Dynamic Loads

Natural (Resonant) Frequency
 
          A key aspect of structural dynamic analysis concerns the behavior of a structure at "resonance." The natural frequency of vibration of a structure --- whether a wood-frame house or a radio tower --- corresponds to that structure's resonant frequency.  If a structure is subjected to vibration at its natural frequency, the displacements of that structure will reach a maximum ("resonance").  The greater the displacements, the greater the stresses that are developed in the framing members and connections of the structure.
          Although it is not a failure due to seismic induced resonance, the Tacoma Narrows Bridge is an example of the destructive effects that resonance can have on a structure.
 

Tacoma Narrows Bridge
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Tacoma Narrows Bridge
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Tacoma Narrows Bridge
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          The following demonstration is often done on a shake table in a laboratory exercise.  It illustrates  the effects of different frequencies of vibration on four different "structures."  Each structure has the same mass at the top and the same size (diameter) rod supporting the mass.  However, the supporting rods have various heights, resulting in increasing structural stiffness as you move from left to right.   As you start the experiment, the hydraulic actuator on the right will cause the table to oscillate left-to-right at a specific frequency.  Note that the displacement of the table associated with the hydraulic actuator frequency of motion is very small and is hardly noticeable.  Below each structure its natural frequency of vibration is listed.  Use the start button to increase the frequency of the motion.
 


Virtual Lab Demonstration
 



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