Experimental Methods in Earthquake Engineering - Dynamic Behavior of Concrete and Seismic Engineering - page 187

As the jacks have to move important masses (a table and model), they are fixed
on a reaction basemat, the mass of which is generally at least 20 times bigger than
the mass of the models to be tested. Furthermore, to avoid generating any vibrations
inside the test hall, the reaction basemat is laid on suspension set at a very low
frequency ( 1 Hz). This suspension consists of springs or anti-vibration masses and
dampers.

Finally, digital acquisition methods enable us to digitize and store all the
responses of the various sensors implanted on the models.

5.5.2. Characteristics and performance of shaking tables

Generally, due to the large dimensions and masses of tested structures, the
loading system consists of one or several electro-hydraulic jacks. In fact, they are
the only equipment that allows the application of high-amplitude motions to
important-mass models at low cost and in a small space. In general, the jack strokes
are about 250 mm, compared with the electro-dynamic strokes of vibrating pots,
which are smaller at 50 mm. The other advantage of jacks is that they allow very
low frequency motions and even continuous strokes. One of their negative aspects is
seen if we consider the frequency response of the servo-valves: the maximum
responses are lower than 100 Hz (500 Hz at the most for some jacks with limited
maximum strokes).

The performance of shaking tables is determined by three limiting factors:
- displacement limit corresponding to the maximum stroke of the jack or the
jacks;
- a speed limit corresponding to the maximum outflow supplied to the jacks. The
outflow is restricted by the maximum flow provided by the hydraulic pumps or by
the maximum flow possible through the servo-valves;
- an acceleration limit that is a function of the mass loaded on the table of the
mass of the table itself and of the maximum force of the jacks.

The maximum force of the jacks depends on the section of the piston, the oil
pressure supplied by the pumps and the head losses in the servo-valves. The pressure
commonly used in hydraulic groups is 210 bars, and at high frequency, head loss in
the servo-valves is about 40 bars.

Thus, the maximum acceleration on the table is given by:

Ȗ = F/ M M

[5.11]

t s