EFFECT 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. In figure this movement is shown as quantity u between the roof and the ground. But, given a free option, columns 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 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 the force. For this reason, these internal forces in the columns are called stiffness forces. In fact, the stiffness force in the columns is the column stiffness times the relative displacement between its ends.

 HORIZONTAL AND VERTICAL SHAKING

         Earthquake causes 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. All structures are primarily designed to carry the gravity loads, i.e. they are designed for a force equal to the mass M (this includes mass due to own weight and imposed loads) times the acceleration due to gravity g acting in vertical downward direction (- Z). The downward force Mg is called the gravity load. The vertical acceleration during ground shaking either adds or subtracts from the acceleration due to gravity. Since factors of safety are used in the design of structures to resist the gravity load, usually most structures tend to be adequate against vertical shaking.

                       However, horizontal shaking along X and Y directions (both + and - directions of each) remains a concern. Structures designed for gravity loads, in general, may not be able to safely sustain the effects of horizontal earthquake shaking.

 FLOW OF INERTIA FORCES TO FOUNDATIONS

                                                   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.

                 Walls or columns are the most critical elements in transferring the inertia forces. But, in traditional construction, floor slabs and beams receive more care and attention during design and construction, than walls and columns. Walls are relatively thin and often made of brittle material like masonry.

Next: How Earthquakes affect Reinforced Concrete Structures?

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