Hello all,
I have free vibration data from an isolated building (isolated with HDRB and flat sliders).The tests are characterized by a static and a dynamic phase.During the static phase the building was displaced statically to the desired displacement amplitude.As soon as this displacement was reached, the building was released.The free vibration oscillation lasted only for 3sec, before the system came to rest.
My question is:
during the static phase (approx. 1,000 sec), the rubber responds with its static properties (k0,k1,Q assuming a bilinear model).When it comes to the dynamic phase(approx. 3sec) the rubber should respond with its dynamic properties (somewhat larger than the static ones).Given the short duration of the dynamic phase (3/1000 the duration of the static phase), I wonder, does the rubber have the time to recover its dynamic properties, or does it keep responding with the static ones?
Thank you, Anastasia
Hi
Dear Anastasia Athanasiou
Assume that the isolation has stiffness characters of k0, k1, ... kn, and damping characters of c1, c2, ... cn. All of these parameters are constant and depend on the material only. However, the reactions and restoring forces depend on the displacement and velocity for the stiffness and damping, respectively. In other words, in the static phase, the contribution of c1, ... cn in restoring forces is almost zero and elastic character of the material is dominant, but in the dynamic phase, due to the vibration and resultant velocity, the forces of damping also step in and the material has viscoelastic character. It is not the matter of recovering a property, but the contributions of each force due to these properties are important.
Best Regards
Saeed Lotfan
Depending on the characteristics of the load-release system, these may also affect the response.
Hi
For the free oscillation to last only 3s this means that damping is rather high and the initial displacement probably small. Is it possible to repeat the test increasing the displacement and comparing results ?
Not really that was a full scale test on an isolated buildingin Italy, it would be hard to rejoin forces and repeat the tests.
Dear Anastasia,
It would be good to note how the duration of the dynamic phase of your experiment compares with the time period of vibration for the isolation level considering superstructure rigid. For HDRB one can consider the secant stiffness at the level of static displacement used to induce free vibrations. It should be noted that for subsequent cycles, since the displacement amplitude is lowered, the effective stiffness will increase, thereby reducing the natural period of vibration The lowest possible time period (highest possible frequency) will correspond to that calculated using K1 value at the isolation level.
The second aspect is what parameter is monitored for quantifying the dynamic effects. If its floor accelerations, the spectral analysis of the response will give an idea of which frequency is excited predominantly. If this corresponds to the free vibration frequency range of your isolation system, you can have better physical explanations. In case you have high frequency predominance in the spectra (higher frequency than that evaluated using K1 value for the isolation level) it indicates that local vibrations due to release mechanism of the test set up is responsible for the records.
If the measurement parameter of dynamic effects are displacements across individual isolators, then it will give a much better physical insight into what actually is happening.
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