BUILDING STIFFNESS AND FLEXIBILITY

                           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).

       Figure shows the Displacement of Building according to their Height & Stiffness

Ductility is the ability to undergo distortion or deformation without resulting in complete breakage or failure.  To see how ductility can improve a building's performance during an earthquake, consider Fig 3.2.b.  In response to the ground motion, the rod bends but does not break.  (of course, metals in general are more ductile than materials such as stone, brick and concrete)  The ductility of a structure is in fact one of the most important factors affecting its earthquake performance.  One of the primary tasks of an engineer designing a building to be earthquake resistant is to ensure that the building will possess enough ductility to withstand the size and types of earthquakes it is likely to experience during its lifetime.

 SEISMIC EFFECTS

          INERTIA FORCES IN STRUCTURES

                                  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. 

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!!!

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 roof is different from that of ground.  

                            Consider a building, whose roof is supported on columns. Coming back to the analogy of yourself on the bus; when the bus suddenly starts, you are thrown backwards as if someone has applied a force on the upper body. Similarly, when the ground moves, even the building is thrown backwards, and the roof experiences a force, called inertia force. If the roof has the mass M and experiences an acceleration a, then from Newton's second law of motion, the inertia force F1 is mass M times acceleration a, and its direction is opposite to that of the acceleration.

"More mass means higher inertia force"

Next: Effect of Deformations in Structures

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