Shaking Table Based Exercises
This extended exercise looks in more detail at the rocking of the structure. It was touched upon in the soil mechanics short lab that, as well as vibration of the structure with horizontal motion, it can also rock from side to side. You may have observed in the short lab that there is a threshold frequency at which the structure will lift-off from the sand, and the amplification factor for rocking increases dramatically.
Things to investigate in this extended exercise include:
- What is the relationship between the amplification factor for rocking and frequency? Is it linear?
- What about the relationship between damping ratio and frequency?
- What is the relationship between shaking amplitude and threshold frequency? (In the short lab you may have had chance to look at rocking at 1mm and 3.5mm amplitude shakes; try some others as well.)
- What form does the rocking motion take? Does the structure rock about a centreline, or another point? What gives evidence of this?
- What practical measures might be used to prevent or reduce rocking? How could you test their effectiveness?
This extended exercise is an activity to examine the damping of the system on a soil base, in more detail.
In the soil mechanics short lab you obtained a value of damping ratio for each floor at a 1mm amplitude shake. Carry out a similar set of tests to obtain the damping ratio at other amplitudes. You may wish to use more data points to get a more accurate curve.
- What happens to the damping ratio? Is there a relationship to the shake amplitude?
- Does the natural frequency of the structure change with amplitude? Why might this be?
- Compare your tests with the provided data for the structure rigidly attached to the shaker table. How can you explain the difference in damping ratio? (There is no need to carry out further tests with the structure rigidly connected to the base).
- Do you think that the soil or the structure makes the most significant contribution to the system damping? Where do you think the shaking energy is being absorbed? How could you test this?
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Last updated 12/10/2012 by hrs@eng