Friday, November 9, 2012

Scaling Earthquakes - the Quanser Way!

Our Shake Table users are familiar with the earthquake scaling file called q_scale. This file has been around a long time, but it has been refined over the years. Special thanks to Dr. Eduardo Jausel from the Massachusetts Institute of Technology for his suggestions on the 'baseline' method and revised the source code.
So what does q_scale do exactly? Given an earthquake acceleration record, q_scale computes the position that the shake table must track in order to obtain the same accelerations - while keeping it below the maximum position specified. This enables an earthquake to be replayed on a bench top shake table and still obtain the same accelerations as recorded.

Take the Northridge earthquake for example. As with many other tremors, the recorded acceleration record can be downloaded from the PEER Ground Motion Database. At a certain measurement station, the peak acceleration and displacement of Northridge reached 0.60 g and 16.9 cm. The trajectory recorded at that station is shown in the two left-hand plots in the figure below under 'Recorded Acceleration' and 'Recorded Position'. 
When processed by q_scale, the acceleration amplitude is maintained (notice the peak acceleration is kept at 0.60 g) and the displacement is scaled-down to 3 cm (i.e. the specified maximum position). The position and acceleration records from q_scale are shown on the right-hand side in the above figure under  the Table Acceleration and Table/Scaled Position plots. Notice that in order to keep the same acceleration, the duration of the tremor is shrunk down from 40.0 sec down to 16.8 sec.
Using ground motions from actual earthquakes makes the lab more interesting. Motivation is key and students would rather see their building stand up to the Northridge earthquake then a pre-defined sinusoidal.
- Mitch Levis, Manager, Curriculum

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