This is the last of two posts that talk about the materials used in Indiana's historic bridges. From iron to steel in the last half of the nineteenth century, to concrete in the early twentieth century.
It only seems natural that the two earlier forms of bridges made of stone and wood would someday be emulated with man-made materials that would attempt to capture the benefits of their predecessors while offering improvements in strength and durability.
Truss bridges would be the first to experience this phenomenon, as the labor-intensive hand hewn wooden beams were replaced with components formed from cast and wrought iron, and later of steel. The benefits were numerous indeed...from allowing for larger and longer span that didn't require a roof and siding to protect the trusses from the elements, to components that could be manufactured in a factory and easily assembled at the site. This allowed for mass production of bridges that would help to improve the transportation infrastructure as a whole.
Where wooden trusses were limited to a handful of "trusted and true" types, the introduction of metal allowed for designers to experiment and look for constant improvements to reduce weight and save materials. Wood had definite limitations in form that was easily solved within it's metal counterpart. Although wooden covered spans would continue to be built on occasion into the 1920's, their heyday had pretty much passed by the early 1880's.
The earliest metal spans started appearing in Indiana by the late 1860's, with Warrick County's 1869 Boner Road Bridge the oldest survivor today. This is a rare example of a Bowstring truss that was popular only into the 1880's. Thomas and Caleb Pratt's truss, on the other hand, would enjoy a popularity that would not be exceeded by any other type to come along. The Pratt truss was found in pony and through trusses and would also be transformed into many other variations over the course of it's existence. The Whipple truss was a Pratt that featured verticals that extended across more than one panel to allow for much longer spans with less weight. When designers looked to reduce material even more, the Camelback and Parker versions of the Pratt were introduced that featured polygonal upper chords. And when the railroads were looking for even stronger versions of the Pratt, they found that sub-dividing the panels within the trusses would give them what they needed. The Baltimore and Pennsylvania trusses were the results of their efforts.
A limited number of metal truss bridges remain in Indiana today. Sadly, the last quarter of the twentieth century saw about two-thirds of these remaining spans obliterated. Now we are scrambling to save the best examples of what remain, and fortunately Indiana is now one of the better states at working to accomplish this.
The introduction of concrete would have even a bigger impact on the future of bridge building that has continued to this day. Concrete first appeared in the sub-structures of wooden and metal spans at about the turn of the century. Cut stone abutments were expensive and often cost more than the bridge superstructure. Concrete had actually been used by the ancient Romans in many of there structures but the technology was basically "lost" for centuries until the mid-1700's. Just why it's use in bridge building wouldn't surface for almost another 150 years is uncertain. It would quickly become evident that concrete abutments and piers offered great savings over their stone siblings.
It didn't take long for designers to discover that concrete could be used for much more than substructures. Concrete by nature is very strong when in compression but falls short under the stress of tension. Reinforcing bars (better known as rebar) were added to the concrete to give it the tensile strength needed for Indiana's first concrete arches. Although a couple pre-1900 examples may survive, large scale construction would begin in the first decade of the twentieth century. Arches were the first type of concrete span to appear, perhaps an emulation of the stone arches that were built in the previous century. But much like the metal trusses to the wooden ones, concrete spans would evolve past the arch design and take on many shapes and sizes even into modern day.
Many wonderful examples of historic concrete bridges remain in the state today, and like the metal trusses some are enjoying a revival through rehabilitation and continued use. Although moving a concrete span is rather impractical compared to a truss bridge, it is hoped that creativity can help to save the best survivors.
I started writing this way back in December and I thank you all for your patience between this and my last one. Sometimes family and work constraints can distract one from the other pleasures of life.