Seismic Behavior of Buildings - Explained
What is seismic behavior?
The perimeter design of a building has a significant impact on seismic behavior. The center of mass will not correspond with the center of resistance if there is significant variation in strength and stiffness around the perimeter, and torsional forces will cause the building to rotate around the point of resistance.
An open-front design in buildings like fire stations and garages, where huge doors allow cars to pass through, is a classic example of an imbalanced perimeter.
What are the effects of earthquakes on buildings?
Inertia Forces in Buildings:
The ground started to shake during an earthquake. Therefore, a structure resting on it will have motion at the base. Despite the building's base moving with the ground, according to Newton's First Law of Motion, the roof tends to hold in its initial position.
However, because it is attached to the walls and columns, they pull the roof along with them. Similar to when a bus you are standing in suddenly starts, your feet go with it but your upper body tends to stay behind, causing you to fall backward! Inertia is the tendency to maintain one's position after changing it.
The building's roof moves differently from the ground because the walls or columns are flexible 👇
Impact of Structure Deformations:
The columns encounter forces as a result of the roof's inertia, which is communicated to the ground through the columns. There is another method to understand the forces produced in the columns. The columns move relative to one another when an earthquake shakes them.
Quantity u ( the relative horizontal displacement between the top and bottom of the column) between the roof and the ground is represented in this movement. However, if given the chance, columns would prefer to return to their original, straight vertical posture; in other words, they oppose deformations. The columns do not transmit any horizontal earthquake force through them when they are vertically aligned.
However, when forced to bend, they produce internal forces. Internal forces within columns increase in magnitude in direct proportion to the relative horizontal displacement u between the top and bottom of the column. Additionally, the magnitude of this force increases with the stiffness of the columns (i.e., column size). These internal forces in the columns are known as stiffness forces as a result. In actuality, a column's stiffness force is equal to the stiffness of the column multiplied by the distance between its ends.