<P> Each structure has natural frequencies . For resonance to occur, it is necessary to have also periodicity in the excitation force . The most tempting candidate of the periodicity in the wind force was assumed to be the so - called vortex shedding . This is because bluff bodies (non-streamlined bodies), like bridge decks, in a fluid stream shed wakes, whose characteristics depend on the size and shape of the body and the properties of the fluid . These wakes are accompanied by alternating low - pressure vortices on the downwind side of the body (the so - called Von Kármán vortex street). The body will in consequence try to move toward the low - pressure zone, in an oscillating movement called vortex - induced vibration . Eventually, if the frequency of vortex shedding matches the natural frequency of the structure, the structure will begin to resonate and the structure's movement can become self - sustaining . </P> <P> The frequency of the vortices in the von Kármán vortex street is called the Strouhal frequency f s (\ displaystyle f_ (s)), and is given by </P> <Dl> <Dd> <Table> <Tr> <Td> <P> f s D U = S (\ displaystyle (\ frac (f_ (s) D) (U)) = S) </P> </Td> <Td> <P> </P> <Table> <Tr> <Td> <P> </P> </Td> <Td> <P> </P> </Td> <Td> <P> </P> </Td> </Tr> <Tr> <Td> <P> </P> </Td> </Tr> </Table> </Td> <Td> <P> (eq. 2) </P> </Td> </Tr> </Table> </Dd> </Dl> <Dd> <Table> <Tr> <Td> <P> f s D U = S (\ displaystyle (\ frac (f_ (s) D) (U)) = S) </P> </Td> <Td> <P> </P> <Table> <Tr> <Td> <P> </P> </Td> <Td> <P> </P> </Td> <Td> <P> </P> </Td> </Tr> <Tr> <Td> <P> </P> </Td> </Tr> </Table> </Td> <Td> <P> (eq. 2) </P> </Td> </Tr> </Table> </Dd>

What is the force exerted by the tacoma narrows bridge