K-Truss Bridge

Before I go into the topic on K-truss bridges, the second of many to come in a series on unusual bridge designs, I would like to tell you how I managed to come across this bridge, located over the Monongahela River near the town of Speers in western Pennsylvania. I did a paper on this subject during my time of my Master’s studies at the University of Jena in eastern Germany and sent out a request on the topic of K-truss bridges to the Bridge Mafia, as Eric DeLony coined it- a group of hundreds of bridge experts with a vast knowledge of bridges, history and preservation.  Already I knew of the number of K-trusses that existed in Oklahoma, according to Wes Kinser’s website on Oklahoma’s historic truss bridges. Yet I did not know that there was a bridge like this, built in the 1920s carrying the Wabash-Erie Railroad. Given its proximity to the I-70 Belle Vernon Bridge located right next to it, combined with many problems trying to get to the bridge for the best shot, I took the chance and hugged the pier of the I-70 Bridge, sliding and creeping around with my body literally glued to it and with almost no space to turn around. Fellow pontists Nathan Holth and Luke Gordon, who were bridgehunting with me at that time, were at the scene and the photos speak for themselves.

KTruss 1
The following two photos were taken by Nathan Holth
K-truss 2
Arriving at Destination: Ready, set, snap!


According to Kara Russell at PennDOT, this bridge is the only one left in the state, and one of only a couple left in the country, whereas the K-truss bridges seen on the roadways in the country are solely through trusses. But what makes a K-truss so unique?  It is clear that the truss type is a cross between a Parker and a Pennsylvania petit but feature two subdivided diagonal beams per panel that meet at the center of the vertical beam, featuring the letter “K” in the alphabet. There are two types of K-trusses that exist: one that features the subdivided beams going outwards away from the center of the span, creating a rhombus shape at the center of the span, as seen in the bridge at Speers. Yet the other type features subdivided beams going inwards, towards the center of the span, creating the letter “X”.  This truss design is one of three that feature diagonal beams resembling a letter in an alphabet. The other two are the Warren (with the W-shape) and the Howe lattice or double-intersecting Warren, which feature the letter X. Technically, a two-panel Warren truss design, resembling the letter V also counts in the mix.

Close-up oblique view of the K-truss as seen with the Speers Railroad Bridge. Note, like the bridges in Oklahoma and Tennessee, the connections on this bridge are riveted.


Example of the first type of K-truss that exists, except the Speers Bridge has a sub-divided Rhombus shape


Little is known about the K-truss bridge design except to mention that it was invented by Phelps Johnson of the Dominion Bridge Company in Montreal, Quebec, Canada. According to information collected to date, the K-truss came into existence in the United States during the age of Standardization in the 1920s. During that time, fancier but structurally deficient truss designs, such as the Thacher, Kellogg and Whipple trusses, were either phased out or modified with heavier truss beams and riveted connections with the goal of handling heavier volumes of traffic. Apart from the bridge near Speers, Tennessee and Louisiana were the first two states that introduced and built K-trusses in the 1920s and 30s.  Oklahoma began to adopt this truss design in the mid-1930s but its construction reached its peak during the 1940s and 50s.  As many as 54 of these K-truss bridges were reportedly built in that state during that time period, three out of four of which are still standing today, making it the state with the highest number of K-trusses. Bridges featuring the K-truss include:

Washita Bridge in Johnston County

Salt Creek Bridge in Osage County

Ralston Bridge

Alaska is the youngest state to implement the usage of K-trusses, as five bridges were built along the two main highways (Parks and Richardson) with the longest being a three-span 1300-foot bridge near Nenana (the main span being 500 feet).  Each of the five bridges were built between 1966 and 68 and are still in use.  And lastly, K-trusses were used on cantilever truss spans, including the Savanna-Sabula Bridge spanning the Mississippi River between Iowa and Illinois. Built in 1932 by the Minneapolis Bridge Company, the Sab-Sav served traffic until a replacement bridge, a through arch built alongside the structure, was opened to traffic in 2017. On March 8, 2018, the old bridge was demolished.

The Sab-Sav Bridge before its replacement came along. Photo taken in January 2015.

Even though Phelps Johnson invented the K-truss, it is unknown when he first used it, for although the first of these trusses were built in the 1920s and 30s, there are reports that K-trusses existes in Europe as well. Two bridges in Germany carry the same truss design and appear to be either as old as the earliest of these truss bridges in the US, if not older. This includes the Rhine River crossing near Mainz at the junction of the Main and Rhine Rivers. The other is a railroad bridge in Passau, spanning the Danube River, connecting the town with an industrial district in Austria. Both bridges carry rail traffic but the Mainz crossing is one of the heaviest used bridges in the greater Frankfurt area, for it serves regional and long distance train services connecting Mainz and Wiesbaden with Frankfurt and its international airport. It is possible that both bridges may have been built after World War II replacing previous structures that were destroyed by Allied bombings. Yet given the pristine condition of the towering portal entries in Mainz combined with the wear and tear of the trusses, it is possible that both these bridges may have existed before the war and somewhat survived the war in tact.   Ironically, while K-truss bridges are no longer being built in the US, they are still being used in Europe and elsewhere as an alternative to more expensive bridge types, such as arch and suspension bridges. This includes the Novi Sad Bridge in Serbia (built in 2000), The Tamur River Bridge in Dharan in Nepal (built in the 1980s), and The Ganga River Bridge in Patna, India (currently being built).

This takes us back to the deck truss railroad bridge near Speers, which is still being used for rail traffic even as this is being posted. While Phelps Johnson invented the truss bridge design, questions remain when the first bridge was built using the design and where. It is impossible to invent the truss type during the time he was alive (he was born in 1849) and not use it, like the other engineers had done with theirs, including the Thacher Truss as seen in the article here.  It is possible that despite a handful of bridges being built in his time that the truss bridge design was shelved for a few decades before being rediscovered and modernized during the age of Standardization in the 1920s, in which it made its comeback through the 1960s before jumping overseas. But more information is needed to answer the questions, mainly:

1. When exactly did Phelps Johnson invent the K-truss bridge? When and where was it first used?

2. When was the K-truss rediscovered and used again during the age of Standardization? (Note: pending on which state, the age started around 1915 and lasted until World War II)

3. Apart from the ones in the United States, where else have K-trusses been used in Europe, Asia and elsewhere and why are they used more commonly there?

4. What do we know about Phelps Johnson and his work as engineer and bridge builder? He had worked for the Wrought Iron Bridge Company prior to taking the presidency at the Dominion Bridge Company.

Any information pertaining to the K-truss can be submitted via e-mail, but also through facebook and LinkedIn. Who knows how many more K-trusses are/were out there but we do know that they were and are still popular for bridge construction. It is just a matter of finding out when it was used and why the design was on hiatus. Good luck with the research and that stunt I told you about: Don’t do this at home. We’re die hard professionals that have a passion for finding the truth and beauty behind these artifacts, no matter what the cost…

N Holth
Photo taken from the pier of the Belle Vernon I-70 Bridge in Speers

It was a good thing that Nathan Holth (pictured here) still had photos of my daring act and sent me a few to be used in the article (I’d had a couple from him earlier that disappeared with an old computer that crashed after the Historic Bridge Weekend in Pittsburgh) and would like to thank him for providing me with the spares to be used and saved for story-telling purposes. 

Another round of thanks goes to the Bridge Mafia for their help in obtaining the K-truss info so far and to Kara Russell at PennDOT for mentioning the Speers Bridge.


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