Vibration of structures and components

The dynamic load dealt with in the preceding section has to be considered as a quasi-static load imposed on the structure as a whole but also as the cause of vibration of the components located inside. It is estimated that the acceleration due to an aircraft impact at the foundation level may reach and overcome the values typical of a design earthquake in a moderately seismic area. 

The response spectrum of the aircraft impact pulse is rather hard , that is dominated by high frequencies. For this reason, the components subjected to the highest loads are the most rigid ones, especially if the plant is located on rigid foundation soil (rock). 

In some designs, the external structures of the plant are mechanically decoupled from the internal ones on which the plant components are fixed. In this way the vibration transmitted to the components is reduced. The decoupling, obviously, is obtained by inserting joints and gaps in the structures. 

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