|Photo from Bridge Hunter|
It is unusual to see a metal bridge using a Howe truss since compression members require more material then tension members and in a Howe truss the longer diagonal members are the ones in compression. Then a description in the HAER for this bridge, "the Howe deck trusses consist of cast and wrought iron and rest on ashlar sandstone piers," reminded me that the manufacturing processes that made steel affordable had yet to be invented in the early 1850s. Cast iron is strong in compression, but wrought iron must be used for tension members.
|Copyleft: Photo by Roantrum (Flickr) [CC BY 2.0], via Wikimedia Commons|
Making wrought iron is significantly more complicated, and thus expensive. By the 1800s, the puddling process would be used to make wrought iron. This heats cast iron in a furnace while stirring the iron until it melts. The stirring oxidizes the impurities of silicon, manganese, sulfur, and phosphorus. Then more fuel is added because the melting temperature increases as the carbon is burned off. Finally, the melting temperature becomes high enough that the iron solidifies into large "puddle balls." In addition to the monetary expense of wrought iron, there was the social expense of most puddlers dying in their 30s.
|Copyleft: Photo by Rainer Halama from Wiki5|
So even though a Howe truss uses more metal than other truss designs, it uses less of the significantly more expensive wrought iron.