Dynamic Loads
| Natural (Resonant)
Frequency |
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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.
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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. |
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Tacoma
Narrows Bridge |
Tacoma
Narrows Bridge Qucktime2 |
Tacoma
Narrows Bridge Mpeg |
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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. |
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Virtual Lab Demonstration |
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