a load moves horizontally across a flat bridge, eg a train, does the time the load is on the bridge affect the vertical force on the bridge. put another way; can a bridge support a load greater than it's maximum load it can support for a stationary object if the load is only on the bridge for a very short time. I ask this because as a kid we had bridge building contests with balsa wood, we determined the best designs by adding a stationary weight until the bridge collapsed. when that same weight was sent across the bridge at speed on a cart the bridge seemed to hold up even when the load increased. flat bridge means no projectile motion.