IIT EQ TIP 10
How flexibility of Buildings affects their Earthquake
Response?
Structures Year 2 Sem 4
Rishabh Dikshit A012
Suprit Raut B011
The flexibility of buildings is a key factor in their seismic response, influencing how they
absorb, distribute, and dissipate seismic forces. A well-designed flexible structure is
more likely to withstand earthquakes and protect both the structure itself and its
occupants.
Oscillations of Flexible Buildings
When the ground shakes, the base of a building moves with the ground, and the building
swings back-and-forth. If the building were rigid, then every point in it would move by the
same amount as the ground. But, most buildings are flexible, and different parts move
back-and-forth by different amounts.
● PULL FORCE - In case of two buildings of different sizes, the longer building will
show more flexibility for the same pull force.
Considering a pull force is applied on a building by tying
a rope on its roof and the other end to a tractor, and the
rope is cut in the process, the building would oscillate
horizontally back and forth for some time and come back
to its original position.
● These oscillations are periodic.
● The time taken (in seconds) for each complete cycle
of oscillation (i.e., one complete back-and-forth
motion) is the same and is called Fundamental
Natural Period T of the building
● Value of T depends on the building flexibility and
mass; more the flexibility, the 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).
Fundamental natural period T is an inherent
property of a building. Any alterations made to the
building will change its T. Fundamental natural
periods T of normal single storey to 20 storey
buildings are usually in the range 0.05-2.00 sec.
Some examples of natural periods of different
structures are shown in Figure
Fundamental natural periods of structures differ
over a large range. The natural period values are
only indicative; depending on actual properties of
the structure, natural period may vary considerably.
Importance of Flexibility
The ground shaking during an earthquake contains a mixture of many sinusoidal waves
of different frequencies, ranging from short to long periods. The time taken by the wave
to complete one cycle of motion is called period of the earthquake wave. In general,
earthquake shaking of the ground has waves whose periods vary in the range
0.03-33sec.
Intensity of earthquake waves at a particular building location depends on a number of
factors:-
● magnitude of the earthquake
● the epicentral distance
● the type of ground that the earthquake waves travelled through before reaching the
location of interest
Loma prieta earthquake
San Francisco 1989
6.9 in magnitude
● During earthquakes buildings undergo larger relative horizontal displacements,
which may result in damage to various structural and nonstructural building
components and the contents.
● Since many buildings that took damage were load bearing brick walls, indicating
the fundamental natural period (T) to be longer leading the structural members to
take devastating damage.
Possible Solutions
● Considering various factors such as the type of soil, the depth of the building's
foundation, and the height of the building. It also requires buildings to have a
flexible structure that can move with the around during an earthquake.
● One of the key features of earthquake resistant buildings is the use of seismic isolation
bearings These bearings allow the building to move horizontally during an earthquake,
reducing the stress on the structure and minimizing damage. Additionally, many
buildings have a reinforced concrete frame, which provides added stability and
protection against collapse.
Yokohama Landmark Tower
Location: Japan
Height: 296.33m(70 floors)
Completion: july 1993
● As for earthquake resistant techniques,
the yokohama landmark tower has a
flexible structure to absorb the force of
earthquakes.
● TUNED MASS DAMPER :The building
contains two tuned mass dampers on the
(hidden) 71st floor on opposite corners of
the building. The tuned mass damper is a
four-storey-tall pendulum with a spherical
mass of stacked steel plates surrounded
by large dampers or shock absorbers.
