Civil Engineering Reference
In-Depth Information
2.6 Cross Checks
Based on the experimental data, several checks were performed. These covered (stress, fatigue) behavior of main girder,
pylon and cables under ambient and jogger loading conditions. We were not directly involved in the respective analytical
procedures but we delivered data like relative cable motion (displacement). This was possible for the cases where the motion
of the pylon fix-point, cable 0.4 L point and bridge deck at the lower cable fix point had simultaneously been measured only.
No stress or fatigue problems could be identified.
2.7 Health Monitoring Using Dynamic Methods?
A commonly promoted procedure to monitor a structure's health is, a least for a level 1 damage detection, to monitor its
natural frequencies. The fact of having “identified” Oberwies Footbridge twice, at the pilot and and the main tests, offers the
opportunity to check the natural frequencie's stability versus temperature. The respective information is presented in
Fig.
2.24
. Temperature effects on natural bridge frequencies are significant for modes where the shape indicates significant
connection of structural elements to soil. This is also discussed in [
2
].
2.8 Summary
Experimental Modal Analysis under ambient excitation is well suited for the identification of a 32-m-twin-span cable-stayed
footbridge with a lot of highway traffic travelling underneath the bridge. Using 10 V/g sensors and choosing a long enough
time window results in a very nice signal-to-noise-ratio. To measure the vibrations of stay cables with a 10-25 m length
under ambient excitation, 1 V/g sensors are well suited.
Oberwies Footbridge exhibits a fundamental natural frequency B1, f
1.19 Hz. This is not critical when it comes to
pedestrian dynamic action. This mode is however well excited through trucks and trailers passing underneath the bridge with
a speed v
¼
100 km/h. The headway is about 1 m which produces a nice air pressure wave during the vehicle passage.
Cross-checks however revealed that the bridge response to such action may be clearly perceptible by humans but is of no
danger to the bridge.
Oberwies Footbridge exhibits a second natural mode B2, f
¼
60
...
2.73 Hz. This is a frequency being nicely excited by
joggers. However, respective tests showed that the bridge response to jogger excitation with f
¼
2.73 Hz is not critical
because the respective frequency is not related to the first but to the second bridge mode. This means: Really critical states
may occur if the exciting frequency corresponds to the structure's fundamental frequency and if the mode shape is similar to
the static deflection shape forced by the exciter only.
Oberwies Footbridge long stay cables exhibit a fundamental natural frequency SL1, f
¼
2.9 Hz. This vibration is easily
excited through a jogger's action. However, the excitation duration is too short to produce a “real” resonance problem.
Finally: The test proved that Oberwies Footbridge is a dynamically active structure without touching some critical limits.
This may also be the reason for the bridge safely surviving 35 years without showing signs of distress.
Due to space restrictions we cannot discuss the attempts undertaken to deriving cable forces from dynamic measurements
here. This is however a very interesting topic. Especially for cables where the clamping conditions are far from those
applying to a string model.
¼
Mode
Juli 20, 2011, 20 deg. C.
Frequency f [Hz]
August 25, 2011 30 deg. C.
Frequency f [Hz]
Δ
f
[%]
1
1.22
1.19
2.5
2
2.73
2.73
0
3
3.08
3.04
1.5
Fig. 2.24
Oberwies
Footbridge natural frequencies
as a function of temperature
4
4.05
4.06
0
5
5.03
5.03
0
