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clarification on modal analysis 3
- Thread starter dammi
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Yes, as said by lisica you can perform a modal analysis to know how your structure vibrates. A multistory building is a M-DOF system, with M > 1. If you have a 10 stories building, you will have a 30 degree of freedom system (if you have a rigid diaphragm, so you get 3 dof/story x 10 stories = 30 dof).
Every dof you have will give you a vibration mode. The first mode is called the fundamental. You need to reach a certain amount of “participating mass” to define the structural behavior (every mode contributes in differently to structural displacement. Longer is the period, largest is the displacement, so with first 3-4 modes you could reach a 98-99% of participating mass). In spatial models you can consider the longest 3 period of X-translational mode, Y-translation mode, rotational mode (ex: T1x = 0.51 sec, T1y = 0.47 sec, tr = 0.45 sec and so on).
So, when you reach the predefined limit of participating mass you can combine the modes and get the “real” behavior of the structure.
I said “real” because other procedures are only simplifications that can be used in certain particular conditions. If you have:
- Regularity in plan and elevation: bidimensional model with static linear analysis;
- Regularity in plan and irregularity in elevation: bidimensional model with dynamic modal analysis;
- Irregularity in plan and regularity in elevation: tridimensional model with static linear analysis;
- Irregularity in plan and elevation: tridimensional model with static linear analysis.
I don’t know what code you are referring to, but nowadays a dynamic modal analysis is always preferable.
Every dof you have will give you a vibration mode. The first mode is called the fundamental. You need to reach a certain amount of “participating mass” to define the structural behavior (every mode contributes in differently to structural displacement. Longer is the period, largest is the displacement, so with first 3-4 modes you could reach a 98-99% of participating mass). In spatial models you can consider the longest 3 period of X-translational mode, Y-translation mode, rotational mode (ex: T1x = 0.51 sec, T1y = 0.47 sec, tr = 0.45 sec and so on).
So, when you reach the predefined limit of participating mass you can combine the modes and get the “real” behavior of the structure.
I said “real” because other procedures are only simplifications that can be used in certain particular conditions. If you have:
- Regularity in plan and elevation: bidimensional model with static linear analysis;
- Regularity in plan and irregularity in elevation: bidimensional model with dynamic modal analysis;
- Irregularity in plan and regularity in elevation: tridimensional model with static linear analysis;
- Irregularity in plan and elevation: tridimensional model with static linear analysis.
I don’t know what code you are referring to, but nowadays a dynamic modal analysis is always preferable.
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