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Building features affecting building during earthquake.

A desire to create an aesthetic and functionally efficient structure drives architects to conceive wonderful and

imaginative structures. Sometimes theshape of the building catches the eye of the visitor, sometimes the structural

system appeals, and in other occasions both shape and structural system work together to make the structure a

marvel. However, each of these choices of shapes and structure has significant bearing on the performance of the

building during strong earthquakes. The wide range of structural damages observed during past earthquakes acrossthe world is very educative in identifying structural configurations that are desirable versus those which must be

avoided.

Size of Buildings: In tall buildings with large height-to-base size ratio (Figure 1a), the horizontal movement ofthe floors during ground shaking is large. In short but very long buildings (Figure 1b), the damaging effects during

earthquake shaking are many. And, in buildings with large plan area like warehouses (Figure 1c), the horizontal

seismic forces can be excessive to be carried by columns and walls.

Vertical Layout of Buildings: The earthquake forces developed at different floor levels in a building need tobe 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 the building. Buildings with vertical setbacks (like the hotel buildings

with a few storeys wider than the rest) cause a sudden jump in earthquake forces at the level of discontinuity (Figure

3a). Buildings that have fewer columns or walls in a particular storey or with unusually tall storey (Figure 3b), tendto damage or collapse which is initiated in that storey. Many buildings with an open ground storey intended for

parking collapsed or were severely damaged in Gujarat during the 2001 Bhuj earthquake. Buildings on slopy ground

have unequal height columns along the slope, which causes ill effects like twisting and damage in shorter columns

(Figure 3c). Buildings with columns that hang or float on beams at an intermediate storey and do not go all the way

to the foundation, have discontinuities in the load transferpath (Figure 3d). Some buildings have reinforced concrete

walls to carry the earthquake loads to the foundation. Buildings, in which these walls do not go all the way to the

ground but stop at an upper level, are liable to get severely damaged during earthquakes.

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Adjacency of Buildings: When two buildings are too close to each other, they may pound on each otherduring strong shaking. With increase in building height, this collision can be a greater problem. When building

heights do not match (Figure 4), the roof of the shorter building may pound at the mid-height of the column of the

taller one; this can be very dangerous.

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How buildings respond to Earthquakes

Ground Acceleration and Building Damage

Comparatively speaking, the absolute movement of the ground and buildings during an earthquakeis not actually all that large, even during a major earthquake. That is, they do not usually undergo

displacements that are large relative to the building's own dimensions. So, it is not the distance

that a building moves which alone causes damage.

Rather, it is because a building is suddenly forced to move very quickly that it suffers damage

during an earthquake. Think of someone pulling a rug from beneath you. If they pull it quickly

(i.e., accelerate it a great deal), then they needn't pull it very far to throw you off balance. On the

other hand, if they pull the rug slowly and only gradually increase the speed of the rug, they can

move (displace) it a great distance without that same unfortunate result.

In other words, the damage that a building suffers primarily depends not upon its displacement,but upon acceleration. Whereas displacement is the actual distance the ground and the building

may move during an earthquake, acceleration is a measure of how quickly they change speed as

they move. During an earthquake, the speed at which both the ground and building are moving will

reach some maximum. The more quickly they reach this maximum, the greater their acceleration.

It's worthwhile mentioning here that in order to study the earthquake responses of buildings, many

buildings in earthquake-prone regions of the world have been equipped with strong motion

accelerometers. These are special instruments which are capable of recording the accelerations of

either the ground or building, depending upon their placement.

Newton's Law

Acceleration has this important influence on damage, because, as an object in movement, the

building obeys Newton' famous Second Law of Dynamics. The simplest form of the equation which

expresses the Second Law of Motion is F = MA.

This states the Force acting on the building is equal to the Mass of the building times

theAcceleration. So, as the acceleration of the ground, and in turn, of the building, increase, so

does the force which affects the building, since the mass of the building doesn't change.

Of course, the greater the force affecting a building, the more damage it will suffer; decreasing F is

an important goal of earthquake resistant design. When designing a new building, for example, it is

desirable to make it as light as possible, which means, of course, that M, and in turn, Fwill be

lessened

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Inertial Forces

It is important to note that F is actually what's known as an inertial force, that is, the force is

created by the building's tendency to remain at rest, and in its original position, even though the

ground beneath it is moving. This is in accordance with another important physical law known

as D'Alembert's Principle, which states that a mass acted upon by an acceleration tends to

oppose that acceleration in an opposite direction and proportionally to the magnitude of the

acceleration (See Figure 2.)

This inertial force F imposes strains upon the building's structural elements. These structuralelements primarily include the building's beams, columns, load-bearing walls, floors, as well as the

connecting elements that tie these various structural elements together. If these strains are large

enough, the building's structural elements suffer damage of various kinds.

figure 3: Simple Rigid Block

To illustrate the process of inertia generated strains within a structure, we can consider the

simplest kind of structure imaginable--a simple, perfectly rigid block of stone. (See Figure 3.)

During an earthquake, if this block is simply sitting on the ground without any attachment to it, the

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block will move freely in a direction opposite to that of the ground motion, and with a force

proportional to the mass and acceleration of the block.

If the same block, however, is solidly founded in the ground and no longer able to move freely, it

must in some way absorb the inertial force internally.

Of course, real buildings do not respond as simply as described above. There are a number of

important characteristics common to all buildings which further affect and complicate a building's

response in terms of the accelerations it undergoes, and the deformations and damages that it

suffers.

Building Frequency and Period

The magnitude of the building response that is, the accelerations which it undergoes depends

primarily upon the frequencies of the input ground motion and the building's natural frequency.

When these are near or equal to one another, the building's response reaches a peak level.

In some circumstances, this dynamic amplification effect can increase the building acceleration

to a value two times or more that of the ground acceleration at the base of the building. Generally,

buildings with higher natural frequencies, and a short natural period, tend to suffer higher

accelerations but smaller displacement. In the case of buildings with lower natural frequencies, and

a long natural period, this is reversed as the buildings will experience lower accelerations but larger

displacements.

Building Stiffness

The taller a building, the longer its natural period tends to be. But the height of a building is alsorelated 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 isn't flexible and vice-versa.)

Stiffness greatly affects the building's uptake of earthquake generated force. Reconsider our first

example above, of the rigid stone block deeply founded in the soil. The rigid block of stone is very

stiff; as a result it responds in a simple, dramatic manner. Real buildings, of course, are more

inherently flexible, being composed of many different parts.

Furthermore, not only is the block stiff, it is brittle; and because of this, it cracks during the

earthquake. This leads us to the next important structural characteristic affecting a building's

earthquake response and performance ductility.

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Ductility

Figure 4

Ductility

Ductility is the ability to undergo distortion or deformation bending, for example without

resulting in complete breakage or failure. To take once again the example of the rigid block in

Figure 3, the block is an example of a structure with extremely low ductility. To see how ductility

can improve a building's performance during an earthquake, consider Figure 4.

For the block, we have substituted a combination of a metal rod and a weight. 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.) Obviously, it is far more desirable for a building

to sustain a limited amount of deformation than for it to suffer a complete breakage failure.

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 earthquakeresistant 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.

Damping

The last of the important structural characteristics, or parameters, which we'll discuss here is

damping. As we noted earlier, ground and building motion during an earthquake has a complex,

vibratory nature. Rather than undergoing a single "yank" in one direction, the building actually

moves back and forth in many different horizontal directions.

All vibrating objects, including buildings, tend to eventually stop vibrating as time goes on. More

precisely, the amplitude of vibration decays with time. Without damping, a vibrating object wouldnever stop vibrating, once it had been set in motion. Obviously, different objects possess differing

degrees of damping. A bean bag, for example, has high damping; a trampoline has low damping.

In a building undergoing an earthquake, damping the decay of the amplitude of a building's

vibrations is due to internal friction and the absorption of energy by the building's structural and

nonstructural elements. All buildings possess some intrinsic damping.

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The more damping a building possesses, the sooner it will stop vibrating--which of course is highly

desirable from the standpoint of earthquake performance. Today, some of the more advanced

techniques of earthquake resistant design and construction employ added damping devices like

shock absorbers to increase artificially the intrinsic damping of a building and so improve its

earthquake performance.