Hi joker, welcome to the board. Usually when someone talks about the natural frequency of something, they're refering to a specific geometry such as a concrete column 2 feet in diameter, 50 feet high and fixed at the base for example. Such a structure vibrates around a natural frequency which is dependant on the stiffness of the structure, it's weight and other things. Could you refine the question for us?
This may be a little crude, but you could ping your structure (hit it with a hammer) and record the vibrations with a microphone. If you record it as, say, a .wav file, you can bring it into MATLAB (for example) and do an FFT to find the frequency content of your system (I guess it would actually be a damped frequency that you would record). If you want a theoretical prediction of the first natural frequency, then as Q_Goest said, you need to know the geometry of your structure and how it is supported.
Hi joker. Ok, so this is an actual structure you need to find the natural frequency of. I'm not big into vibrations, but I suspect there are ways of measuring this as Jamerc suggests. I doubt you'd even have to ping it though. It sounds like you're talking about a bridge that is used for motor vehicles. Is this in Europe? England? Anyway, I would guess there are accelerometers you could attach to the structure, probably made specifically for this purpose. For example, I design reciprocating machinery, and there are specific vibration meters I use to measure amplitude and frequency of that machinery. They can also be used to determine resonant or natural frequency, though I've never done that.
I have to believe there are similar things for bridges. You just need to search a little and find out what companies specialize in that kind of equipment.
This is a tough nut. You are doing this on a complex system. Finding the natural frequencies are usually done numerically. Are you specifically looking at the concrete drive path on the bridge? Are you sure you're really looking for the natural frequencies or just the common forcings that the bridge experiences?
I don't see any other way to do this other than instrumenting the bridge in the areas of interest. However, how are you going to tell when you actually have reached a mode?
I think you need to explain what it is you are trying to actually do here.