Experimental Methods in Earthquake Engineering - Dynamic Behavior of Concrete and Seismic Engineering - page 168

concentrated) is calculated for a short time step using an appropriate time integration
algorithm. These displacements are then imposed on the structure via servo-
controlled hydraulic jacks leaning on a reaction wall. Load cells measure the
load level needed to impose the required displacement on each floor, and these
values are fed back to the computer to be used at the next time step.
Because the inertia is modeled, the experiments do not have to be carried out in
real time, which allows us to test large size structures with relatively low hydraulic
power. In this regard, PSD tests are complementary to those carried out with shaking
tables which are conducted in real time on extremely reduced structure components
or structure models.
5.1.2. History of the PSD method
The PSD method was created in Japan [TAK 75] over 30 years ago, and the
basic implementations, as well as numerous improvements, were made. Initially, the
integration methods used were the central finite difference method (explicit
Newmark's method), then an implementation with operator splitting of the
trapezoidal method (implicit Newmark's method) [NAK 90]. The latter allows the
use of an unconditionally stable scheme for cases including stiff structures and a
certain number of degrees of freedom.
The PSD method was further developed in the USA [SHI 84]. There, a host of
improvements were introduced, including the use of iterative implicit schemes [SHI
91].
The experience accumulated in both Japan and the USA has shown that success
is determined by the quality of the implementation of the PSD method (see [ELB
89], [NAK 90] and [SHI 87]). Many elements in this implementation have a strong
tendency to bring about errors. The measurement or control errors tend to
accumulate [SHI 87], and if they get too high, they can eventually overcome the
response [NAK 87].
All of this takes place as if a differential equation system was integrated with a
computer that would contain only a small number of significant digits.
5.1.3. The ELSA laboratory
The European Laboratory of Structural Assessment (ELSA) [DON 92] is a
laboratory of the European Community Research Common Center (see Figure 5.2)
and was inaugurated in 1992. It consists of a 16 m high and 21 m wide reaction wall