Indiana's Iron Truss Bridges

The following article, written by James Barker, appeared in the March, 1976, issue of Outdoor Indiana Magazine: Mr. Barker affirms that his 1976 views haven't changed much.

Antique bridges, like antique automobiles, have a certain fascination, a peaceful and happy beauty. Such bridges are machines from the horse-and-buggy era that are still performing their job in a time of Moon landings and nuclear energy. Coming across a fancy old bridge on a country road is like seeing a Model A being driven by an old man and sensing that neither has been restored -- they've just never stopped working!

Covered bridges are rare and almost unique. Few exist outside North America. Their value is widely respected, particularly by Hoosiers. Less often noticed are the lacy iron truss bridges. The ones with all the rods running a funny angles. The ones that won't let you quite figure out how they work.

A giant old truss is something special. It is stately and proud. It's a product of cheap labor -- a sort of industrial archaeological specimen. It is fancy and delicate and a little mysterious, and at the same time simple and weary from years of standing there spanning the floods.

These bridges can be majestic. Their rods and girders sometimes tower high into the air, catching the sunlight, and looking -- from within -- like a pile of tremendous pickup sticks dropped at random. Beneath the grime of decades of neglect there is real beauty is such bridges. Indiana is graced with more than its share of these structures. We are lucky.

In the West the early boomers and gold-seekers built quicky bridges that didn't last. Indiana's early settlers were farmers who stayed with their land. They knew that their great-grandchildren would need bridges some day, so they built to endure.

Each County listened to different salesmen, and each salesman had his own favorite patented truss design. As a result our State has a great variety of these old bridges. Most of them will be found languishing on the backroads of those Counties which are too loving or too poor to replace them.

There are Whipple trusses, Baltimore trusses, Petit trusses, Warrens, bowstrings, Burrs, Howes, and even Fink trusses. There are ordinary Pratts, and "bedstead" Pratts, and half-hip Pratts and camelback Pratts. Yes. Caleb Pratt hit a good thing when he patented his truss!

Just as the arch was a Roman/European development, the truss was pretty much American. Prior to 1850 few trusses were built. No one understood how they worked. Then, in 1847, Squire Whipple from New York (a commoner, his first name just happened to be Squire, like Squire Boone) discovered how to scientifically analyze the complicated framework.

Whipple's technique was a rigorous application of a simple principle: the total force acting downward on any piece in the truss would be balanced by an equal force acting upward. Otherwise the piece would start to move, usually down. A similar balance-of-forces equation holds in the horizontal direction.

This is really just a special application of Isaac Newton's great Second Law: Force = Mass x Acceleration. Where movement (i.e. acceleration) is 0, make the total net force (one the other side of the equals sign) also 0.

Using Squire Whipple's relations the forces acting in each truss rod could be determined, and the rod's required size thus calculated.

With this breakthrough the "golden age" of iron truss bridges began. Iron was used instead of wood because it was stronger (pound for pound) and easier to erect. The connections of wooden tension members gave trouble because the bolts tended to pull out. Iron eye-bars with pin connections were simpler.

Because cast iron was brittle and sometimes cracked, the more ductile form -- wrought iron-- was used in bridges. Wrought iron was expensive, and trusses used it efficiently.

Individual truss members could be hauled over primitive roads and put together at the site. Trusses were complicated to erect, but labor was cheap, so it didn't matter. Trusses were built in profusion, and the relative merits of each style was a topic of much debate.

As decades passed, steel gradually replaced wrought iron. Steel is a stronger material although it rusts faster. The fancy filigree portal bracing so characteristic of iron bridges was seldom carried over on steel trusses.

Times were changing. Labor grew more and more expensive, and the price of steel declined. Erection costs had to be reduced, so heavy beams and plate girders became prevalent.

Bridge truss construction faded. By 1930 Indiana counties had stopped building trusses. Today truss bridges are constructed only rarely, across the largest rivers.

Our inheritance of old trusses dwindles each year. Life is hard on bridges, and repairing doesn't always get done. State-administered funds that help counties build new bridges cannot be used to repair or maintain existing ones [this has changed for the better since this article was written]. Yet if bridges were better maintained they would last longer. Resource conservation concepts are slow in coming to some segments of the construction industry.

The No. 1 maintenance need of old trusses is protection from rust. While streams nibble the foundations, rust eats away at the superstructure.

Exposed metal should be cleaned and painted about every 15 years. Cleaning is especially important because dirt holds the moisture that speeds rust. Stream-scour under abutments is an occasional problem, but extra concrete, wisely placed, can often take care of this.

Load limit signs are critical, too. Truck and schoolbus weights have been steadily growing over the years. This endangers many lacy trusses.

Quite a few will simply have to be replaced. But in some cases strengthening repairs can increase the load limit sufficiently, or slight road alignment can bypass an old bridges. These alternatives should always be considered.

Bridges such as shown in the accompanying pictures are wonderful things, possessing beauty and history and public-spirited qualities far beyond strict functionalism. At their best they are almost art. Yet they can be climbed on or fished from. They help make the world very exciting for a youngster.

These bridges are old yet they express a certain sturdy Hoosier optimism. They are content to do humble but helpful jobs in good style. They remind us that our grandparents built for the future--for us-- with the charity to provide what their descendants would need.

Indiana's iron truss bridges are public heirlooms. They deserve to be treated kindly. Maybe the preservation of one of them will become your County's Bicentennial project.