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Learning

10 Earthquake Design
Earthquake Tip and

Construction

How flexibility of Buildings affects their Earthquake Response?

Oscillations of Flexible Buildings Fundamental natural period T is an inherent
When the ground shakes, the base of a building property of a building. Any alterations made to the
moves with the ground, and the building swings back- building will change its T. Fundamental natural
and-forth. If the building were rigid, then every point periods T of normal single storey to 20 storey
in it would move by the same amount as the ground. buildings are usually in the range 0.05-2.00 sec. Some
But, most buildings are flexible, and different parts examples of natural periods of different structures are
move back-and-forth by different amounts. shown in Figure 2.
Take a fat coir rope and tie one end of it to the roof
of a building and its other end to a motorized vehicle Single Storey
(say a tractor). Next, start the tractor and pull the Building:
0.05 sec
building; it will move in the direction of pull (Figure
1a). For the same amount of pull force, the movement
is larger for a more flexible building. Now, cut the
Low-rise
rope! The building will oscillate back-and-forth Building:
horizontally and after some time come back to the 0.4 sec
original position (Figure 1b); these oscillations are
periodic. The time taken (in seconds) for each complete 15 Storey Building:
1 sec
cycle of oscillation (i.e., one complete back-and-forth Reinforced
motion) is the same and is called Fundamental Natural Concrete
Period T of the building. Value of T depends on the Chimney:
building flexibility and mass; more the flexibility, the 2 sec
longer is the T, and more the mass, the longer is the T.
In general, taller buildings are more flexible and have
larger mass, and therefore have a longer T. On the
contrary, low- to medium-rise buildings generally
have shorter T (less than 0.4 sec). Elevated Water Tank: 4 sec

Large
Concrete Gravity Dam:
0.8 sec

(a) Building pulled with a rope tied at its roof

Roof T T
Displacement T T
Time
0

Suspension Bridge: 6 sec
Adapted from: Newmark, (1970), Current trends in the Seismic
Analysis and Design of High Rise Structures, Chapter 16, in
Wiegel, (1970), Earthquake Engineering, Prentice Hall, USA.

Figure 2: Fundamental natural periods of
Inverted Pendulum Model structures differ over a large range. The
(b) Oscillation of building on cutting the rope natural period values are only indicative;

Figure 1: Free vibration response of a building: depending on actual properties of the structure,
the back-and-forth motion is periodic. natural period may vary considerably.
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