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

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.

“If we have a poor configuration to start with, all the engineer can do is to provide a band-aid – improve a basically poor solution as best as he can. Conversely, if we start-off with a good configuration and reasonable framing system, even a poor engineer cannot harm its ultimate performance too much”.

Size of Buildings

Size of Buildings

In tall buildings with large weight-to-base size ratio the horizontal movement of the floors during ground shaking is large. In short but very long buildings, the damaging effects during earthquake shaking are many. And, in buildings with large plan area, the horizontal seismic forces can be excessive to be carried by columns and walls.

Horizontal Layout of Buildings

Buildings with simple geometry in plan perform well during strong earthquakes. Buildings with re-entrant corners, like U, V, H and + shaped in plan sustain significant damage. The bad effects of these interior corners in the plan of buildings are avoided by making the buildings in two parts by using a separation joint at the junction.

Vertical Layout of Buildings

Vertical Layout of Buildings

Earthquake forces developed at different floor levels in a building need to be brought down along the height to the ground by the shortest path, any deviation or discontinuity in this load transfer path results in poor performance of building. Buildings with vertical setbacks cause a sudden jump in earthquake forces at the level of discontinuity. Buildings that have fewer columns or walls in a particular storey or with unusually tall storey tend to damage or collapse which is initiated in that storey. Buildings on sloppy ground have unequal height columns along the slope, which causes twisting and damage in shorter columns that hang or float on beams have discontinuity in load transfer. Buildings in which RC walls do not go all the way to the ground but stop at upper levels get severely damaged

Adjacency of Buildings

Adjacency of Buildings

When two buildings are close to each other, they may pound on each other during strong shaking. When building heights do not match the roof of the shorter building may pound at the mid- height of the column of the taller one; this can be very dangerous.

Brittle and Ductile Building Materials

Construction Materials

I. Masonry

Masonry is made up of burnt clay bricks and cement or mud mortar. Masonry can carry loads that cause compression (i.e. pressing together) but can hardly take load that causes tension (i.e. pulling apart). Masonry is a brittle material, these walls develop cracks once their ability to carry horizontal load is exceeded. Thus infill walls act like sacrificial fuses in buildings: they develop cracks under severe ground shaking but they share the load of the beams and columns until cracking.

II. Concrete

Concrete is another material that has been popularly used in building construction particularly over the last four decades. Cement concrete is made of crushed stone pieces (called aggregate), sand, cement and water mixed in appropriate proportions. Concrete is much stronger than masonry under compressive loads, but again its behavior in tension is poor. The properties of concrete critically depend on the amount of water used in making concrete, too much and too little water both can cause havoc.

III. Steel

Steel is used in masonry and concrete buildings as reinforcement bars of diameter ranging from 6mm to 40mm. reinforcing steel can carry both tensile and compressive loads. Moreover steel is a ductile material. This important property of ductility enables steel bars to undergo large elongation before breaking. Concrete is used with steel reinforcement bars. This composite material is called as reinforced cement concrete. The amount and location of steel in a member should be such that the failure of the member is by steel reaching its strength in tension before concrete reaches its strength in compression. This type of failure is ductile failure, and is preferred over a failure where concrete fails first in compression. Therefore, providing more steel in R.C. buildings can be harmful even!!

]]> http://articles.architectjaved.com/earthquake_resistant_structures/building-construction-materials-for-earthquake-resistance/feed/ 8 http://articles.architectjaved.com/earthquake_resistant_structures/concept-of-earthquake-resistant-engineering/ http://articles.architectjaved.com/earthquake_resistant_structures/concept-of-earthquake-resistant-engineering/#comments Tue, 15 Jun 2010 00:05:35 +0000 Architect http://articles.architectjaved.com/earthquake_resistant_structures/?p=67 If two bars of same length and same cross-sectional area – one made of ductile material and another of a brittle material. And a pull is applied on both bars until they break, then we notice that the ductile bar elongates by a large amount before it breaks, while the brittle bar breaks suddenly on reaching its maximum strength at a relative small elongation. Amongst the materials used in building construction, steel is ductile, while masonry and concrete are brittle.

Comparison of Brittle and Ductile Building materials

The correct building components need to be made ductile. The failure of columns can affect the stability of building, but failure of a beam causes localized effect. Therefore, it is better to make beams to be ductile weak links then columns. This method of designing RC buildings is called the strong-column weak-beam design method. Special design provisions from IS: 13920-1993 for RC structures ensures that adequate ductility is provided in the members where damage is expected.

Quality Control in Construction

The capacity design concept in earthquake resistant design of buildings will fail if the strengths of the brittle links fall below their minimum assured values. The strength of brittle construction materials, like masonry and concrete is highly sensitive to the quality of construction materials. Workmanship, supervision, and construction methods. Similarly, special care is needed in construction to ensure that the elements meant to be ductile are indeed provided with features that give adequate ductility. Thus, strict adherence to prescribed standards, of construction materials and processes is essential in assuring an earthquake resistant building. Regular testing of materials to laboratories, periodic training of workmen at professional training houses, and on-site evaluation of the technical work are elements of good quality control.

Popular Earthquake Resistant Techniques

Conventional seismic design attempts to make buildings that do not collapse under strong earthquake shaking, but may sustain damage to non-structural elements (like glass facades) and to some structural members in the building. This may render the building non-functional after the earthquake, which may be problematic in some structures, like hospitals, which need to remain functional in the aftermath of earthquake. Special techniques are required to design buildings such that they remain practically undamaged even in a severe earthquake. Buildings with such improved seismic performance usually cost more than the normal buildings do.

Two basic technologies are used to protect buildings from damaging earthquake effects. These are Base Isolation Devices and Seismic Dampers. The idea behind base isolation is to detach (isolate) the building from the ground in such a way that earthquake motions are not transmitted up through the building or at least greatly reduced. Seismic dampers are special devices introduced in the buildings to absorb the energy provided by the ground motion to the building (much like the way shock absorbers in motor vehicles absorb due to undulations of the road)

Base Isolation Technique

The concept of base isolation is explained through an example building resting on frictionless rollers. When the ground shakes, the rollers freely roll, but the building above does not move. Thus, no force is transferred to the building due to the shaking of the ground; simply, the building does not experience the earthquake.

Now, if the same building is rested on the flexible pads that offer resistance against lateral movements (fig 1b), then some effect of the ground shaking will be transferred to the building above. If the flexible pads are properly chosen, the forces induced by ground shaking can be a few times smaller than that experienced by the building built directly on ground, namely a fixed base building (fig 1c). The flexible pads are called base-isolators, whereas the structures protected by means of these devices are called base-isolated buildings. The main feature of the base isolation technology is that it introduces flexibility in the structure.

As a result, a robust medium-rise masonry or reinforced concrete building becomes extremely flexible. The isolators are often designed, to absorb energy and thus add damping to the system. This helps in further reducing the seismic response of the building. Many of the base isolators look like large rubber pads, although there are other types that are based on sliding of one part of the building relative to other. Also, base isolation is not suitable for all buildings. Mostly low to medium rise buildings rested on hard soil underneath; high-rise buildings or buildings rested on soft soil are not suitable for base isolation.

Concept of Base Isolation

Lead-rubber bearings are the frequently-used types of base isolation bearings. A lead rubber bearing is made from layers of rubber sandwiched together with layers of steel. In the middle of the 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.

How it Works

To get a basic idea of how base isolation works, first examine the above diagram. This shows an earthquake acting on base isolated building and a conventional, fixed-base, building. As a result of an earthquake, the ground beneath each building begins to move. . Each building responds with movement which tends towards the right. The buildings displacement in the direction opposite the ground motion is actually due to inertia. The inertia forces acting on a building are the most important of all those generated during an earthquake.

In addition to displacing towards right, the un-isolated 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 upon it.

Response of Base Isolated Buildings

The base-isolated building retains its original, rectangular shape. The base isolated building itself escapes the deformation and damage-which implies that the inertial forces acting on the base isolated building have been reduced. Experiments and observations of base-isolated buildings in earthquakes to as little as ¼ of the acceleration of comparable fixed-base buildings.

Acceleration is decreased because the base isolation system lengthens a buildings period of vibration, the time it takes for a 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.

Spherical Sliding Base Isolation

Spherical Sliding Base Isolation

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. 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. The forces 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 bearings curved surface, this property can be used to design bearings that also lengthen the buildings period of vibration

Energy Dissipation Devices

Commonly used Seismic Dampers

1. Viscous Dampers (energy is absorbed by silicone-based fluid passing between piston cylinder arrangement),
2. Friction Dampers (energy is absorbed by surfaces with friction between them rubbing against each other),
3. Yielding Dampers (energy is absorbed by metallic components that yield).
4. Viscoelastic Dampers (energy is absorbed by utilizing the controlled shearing of solids).

Thus by equipping a building with additional devices which have high damping capacity, we can greatly decrease the seismic energy entering the building.

How it Works?

How Dampers Work

The construction of a fluid damper is shown in (fig). It consists of a stainless steel piston with bronze orifice head. It is filled with silicone oil. The piston head utilizes specially shaped passages which alter the flow of the damper fluid and thus alter the resistance characteristics of the damper. Fluid dampers may be designed to behave as a pure energy dissipater or a spring or as a combination of the two.

A fluid viscous damper resembles the common shock absorber such as those found in automobiles. The piston transmits energy entering the system to the fluid in the damper, causing it to move within the damper. The movement of the fluid within the damper fluid absorbs this kinetic energy by converting it into heat. In automobiles, this means that a shock received at the wheel is damped before it reaches the passengers compartment. In buildings this can mean that the building columns protected by dampers will undergo considerably less horizontal movement and damage during an earthquake.

Fluid Viscous Dampers

New Breed of Energy Dissipation Devices

The innovative methods for control of seismic vibrations such as frictional and other types of damping devices are important integral part of seismic isolation systems as they severe as a barrier against the penetration of seismic energy into the structure. In this concept, the dampers suppress the response of the isolated building relative to its base.

The novel friction damper device consists of three steel plates rotating against each other in opposite directions. The steel plates are separated by two shims of friction pad material producing friction with steel plates.

When an external force excites a frame structure the girder starts to displace horizontally due to this force. The damper will follow the motion and the central plate because of the tensile forces in the bracing elements. When the applied forces are reversed, the plates will rotate in opposite way. The damper dissipates energy by means of friction between the sliding surfaces.

The latest Friction-ViscoElastic Damper Device (F-VEDD) combines the advantages of pure frictional and viscoelastic mechanisms of energy dissipation.  This new product consists of friction pads and viscoelastic polymer pads separated by steel plates.  A prestressed bolt in combination with disk springs and hardened washers is used for maintaining the required clamping force on the interfaces as in original FDD concept.

People standing in swaying train or bus try to maintain balance by unintentionally bracing their legs or by relaying on the mussels of their spine and stomach. By providing a similar function to a building it can dampen immensely the vibrations when confronted with an earthquake. This is the concept of Dynamic Intelligent Building (DIB).

The philosophy of the past conventional a seismic structure is to respond passively to an earthquake. In contrast in the DIB which we propose the building itself functions actively against earthquakes and attempts to control the vibrations. The sensor distributed inside and outside of the building transmits information to the computer installed in the building which can make analyses and judgment, and as if the buildings possess intelligence pertaining to the earthquake amends its own structural characteristics minutes by minute.

See Active Structural Control Research at Kajima Corporation (PDF)

Active Control System

The basic configuration of an active control system is schematically shown in figure. The system consists of three basic elements:

1. Sensors to measure external excitation and/or structural response.
2. Computer hardware and software to compute control forces on the basis of observed excitation and/or structural response.
3. Actuators to provide the necessary control forces.

Thus in active system has to necessarily have an external energy input to drive the actuators. On the other hand passive systems do not required external energy and their efficiency depends on tunings of system to expected excitation and structural behavior. As a result, the passive systems are effective only for the modes of the vibrations for which these are tuned. Thus the advantage of an active system lies in its much wider range of applicability since the control forces are worked out on the basis of actual excitation and structural behavior. In the active system when only external excitation is measured system is said to be in open-looped. However when the structural response is used as input, the system is in closed loop control. In certain instances the excitation and response both are used and it is termed as open-closed loop control.

Control Force Devices

Many ways have been proposed to apply control forces to a structure. Some of these have been tested in laboratory on scaled down models. Some of the ideas have been put forward for applications of active forces are briefly described in the following:

Active-tuned Mass Dampers (TMD)

these are in passive mode have been used in a umber of structures as mentioned earlier. Hence active TMD is a natural extension. In this system 1% of the total building mass is directly excited by an actuator with no spring and dash pot. The system has been termed as Active Mass Driver (AMD). The experiments indicated that the building vibrations are reduced about 25% by the use of AMD.

Tendon Control

Various analytical studies have been done using tendons for active control. At low excitations, even with the active control system off, the tendon will act in passive modes by resisting deformations in the structures though resulting tension in the tendon. At higher excitations one may switch over to Active mode where an actuator applies the required tension in tendons.

Other Methods

The liquid sloshing during earthquakes has assumed significance importance in view of over flow of petroleum products from storage tank in post earthquakes. One of the important consideration with sloshing is that is associated with a very low damping. The wave height was controlled through force applied to the side wall by a hydraulic actuator. The active control successfully reduced wave heights to the level of 6% of those without control, for harmonic excitations at sloshing frequency. For earthquake type excitation the wave heights were reduced to 19% level.

Conclusion

Conventional approach to earthquake resistant design of buildings depends upon providing the building with strength, stiffness and inelastic deformation capacity. But the new techniques like Energy Dissipation and Active Control Devices are a lot more efficient and better.