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This report describes how the system may be exposed to oscillations and how this can lead to resonance. Resonance is a phenomenon that occurs when a system is affected by external periodic oscillations with frequencies close to the system's natural frequencies. Structures subjected to resonance can risk great damage and destruction, why resonance is something that is important to take into account when designing structures. Alternative ways to counteract resonance is to supply damping in the system. Damping means that the resonance decreases instead of letting the amplitude increase.The report discusses some examples on constructions that have experienced the effect of resonance. The control tower in Stockholm's Arlanda Airport was swaying so much that water tanks had to be installed as resonance dampers, where the water swung in opposite phase compared to the tower. The oscillations probably started due to the fact that the tower was too lightweight in relation to its height. Tacoma Narrows Bridge in Washington, USA, oscillated in the amount that the bridge collapsed. The reason was that the bridge weighed too little to withstand the strong wind forces. The stadium Ullevi in Gothenburg had to be strengthened due to the fact that a jumping crowd during a concert caused resonance oscillations which endangered parts of the construction. After the concert, beams with decent damping attributes were installed against the rooftops to reduce the risk of damage to these. Broughtons Suspension Bridge in Britain fell apart when soldiers marched in step and in this way contributed to the oscillations of the bridge. The bridge also had structural parts whose strength was not sufficient. |
