a force equal to the mass M (this includes mass due to own weight and imposed loads) times the acceleration due to gravity g acting in vertical downward direction (- Z). The downward force Mg is called the gravity load. The vertical acceleration during ground shaking either adds or subtracts from the acceleration due to gravity. Since factors of safety are used in the design of structures to resist the gravity load, usually most structures tend to be adequate against vertical shaking.

However, horizontal shaking along X and Y directions (both + and – directions of each) remains a concern. Structures designed for gravity loads, in general, may not be able to safely sustain the effects of horizontal earthquake shaking.

Flow of Inertia Forces to Foundation

Under horizontal shaking of ground, horizontal inertia forces are generated at a level of the mass of the structure (usually situated at the floor levels). These lateral inertia forces are transferred by the floor slab to the walls or the columns, to the foundations, and finally to the soil system underneath. So, each of this structural elements (floor slabs, walls, columns, and foundations) and the connections between them must be designed to safely transfer these inertia forces through them.

Walls or columns are the most critical elements in transferring the inertia forces. But, in traditional construction, floor slabs and beams receive more care and attention during design and construction, than walls and columns. Walls are relatively thin and often made of brittle material like masonry.