Bridge Building Unit
History of bridges:
http://en.wikipedia.org/wiki/Clapper_bridge http://en.wikipedia.org/wiki/Arch_bridge http://www.makingthemodernworld.org.uk/learning_modules/math s/02.TU.03/?section=4 http://www.britannica.com/EBchecked/topic/243775/Great-StoneBridge#assembly=url~http%3A%2F%2Fwww.britannica.com%2F EBchecked%2Ftopic-art%2F243775%2F402%2FThe-Great-StoneBridge-built-between-589-and-618in&tab=active~checked%2Citems~checked&title=Great%20Ston e%20Bridge%20--%20Britannica%20Online%20Encyclopedia http://en.wikipedia.org/wiki/Image:Ironbridge_6.jpg http://en.wikipedia.org/wiki/Image:Britannia_Bridge__circa_1852.jpg http://en.wikipedia.org/wiki/Image:Brittania_Bridge_Train_crossing _3.JPG http://en.wikipedia.org/wiki/Image:Larrys_Creek_Covered_Bridge.J PG http://en.wikipedia.org/wiki/Image:Brooklyn-Bridge-Panorama.jpg http://en.wikipedia.org/wiki/Image:Brooklyn_Bridge_New_York_City _1896.jpg http://bridges.wikispaces.com/Beam-Type+Bridge