Cross bracing is a form of reinforcement in construction that involves putting braces from the right-hand corner of the floor to the left-hand corner of the ceiling, then the same again for the converse. The result is an X-shaped brace that pushes the floor and ceiling against one another, thus increasing the structure's stability.
Floors and Ceilings
A steel structure without cross bracing has its floors and ceilings very separate. While they are connected to one another, they do not support one another as much as they could. Cross bracing takes the large amount of support the heavy floors and ceilings offer and transfers it to one another. Rather than build expensive auxiliary support structures, cross bracing utilizes and applies the support already available.
Cross-bracing is used most frequently in areas like California where earthquakes are common. This is because buildings in these areas need to be more stable than buildings in other areas; after all, a building in, say, Iowa, is probably not going to have to withstand seismic activity.
This means that cross-bracing is essentially an emergency measure. Buildings can function without it on a day-to-day basis, but should an earthquake destabilize a building it is the cross-bracing that will keep it standing.
To understand how cross bracing works, think of a child climbing in a doorway by pushing his feet against one side and his butt against the other. He is able to sit in the top of the doorway because he is pushing against both sides, applying the weight of the door and the force his body can generate to create stability. Cross-bracing works on the same principle — pushing two surfaces against one another to hold something in between them, except where the child uses a doorframe cross-bracing uses a building's floor and ceiling.