what do you mean by stractural resonance?

Answer 1

Structural resonances - This is far more common than a critical speed problem. It becomes a problem when some forcing frequency comes close (+/- 10%) to the resonant (natural) frequency of a structure.
The structure can be the machine housing itself or some nearby structure such as a hand rail or I-beam.
A common example of this is a vertical pump. Due to the lack of a support at the top of the unit, these typically have very low resonant frequencies (~ 300 cpm). While running, this is not a problem but during start-up or coast-down, the unit experiences a "shudder" as it passes through the structural resonance (this is not a critical speed - it is a structural resonant frequency).
The structure itself will vibrate excessively - do not confuse with a critical speed.
The "shape" of the structure's vibration is an important clue and is known as a "mode shape".
Testing for the structure's natural frequency is crucial (required) to confirming a resonance problem.

Answer 2

Structural resonance is when an object (eg a bridge) is vibrated by a force (wind etc) at a frequency close to or on its natural frequency. At natural frequency, the input force is magnified several or many times more, which greatly increases the stress on the object. The object then 'resonates', and can literally shake itself apart.

The input force itself statically may not be high enough to induce failure, but the magnification causes high amplitude and force vibrations.

Answer 3

Many objects have what's called a natural frequency: a tuning fork is an obvious example, a wine glass if you tap it. These small objects have a high natural frequency, but large objects, like a bridge, will also have a natural frequency, which may be really low, like one Hz (vibration per second) or lower. This was recognised a couple of thousand years ago and explains why armies stopped marching over a bridge and just walked 'out of step' in case their marching frequency matched the natural frequency of the bridge, which could cause it to break up.