A surprising number of bridges collapse as they're being built. You might think these types of collapses aren't as serious because no one was driving on the bridge at the time of the collapse. Unfortunately, some of the deadliest bridge collapses in history have occurred during the bridge's construction. While a functional bridge may only have a few vehicles on it when it collapses, it takes hundreds of workers to build a bridge -- all of whom may be in dangerous positions in case of collapse.
The 1907 collapse of the Quebec Bridge crossing the St. Lawrence River at Quebec City shows how engineering miscalculations can lead to disaster. The bridge was only partially constructed, but parts were already bending and breaking from the weight of the bridge itself. Engineers were concerned, but unable to take action swiftly enough. When it collapsed, 74 workers were killed [source: Structurae]. Amazingly, when the bridge was being rebuilt in 1916, it collapsed again, killing 13 more workers. It was finally completed in 1917 and remains in use today.