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A topical example for a vibration-sensitive structure is the suspension pedestrian bridge in Forchheim,
a town in the south of Germany (Fig. 4.1). The bridge deck was installed in summer 2002 and
investigations about the dynamic behaviour of the bridge are made at the moment (08/2002).
The deck is supported by 8 cables, meeting in the top end of an inclined steel pier. All main structural
members (pier and superstructure beams) are made of steel, whereas the pedestrian deck is covered
with wooden planks. The total length is 117.5 m, the width of the deck is 4.25 m. The height of the
inclined pier is app. 30 m.
Fig. 4.1 Sketch of the footbridge Forchheim, Germany
A numerical modal analysis of the bridge showed the natural frequencies in the range between 1,0 Hz
and 3,5 Hz. Experimental testing at the bridge pointed out that the structure is not only sensitive to one
natural frequency, but at least two of them could be excited by a group of people crossing the bridge.
This is the reason why the installation of one passive device would not be sufficient.
For maximum efficiency the properties of a TMD should be chosen according to the real dynamic
characteristics of the bridge which are usually obtained by experimental tests. However, in case the
system properties may change (e.g. variable bridge mass due to variable live loads) or several different
frequencies with similar eigenforms shall be dampened, the properties of the damper shall vary. The
effectiveness of a passive TMD is considerably reduced in case the system properties deviate from the
design properties (e.g. deviation in frequency). Therefore, instead of the conventional passive device, a
semi-active magnetorheological damper is installed in the TMD in order to adopt the latter to the
current frequency and to increase the device's effectiveness in different load cases.
4.2 Characteristics of the TMD
The mechanical properties of the semi-active magnetorheological TMD will be identical to an optimal
passive TMD, according to the Den Hartog criteria. The passive viscous damping device will be
replaced by a magnetic field-controlled MR damper.
To avoid transverse movements and rotations of the oscillating damper mass, two MR-dampers are
installed in the TMD. The damping mass of the TMD can be varied by the number of steel plates,
inserted into the damper (Fig 4.2). This step allows a variation of the damper frequency in order to test
the efficiency of the semi-active behaviour.