Experiments on Large Structures - Dynamic Behavior of Concrete and Seismic Engineering - page 218

Table 6.2 presents the average dynamic amplitude factor values corresponding to
strains measured on the main trusses, for the displacements measured (LVDT at
position 14), and the integration of the average accelerations recorded for all
positions. The highest values were obtained for a single lorry, whereas strain
dynamic amplification factor values were higher than the values obtained for
displacements. This can be explained by the fact that strains are affected more by the
contribution of higher modes than displacements, and also because the lowest
vibration frequencies are below the 2 to 5 Hz range generally associated with
commercial vehicles.
Figure 6.14. Separation between torsion and bending modes
Ambient vibration tests (steady traffic) were also carried out. As many as 70
ambient vibration response measurements were recorded for 120 seconds with a 100
Hz sampling frequency. The frequency content and coherence functions for vertical
acceleration are shown in Figure 6.14. The responses of both sides of the bridge
deck were added and subtracted to separate bending and torsion modes, which are
often quite close to each other. Graphs on the right illustrate the frequency contents
of the added data leading to the identification of bending modes, whereas those on
the left illustrate the results obtained with subtracted answers where torsion modes
are isolated. A total of 23 bridge deck modes (10 in bending, 8 in torsion, 4 lateral, 1
longitudinal) and 8 tower modes could be identified.