Page 10 - EQTips_Eng

Learning

5 Earthquake Design
Earthquake Tip and

Construction

What are the Seismic Effects on Structures?

Inertia Forces in Structures would like to come back to the straight vertical
Earthquake causes shaking of the ground. So a position, i.e., columns resist deformations. In the
building resting on it will experience motion at its straight vertical position, the columns carry no
base. From Newton’s First Law of Motion, even though horizontal earthquake force through them. But, when
the base of the building moves with the ground, the forced to bend, they develop internal forces. The larger
roof has a tendency to stay in its original position. But is the relative horizontal displacement u between the
since the walls and columns are connected to it, they top and bottom of the column, the larger this internal
drag the roof along with them. This is much like the force in columns. Also, the stiffer the columns are (i.e.,
situation that you are faced with when the bus you are bigger is the column size), larger is this force. For this
standing in suddenly starts; your feet move with the bus, reason, these internal forces in the columns are called
but your upper body tends to stay back making you fall stiffness forces. In fact, the stiffness force in a column is
backwards!! This tendency to continue to remain in the the column stiffness times the relative displacement
previous position is known as inertia. In the building, between its ends.
since the walls or columns are flexible, the motion of Inertia Force
the roof is different from that of the ground (Figure 1).
u
Roof

Column

Foundation

Figure 1: Effect of Inertia in a building when
shaken at its base Soil

Consider a building whose roof is supported on Acceleration
columns (Figure 2). Coming back to the analogy of Figure 2: Inertia force and relative motion within
yourself on the bus: when the bus suddenly starts, you are a building
thrown backwards as if someone has applied a force on the
upper body. Similarly, when the ground moves, even Horizontal and Vertical Shaking
the building is thrown backwards, and the roof Earthquake causes shaking of the ground in all
experiences a force, called inertia force. If the roof has a three directions – along the two horizontal directions
mass M and experiences an acceleration a, then from (X and Y, say), and the vertical direction (Z, say) (Figure
Newton’s Second Law of Motion, the inertia force F I is 3). Also, during the earthquake, the ground shakes
mass M times acceleration a, and its direction is randomly back and forth (- and +) along each of these X,
opposite to that of the acceleration. Clearly, more mass Y and Z directions. All structures are primarily
means higher inertia force. Therefore, lighter buildings designed to carry the gravity loads, i.e., they are
sustain the earthquake shaking better.
designed for a force equal to the mass M (this includes
Effect of Deformations in Structures mass due to own weight and imposed loads) times the
The inertia force experienced by the roof is acceleration due to gravity g acting in the vertical
transferred to the ground via the columns, causing downward direction (-Z). The downward force Mg is
forces in columns. These forces generated in the called the gravity load. The vertical acceleration during
columns can also be understood in another way. ground shaking either adds to or subtracts from the
During earthquake shaking, the columns undergo acceleration due to gravity. Since factors of safety are
relative movement between their ends. In Figure 2, used in the design of structures to resist the gravity
this movement is shown as quantity u between the loads, usually most structures tend to be adequate
roof and the ground. But, given a free option, columns against vertical shaking.
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