Here are the dimensions of the bridge, as described by Kaladin, "around thirty feet long, eight feet wide". This limits the maximum size of the chasms crossed by the armies at around 15ft across. This also means that you definitely cannot have a four abreast cavalry charge. You might be able to get four horses on the bridge, but even two side by side would be a very tight fit. The bridge would probably have to be at least 8-12 inches thick as well because it would have to support at least two more likely four, probably medium to heavy cavalry, which could exert a force of at least 3700-4700N each (adjusted for Roshar gravity). I have no idea how much force a horse kicks off the ground with, but an average medium warhorse weighs around 400kg on earth plus a rider of between 80-120kg depending on size of person and weight of armour so it seems about reasonable. Assuming dimensions of around 10m x 2.5m x 0.25m, this gives us a volume of 6.25m^3. Since they really need about 25 bridgemen to carry a bridge in battle, the maximum any of them can carry while running is probably around 40-60kg on Earth. 60x25x0.7 is 1050kg. This gives the wood a density of 168kg/m^3, about the same as balsa wood.