Walnut Street Bridge

Calvin,

Sorry for the delayed response. Apparently my initial response didn't load.

I have no problems with disagreements and welcome discussions. Writing my response cleared out some cobwebs in my memory banks :^)

I look forward to seeing your other photos and your input/insight.

James,

There are many errors that can be perpetuated by being written down. The issue is: What was the basis of that chart. Its not wrong but was it industry standard terminology or is it what the guy who made the chart knew? The trick is getting general agreement on terminology for this site with the an aim of adopting the most commonly used terms. While I'm OK tying the truss types to that chart, I personally don't think its necessary.

To me, the original list is fine and correct for our purposes. If we start renaming the configurations, things will get confusing e.g. a Baltimore can also be called a subdivided Pratt or, according the same chart, a Petit.

Tony,

Of course you'd find the oddball :^)

My understanding of the subdivided panel rigging is that it increases stability especially under a dynamic load.

One of the problems with a Pratt is how parts of the bridge take and release strain as the load travels across it. If the load (vehicle/horse/etc.) travels too fast, the strains can go negative in some panels causing instability and failure. Which is why the old bridges had signs that all said 'cross at a walk' or similar.

By subdividing the panels, the elements are stabilized and stay aligned even at increased length allowing higher static and dynamic loads. (think wheelie bars or axle straps on a hot rod).

I agree with you that a bridge with any panels that are subdivided should be in the Petit family (Baltimore/Pennsylvania).

Regards to all,

Art S.

And certainly no disrespect meant to you Calvin!

I have seen these dubbed as "Pennsylvania Truss-Camelback format". This is to differentiate them from the much more common Parker style (more than 5 slope) endpost/upper chord format. But slopes are not trusses, and it's just muddying the waters to say this is a Camelback... but yet it's a Pennsylvania. Throw Pratt together with either of those two and it makes sense.

The Black River Bridge in Posey County, IN is a Camelback with a sub-divided center panel: http://bridgehunter.com/in/posey/griffin-road/. All of the other panels are traditionally composed.

So I guess from a terminology standpoint there will always a level of dissension here. To me it is, and always will be, a Pennsylvania Truss.

No problem, Art. I'm not trying to get into a disagreement. I've photographed hundreds of bridges that I have yet to post on Bridgehunter, several of them gone now.

I enjoy a healthy discussion with fellow bridge lovers.

Stirring the pot some more...

According to the HAER poster of truss types, this is a "Camelback with subdivided panels". It's defined as "a Pennsylvania truss with a polygonal top chord of exactly five slopes." It's shown separately from the regular Pennsylvania truss.

I've never noticed that distinction before:

https://www.nps.gov/hdp/samples/HAER/truss.htm

Calvin,

I appreciate your desire for accuracy to the extent of checking with experts before posting. One of the interesting things you will encounter is that most engineers involved with bridges today deal in cable stays and pre-stressed concrete. They are true experts in these areas. Few of these engineers also have a deep knowledge of trusses.

An equivalent scenario is the engineers and crews that design, build and maintain modern railroads may not have the same knowledge of steam locomotive design and operation as they do about diesel-electric locomotives.

This is also why bridge enthusiasts are a bit of a thorn in DOTs' & DPWs' sides. We spout on poetically about these wonderful old structures in a similar manner to foamers going on about old engines. They want to get on with the business of transportation and are not necessarily knowledgeable about, or set up to maintain, these obsolete elements.

Its also the reason most of these old trusses are a genuine headache for these agencies.

Regards,

Art S.

PS. I am in no way trying to pour cold water on your enthusiasm nor was what I wrote intended as any sort of attack. Why not google these designs yourself? The basic engineering of most trusses is quite straightforward. You can figure out the strains without an engineering degree.

PPS. I'd be curious to know what your friend/acquaintance at T-DOT would say if you shared this and my prior post with them.

The note I received verbatim from the Regional Bridge Inspector with the Tennessee Department of Transportation, who confirmed the information with his supervisors at T-DOT, Nashville:

Calvin: Walnut Street is a pratt camelback. The approaches are just “riveted cross frame supports” or sometimes called a “rigid frame support”.

Let me know if you need any more information.

Art, I appreciate your descriptions. I will go with what the people who build and repair them tell me they are, and I withdraw anything I have put on this particular bridge. You all are free to call them whatever you feel is correct.

Calvin,

Your understanding of truss types is incorrect, Tony is correct. This is a Pennsylvania.

A Pratt is a truss that has a simple web with diagonals in tension (being stretched) and verticals in compression (being squeezed).

A Parker is a Pratt (same loading characteristics + simple web) with a polygonal top chord (not flat between endposts).

A Camelback is a specific Parker configuration where the top chord is limited to three slopes (five including the endposts) thus each slope extends over multiple panels.

A Pennsylvania is any configuration of Parker, including Camelback, where the web is complex or subdivided in the manner of a Baltimore truss (connections also occur away from the panel points).

One way to think about it is a Pennsylvania is a Parker that is rigged like a Baltimore truss rather than like a Pratt truss.

Since the rigging defines the Pennsylvania truss, there really isn't anything to debate. There may be a sub variety / call-out specifying a five slope Pennsylvnia as a 'Camelback' Pennsylvania - as far as I know this is not universally regarded as its own sub variety/type.

Sincerely,

Art S.

They look like Pennsylvania trusses to me Calvin. Yes, they do have the 5-slope endpost/upper chord configuration of a Camelback, but the trusses themselves are sub-divided Pennsylvania trusses.

This bridge was found unsafe for vehicular traffic and stood abandoned and unused for more than twenty years, despite repeated petitions by the Coast Guard to either repair it or remove it. Community interest in preserving this bridge led the City of Chattanooga to include the restoration of the bridge as a key part of the Tennessee Riverpark. Most folks today do not realize how close we came to losing this bridge.

This bridge is a popular spot for college students from UTC as well as Lee University who are looking for a romantic setting for a walk with a date.