Find the mass of the structure (m) , find the stiffness (k) of the material, the bridge is made up of and put them in the equation below:
F = (1 / 2π) * √ (k / m)
The stiffness of the material can be found from a stiffness table. The use of units must also be consistent throughout i.e. If you use Kg for mass, use Nm for stiffness.