But no matter what the string's length, the pendulum will exhibit a resonance. The frequency at which this resonance occurs is all that changes. Period is the time it takes for a resonant system to complete one cycle of its motion. For example, if a pendulum takes two seconds to swing over and back, then its period is two seconds. For example, if the pendulum swings one meter to the left of center and then one meter to the right of center, its amplitude of motion is one meter.
Frequency is the number of cycles a resonant system completes in a certain amount of time.
Apparently not, because we've never seen or heard of one doing it. To prevent that sort of thing from happening, the bridge builders probably do two things. First, they damp all of the resonance in the bridge. By this, I mean that they introduce energy loss mechanisms that sap the energy out of all the resonant motions. For example, they could add plates that slide against one another as the bridge bends so that sliding friction will waste energy and spoil the resonant motion.