Cable-Stayed Footbridge: Investigation into Superstructure and Cable Dynamics - Topics in Dynamics of Bridges - page 18

Fig. 2.15 (continued)

2.3.7 Modal Test Result Discussion, Bridge

The first mode B1 is dominated by a horizontal longitudinal pylon movement and an out-of-phase vertical motion of the two main
girder spans. Its frequency f ¼ 1.19 Hz is too low to be excited by walking people but it seems to be well suited to be excited by
trucks passing underneath the bridge. This is quite straightforward. A truck of 20 m length travelling with 20 m/s (about 80 km/h)
will produce an impulse with a length td 1s.Inhisfamousbook[ 1 ] from 1964, MIT-Professor JohnM. Biggs tells us that this is

a very nice situation to produce a maximum system response of the Oberwies Footbridge exhibiting a fundamental period
T

¼

0.84 s (Fig. 2.16 ).
Checking the bridge modes for susceptibilities versus pedestrian actions yields that, on the one hand, the bridge natural
frequencies lie outside of the critical range for walking people (1.6

¼

2.4 Hz) but, on the other hand, that modes B2 and B3

...

might be susceptible versus the action of joggers (2.5 3.5 Hz). As this had already become clear from the Pilot Test, jogger

tests were planned to be included into the modal tests (see Chap. 5 ) .

...

2.4 Experimental Modal Analysis of a Long and a Short Cable

Of course, it would have been possible to derive cable natural frequencies from the tests described in Chap. 3 . In an attempt
to determine the actual cable force through dynamic methods, special modal tests under ambient excitation were performed
the day after the modal tests described in Chap. 3 . The instrumentation is shown in Fig. 2.17 . The sampling rate was again
sR

200 Hz, the time window length per setup started at 20 min and had to be reduced to 15 min due to the tight time
schedule. The time pressure arose because the bottom point (No. 3 in Fig. 2.17 ) was measured in three different positions: at

¼