i recorded the vehicle induced vibrations and then processed the vibration data in matlab. i applied windowed fourier transform to get fundamental frequency. but the problem is that frequencies are increasing in the portion of vibration data where heavy vehicle is passing but this is against the laws of physics. physics says that fundamental frequency is f= 1/2π √.k/m . so when mass will increase the frequency will decrease. what possible factors are effecting the fundamental frequency.
this is i shaped girder bridge with16 spans and each span is 140ft
Sounds to me like you're seeing harmonics of the fundamental frequency being excited by the fundamental. Are the high frequencies related to the fundamental? (like 2f, 3f, 4f etc)
From your vague description, it looks like you recorded vibrations of something different than a simple oscilator, and that 'something' does not include the vehicles. I this case, you should not use masses of the vehicles. Moreover, a vehicle mass is connected to the road by the vehicle suspention system, different for different vehicles. If you try to consider this situation as interacting oscilators, you were to take into account this difference of k. You do not do that in your consideration. Generally, you use a wrong model for your measured data.
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