Page 14 - EQTips_Eng

Learning

7 Earthquake Design
Earthquake Tip and

Construction

How Buildings twist during Earthquakes?

Why a Building Twists Uniform Movement
In your childhood, you must have sat on a rope of Floor
swing - a wooden cradle tied with coir ropes to the
sturdy branch of an old tree. The more modern
versions of these swings can be seen today in the
children’s parks in urban areas; they have a plastic
cradle tied with steel chains to a steel framework.
Consider a rope swing that is tied identically with two
equal ropes. It swings equally, when you sit in the
middle of the cradle. Buildings too are like these rope Earthquake

swings; just that they are inverted swings (Figure 1). Identical Vertical Ground
The vertical walls and columns are like the ropes, and Members Movement
the floor is like the cradle. Buildings vibrate back and

forth during earthquakes. Buildings with more than Figure 2: Identical vertical members placed
one storey are like rope swings with more than one uniformly in plan of building cause all points
cradle. on the floor to move by same amount.
Again, let us go back to the rope swings on the
tree: if you sit at one end of the cradle, it twists (i.e.,
moves more on the side you are sitting). This also
happens sometimes when more of your friends bunch
together and sit on one side of the swing. Likewise, if
the mass on the floor of a building is more on one side
(for instance, one side of a building may have a storage
or a library), then that side of the building moves more
under ground movement (Figure 3). This building
moves such that its floors displace horizontally as well
as rotate.

Twist

(a) Single-storey building (b) Three-storey building

Figure 1: Rope swings and buildings, both swing

back-and-forth when shaken horizontally. The Light Side

former are hung from the top, while the latter of Building

are raised from the ground.

Thus, if you see from sky, a building with identical Heavy Side
Earthquake
vertical members and that are uniformly placed in the Ground Shaking of Building
two horizontal directions, when shaken at its base in a
certain direction, swings back and forth such that all Figure 3: Even if vertical members are placed
uniformly in plan of building, more mass on
points on the floor move horizontally by the same one side causes the floors to twist.
amount in the direction in which it is shaken (Figure 2).
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