Part 1: https://www.youtube.com/watch?v=GWZr8YX1bW8
Building Bradshawgate: http://youtu.be/ZxKqYuJBOhk
Building Orlando Street bridge: https://www.youtube.com/watch?v=HiK0ICYxZoU
In part 2 Jenny discusses the construction of a small footbridge using ready to buy parts from two Dapol signal gantry kits. These are very versitile, and the same technique can be used to create larger bridges as well as pipe gantries.
Jenny also covers the building of the Bradshawgate tunnel which shows techniques to create both cast iron girders and steel girders. Whilst strictly not a bridge, these techniques are perfectly adaptable to a vast plethora of different bridge applications in the model world.