I was looking at the Sydney Harbour Bridge the other day, trying to work out how the weight of the roadway was transmitted to the ground. Here's a picture. You can see that there is an inner (lower) and outer (upper) arch. From underneath the bridge I saw that the lower arch abuts against the big granite pylons. So I concluded that the lower arch supports the weight of the road, via the vertical girders from the lower arch down to the roadway. But I couldn't work out what the upper arch is for. It just ends without abutting against anything massive. What does it do? Could the bridge have been built just as effectively without the upper arch?