Foundation, Concrete and Earthquake Engineering


Earthquake causes shaking of the 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 building moves with the ground, the roof has a tendency to stay in its original position. But since the column are connected to it, they drag the roof along with them. This tendency to continue to remain in the previous position is known as Inertia. In the building, since the walls or columns are flexible, the motion of the roof is different from that of the ground.

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. This movement is shown as quantity u between the roof and the ground . But, given a free option, column would like to come back to the straight vertical position, i.e., columns resist deformations. In the straight vertical position, the columns carry no horizontal earthquake force through them. But, when forced to bend, they develop internal forces. The larger is the relative horizontal displacement u between the top and bottom of the column, the larger this internal force in columns. Also, the stiffer the columns are ( i.e.,bigger is the column size), larger is this force. For this reason, these internal forces in the columns are called stiffness forces. In fact, the stiffness forces in a column is the column stiffness times the relative displacement between its ends.

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