Why Modal Analysis Is Important for Designing of High-rise Structures?

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  1. Modal analysis is linear dynamic analysis method which provides the greatest contribution to structural response and behavior of structure after hitting earthquake.
  2. According to IS 1893-2016 CLOUSE NO.7.7.5.2 and table no. 6 the first two-mode of vibration is always translation mode unless and until building shape is very irregular. The time difference between translation and immediate rotational mode is always greater than 10 %
  3. The modal analysis gives the actual behavior of building like participation of modal mass in first 3 modes of vibration and last mode of vibration
  4. The modal analysis also contributes to various other types of analysis like response spectrum and time history analysis
  5. Number of modes to be considered The number of modes Nā€ž, to be used in the analysis for earthquake shaking along a considered direction, should be such that the sum total of modal masses of these modes considered is at least 90 percent of the total seismic mass.
  6. If modes with natural frequencies beyond 33 Hz are to be considered. the modal combination shall be carried out only for modes with natural frequency less than 33 H-z: the effect of modes with natural frequencies more than 33 Hz shall be included by the missing mass correction procedure following established principles of structural dynamics. I f justified by rigorous analysis. designers may use a cut off frequency other than 33 Hz.
  7. The first three modes contribute less than 65 percent mass participation factor in each principal plan direction. and
    the the fundamental lateral periods of the building in the two principal plan directions are closer In each other by 10 percent of the larger value,
  8. In buildings located in Scenic Zones II & III. It shall be ensured that the first three modes together contribute at least 65 percent mass participation factor in each principal plan direction. And, in buildings located in Seismic Zones IV & V, it shall be ensured that
    1. the first three modes together contribute at least 65 percent mass participation factor in principal plan direction, and
    2. the fundamental lateral natural period of the building in the two principal plan directions are away from each other by at least 10 percent of the larger value.
  9. Natural Period (Tk) in Mode (k) of Oscillation – The time taken (in second) by the structure to complete one cycle of oscillation in it’s natural mode k of oscillation.

    1. Fundamental Lateral Translational Natural Period (T1) ā€” It is the longest time taken (in second) by the structure to complete one cycle of oscillation in its lateral translational mode of oscillation in the considered direction of earthquake shaking. This mode of oscillation is called the fundamental lateral translational natural mode of oscillation. A three-dimensional model of a structure will have one such fundamental lateral translational mode of oscillation along each of the two orthogonal plan directions.

    2. Normal Mode of Oscillation ā€” The mode of oscillation in which there are special undammed fine oscillations in which all points on the structure oscillate harmonically at the same frequency (or period). such that all these points reach their individual maximum responses simultaneously.

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