Projefct 1_Comparative study of different storey buildings for Seismic forces
Factors influencing the dynamic characteristics of a building
Buildings oscillate during earthquake shaking. The oscillation causes inertia force to be induced in the building.
The intensity and duration of oscillation, and the amount of inertia force induced in a building depend on features of buildings, called their dynamic characteristics, in addition to the characteristics of the earthquake shaking (beyond the control of an engineer) itself.
The important dynamic characteristics of buildings are modes of oscillation and damping (assumed constant in most practical cases). A mode of oscillation of a building is defined by associated Natural Period and Deformed Shape in which it oscillates.
Every building has a number of natural frequencies (how many?), at which it offers minimum resistance to shaking induced by external effects (like earthquakes and wind) and internal effects (like motors fixed on it).
Each of these natural frequencies and the associated deformation shape of a building constitute a Natural Mode of Oscillation.
The mode of oscillation with the smallest natural frequency (and largest natural period) is called the Fundamental Mode; the associated natural period T₁ is called the Fundamental Natural Period.
Regular buildings held at their base from translation in the three directions, have two fundamental translational natural periods, Tx1 and Tyi, associated with its horizontal translational oscillation along X and Y directions, respectively, and one fundamental rotational natural period Te1 associated with its rotation about an axis parallel to Z axis.
Factors influencing the Natural Period of a building
2story building
5storey building natural period
5storey building x axis orientation natural period
5storey building y axis orientation natural period
10 storey building same column
10 storey building with two layers of different columns
25 storey building
25 storey with three layer of different columns
25 storey with less than 10% of live load
25 storey with lees then 20% of live load
Factors influencing the Mode shape of oscillations
Mode shape of oscillation associated with a natural period of a building is the deformed shape of the building when shaken at the natural period. Hence, a building has as many mode shapes as the number of natural periods.
For a building, there are infinite numbers of natural period. But, in the mathematical modeling of building, usually the building is discretized into a number of elements. The junctions of these elements are called nodes. Each node is free to translate in all the three Cartesian directions and rotate about the three Cartesian axes. Hence, if the number of nodes of discretization is N, then there would be 6N modes of oscillation, and associated with these are 6N natural periods and mode shapes of oscillation.