PSI - Issue 14

Angitha Vijayan et al. / Procedia Structural Integrity 14 (2019) 696–704

703

Angitha Vijayan et al. / Structural Integrity Procedia 00 (2018) 000 – 000

8

5. Results and discussion

Fig. 6 shows the FRF curves for varying crowd size. The modal properties and response of the crowd structure system are summarized in Table 8. The comfort level of the people standing on the structure is also estimated. It is observed that with the change in activity, size and location of crowd, modal properties and response of the CS system change. This behavior can be attributed to the crowd interaction with the structure. It is observed that the natural frequency of the CS system is less than that of the empty structure. As the crowd size increases, the natural frequency of the CS system decreases whereas the damping ratio increases. The peak acceleration level is observed to increase with the increase in number of people. It is seen that the type of activity does not affect the natural frequency and damping of the crowd structure system whereas it affects the vibration level. Vibration level at resonance is found to be more for walking crowd than running crowd. The comfort level during resonance of the system is found to be either disturbing or unacceptable for all the cases studied. It is further observed that the trend of change in natural frequency of the occupied structure for change in crowd properties differs with change in modal properties of the empty structure. At a natural frequency of 4.440 Hz for the empty structure, the frequency of the CS system increases with increase in crowd size as seen in Table 1. At a frequency of 3.356 Hz for the empty structure, frequency of the CS system decreases with increase in crowd size as seen in Table 8.

Table 8. Modal properties and response of CS system for varying activity, size and location of crowd Activity Location No. of people f cs (Hz) ζ cs (%) ̈ , (m/s 2 ) 1 3.352 1.35 1.04 10.58

Vibration level (% of g)

Comfort level

Disturbing Disturbing

3 5 7 2 4 6 1 3 5 7 2 4 6

3.350 3.347 3.345 3.342 3.350 3.345 3.342 3.350 3.345 3.342 3.352 3.350 3.347 3.345 3.342 3.350 3.345 3.342 3.350 3.345 3.342

2.16 2.97 3.83 5.16 2.16 3.83 5.64 2.16 3.83 5.64 1.35 2.16 2.97 3.83 5.16 2.16 3.83 5.64 2.16 3.83 5.64

1.61 1.84 1.96 2.06 1.34 1.56 1.65 1.89 2.20 2.33 0.82 1.27 1.44 1.54 1.62 1.05 1.23 1.30 1.49 1.73 1.83

16.45 18.74 19.98 21.04 13.63 15.90 16.81 19.27 22.48 23.77 12.94 14.74 15.72 16.55 10.72 12.50 13.22 15.16 17.68 18.69 8.32

Along the span

Unacceptable Unacceptable Unacceptable

10

Walking

Disturbing Disturbing Disturbing

In a circle at midspan

2+2 4+4 6+6

Unacceptable Unacceptable Unacceptable

In a circle at 1/4 th + 3/4 th of span

Disturbing Disturbing Disturbing Disturbing Disturbing Disturbing Disturbing Disturbing Disturbing Disturbing

Along the span

10

In a circle at midspan

Running

2+2 4+4 6+6

In a circle at 1/4 th + 3/4 th of span

Unacceptable

6. Conclusions

The behavior of structures when occupied with crowd is studied in detail. It is observed from the results that, interaction of crowd with the structure increases with the increase in crowd size. As a result, modal properties,