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Structural Vibration

Structural Vibration

Structural vibration is a sub category of structural dynamics. It is a type of structural analysis which covers the behaviour of a structure subjected to dynamic loading. Dynamic loads include people, wind, waves, traffic, earthquakes, and blasts. Any structure can be subjected to dynamic loading. Dynamic analysis can be used to find dynamic displacements, time history, and modal analysis.

Structural analysis is mainly concerned with finding out the behaviour of a physical structure when subjected to force. This action can be in the form of a load due to the weight of things such as people, furniture, wind, snow, etc. or some other kind of excitation such as an earthquake, shaking of the ground due to a blast nearby, etc. In essence all these loads are dynamic, including the self-weight of the structure because at some point in time these loads were not there. The distinction is made between the dynamic and the static analysis on the basis of whether the applied action has enough acceleration in comparison to the structure’s natural frequency. If a load is applied sufficiently slowly, the inertia forces (Newton’s first law of motion) can be ignored and the analysis can be simplified as static analysis.

static load is one which varies very slowly. A dynamic load is one which changes with time fairly quickly in comparison to the structure’s natural frequency. If it changes slowly, the structure’s response may be determined with static analysis, but if it varies quickly (relative to the structure’s ability to respond), the response must be determined with a dynamic analysis.

Dynamic analysis for simple structures can be carried out manually, but for complex structures finite element analysis can be used to calculate the mode shapes and frequencies.

In structural engineering, modal analysis uses the overall mass and stiffness of a structure to find the various periods at which it will naturally resonate. These periods of vibration are very important to note in earthquake engineering, as it is imperative that a building’s natural frequency does not match the frequency of expected earthquakes in the region in which the building is to be constructed. If a structure’s natural frequency matches an earthquake’s frequency, the structure may continue to resonate and experience structural damage.

Modal analysis is important in structures such as bridges where the engineer should attempt to keep the natural frequencies away from the frequencies of people walking on the bridge. This may not be possible and for this reason, when groups of people are to walk along a bridge, for example a group of soldiers, the recommendation is that they break their step to avoid possibly significant excitation frequencies.

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