SAMPE Journal - January/February 2021 - 24

FEATURE / REINFORCED POLYMER HANGERS

Figure 2. April 2020. The photo on left shows the completely installed network arch aside of the highway. The CFRP hangers are already pre-stressed. In the night of May 3rd the bridge will be slid in over
the highway. The photo on right demonstrates the slenderness of the CFRP hangers.

to the system-related low deformations under traffic
loads. Nevertheless, so far only about two dozen of
them exist worldwide8. This might be largely due to
the challenges attributed to the high fatigue loads
from rail traffic.
The use of hangers made of CFRP fundamentally changes the boundary conditions used in the
selection of a hanger layout and thus influences the
load bearing system of the network arch8,9. Due to
the excellent fatigue resistance, the layout of carbon
hangers can be tuned ignoring any criteria for fatigue stress range limits, allowing the selection of the
tendon to be based entirely on the criteria of the utilization of the tensile load capacity. As a result, the
required hanger cross sectional area can be reduced
to less than one quarter of what would be required
in steel. This together with the smaller young's-modulus of CFRP leads to significantly larger elongations
of the hangers, which resolves the issue of the hangers going slack. By reducing the mass and simultaneously increasing the relative pre-stress in the hanger
cross-section (same force in relation to a significantly smaller cross-sectional area) the natural frequencies of the hangers shift significantly upwards. This
avoids the frequency range in that rain-wind-induced vibrations occur. The use of round cross sections is therefore in this context not a disadvantage.
Wind-induced vibrations of CFRP tendons above
a frequency of 10Hz are since 25 years well known
in marine applications. They do not reduce the service life due to the high fatigue resistance of CFRP in
the case of a suitable design of the connections.8-10
By using carbon tension members as hangers, it is
possible to realize a high aesthetic quality and at the
same time very efficient hanger arrangements with
comparatively steep hangers avoiding hangers getting slack and without impairing the service life as a
result of wind- induced vibrations.

SAMPE JOURNAL

J A N UA R Y/ F E B R UA R Y 2 0 2 1

FABRICATION OF THE CFRP HANGERS
Since twenty years, the EMPA spin-off company
Carbo-Link is producing pin loaded CFRP straps as
backstays for huge cranes and the rigging in marine
applications. Therefore, it was for the bridge engineers of schlaich bergermann partner (sbp) in Stuttgart obvious to get in touch with this company.
The production process is highly automated. A
tape-laying machine (Figure 3) is winding an " endless " 50 mm wide unidirectional carbon fibre/epoxy
prepreg around two milled titanium thimbles. As
soon as the required cross section is reached, the
whole assembly is rotated around the longitudinal
axis similar to a classical filament-winding machine.
Figure 3. Tape-laying/winding machine.

w w w. s a m p e . o r g

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SAMPE Journal - January/February 2021

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