Building Stiffness and Flexibility | Earthquake Engineering
The taller a building, the longer its natural period tends to be. But the height of a building is also related 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).
Displacement of Building according to their Height & Stiffness
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Inertial Forces in a Structure
An earthquake causes shaking of ground. So a building resting on it will experience motion at its base. From Newton’s first law of motion, even though the base of the building moves with the ground, the roof has a tendency to stay in its original position. But since the walls and columns are connected to it, they drag the roof along with them.
Inertial Forces in a Structure
This is much like the situation that you are faced with when the bus you are standing in suddenly starts, your feet move with the bus, but your upper body tends to stay back making you fall backwards!
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Effects of Deformations in Structures
The inertia force experienced by the roof is transferred to the ground via the columns, causing forces in columns. These forces generated in the columns can also be understood in another way. During earthquake shaking, the columns undergo relative movement between their ends.
Deformation in a Structure
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Horizontal and Vertical Shaking of a Structure
Earthquake cause shaking of ground in all three directions – along the two horizontal directions (X and Y, say), and the vertical direction (Z, say). Also during the earthquake, the ground shakes randomly back and forth (- and +) along each of this X, Y and Z directions.
Horizontal and Vertical Shaking
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Flow of Inertia Forces to Foundations
Flow of Inertia Forces to Foundation
Under horizontal shaking of ground, horizontal inertia forces are generated at a level of the mass of the structure (usually situated at the floor levels). These lateral inertia forces are transferred by the floor slab to the walls or the columns, to the foundations, and finally to the soil system underneath. So, each of this structural elements (floor slabs, walls, columns, and foundations) and the connections between them must be designed to safely transfer these inertia forces through them.
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