The leading end would start to dip down about half-way to just over half-way across the chasm. That would only work if the width of the chasm is less than half the length of the bridge. The chasms couldn't all be that narrow. Also, the plateaus couldn't all be the exact same height, either. If a chasm width was less than half the length of the bridge, but the opposite plateau was just a foot higher, how do they get the leading end of the bridge up high enough?

It just doesn't make any physical sense. I'd like to hear Sanderson explain how this is done. Considering how important the bridges are to the story, this is a detail that can't just be glossed over.