Improved Ribbon Bridges are a mobility marvel for the Army, allowing troops to move 70-ton tanks across large rivers of flowing water and take the fight to the bad dudes on the other side.
The IRB is a pontoon bridge that is put in place by multirole bridge companies. The MRBCs use small bridge erection boats to navigate the rivers and position the structure.
Once the engineers and their boats are in the water, the bridge bays are launched. These are floating sections that are unfolded and propelled into position. Each MRBC is equipped with 42 of the bays and can build up to 210 meters of span.
The bridge bays can be launched by truck or helicopter. The MRBCs have their own trucks, but require support from aviation units to launch by helicopter.
If there aren't enough bays to bridge the entire river, the MRBC can use them as rafts. They position the bay on one side of the river and drop the ramp onto the bank. After the vehicles drive on, engineers lift the ramp and begin crossing the river.
Once near the opposite shore, the engineers drop down a ramp against the bank and the vehicles can drive off.
Of course, vehicles can cross much faster if the engineers are able to complete and position one unbroken span. In that case, tanks, fighting vehicles, and humvees can simply drive across. Humvee traffic can pass in two directions at once, but tanks and most fighting vehicles have to cross near the center, making it a one-way bridge.
Watch engineers deploy the bridge and wave tanks across in the video below.