Influence of longitudinal forces on construction

It is one of my first posts, so I think it is time to introduce myself. My name is Volker and I’m living in Germany.

I’m collecting American HO-scale diesel locomotives illustrating their history and planning a HO layout in the 1920s era.

It is a nicely built and good looking bridge and represents an early bridge nicely. Deviations from the prototype are only noticeable at a close distance.

But I would suggest a change in the bottom chord as the bridge has to carry quite heavy traffic according to the second photo.

As you built it the bridge represents an early bridge built for light traffic. The two six-axle diesels and their train are not light and produce quite substantial longitudinal compressive forces in the bottom chord from braking and accelerating. In those panels where these compressive forces exceed tension caused by vertical loads the resulting compressive forces will cause the eye bars to buckle.

This can happen in the last two panels at both supports in this type and length of bridge under the shown load. I would change the bottom cords in the two last panels on each side to cross sections like the posts of your bridge which are compression-proof. They would still be pin-connected.

But it is your model railroad and you make the rules. It is just comment from a professional point of view.

Regards, Volker

Pin connected bridge

There are still many pin connected bridges with eye bars in main line service in the US, especially the eastern US.  The B&O was especially fond of them, and this was built to give it a B&O look.  I do appreciate the engineering aspect of what you are saying, you can watch the eye bars flap around on some of the prototype bridges when a train is crossing.  I guess some of the heavier pin-connected have more eye bars per section and maybe that gives them more rigidity.  Plus you have the girder assembly under the track which would counteract the longitudinal compressive forces it would seem.  For me the point was to model something that looked like a B&O bridge, and had an older more fragile look to it.  And as I said, there are still many of these bridges in service today carrying heavy loads such as coal trains.  I think in Europe (especially Germany) the bridge physical plant probably falls into two categories, heavy stone or concrete structures that survived the war, or steel structures that were destroyed and replaced with more modern examples after the war.  Thanks for your comments, I have always found bridges to be fascinating.

Jim

Working on the same project

I am also about to start on the superstructure to convert the CV  bridge to a pin connected truss, based on DRGW narrow gauge prototypes. I am shortening my bridge by one section to match my prototype. Question: How did you connect the pins? And what did you use for the spacers between the pins? Your bridge is excellent! Nice work!

Bridge construction

When I converted my bridge I actually narrowed the entire superstructure so that the pins on the center cross frames became the pins to connect the eye bars.  This allowed me to keep the eye bars on the inside of the vertical posts to hide the fact that they did not include an actual 'eye' at the end. I have attached a construction pic that shows more detail.   I would really like to build one that had the correct eye bars so they could be positioned outside the vertical posts.

hope this helps.

Jim

Pin connected

Here's one on the old SP at Capitola Ca.  with the tracks running up on  the top instead of thru the trusses.It looks spindly but held up multi units diesels and loaded coal and cement trains....DaveB 

Capitola Bridge

There is no danger that the tension (eye) bars of the bottom chord of this bridge have to carry compressive forces. The longitudinal forces are led to the top chord which is naturally designed for compressive forces. Through the diagonal end struts the forces are carried to the pier.That is true for all deck truss bridges.

@ Tonka: Do you have a photo of your prototype? I am sure there are a lot of pin-connected bridges still in use in mainline service in the USA. Southern Pacific's Shasta line comes to mind. But those short bridges I have seen all have the compression resistant bottom chord in the end panels.

If the bridge is long enough (perhaps 250+ ft) the compression resistant chords are not needed as the tensions from the vertical loads are larger than the compression forces. As I said before you make the rules.

We still have old steel truss bridges in Germany, not all were hit in WWII. I must confess there were not many pin-connected ones. They didn't get much favor in Germany. Today pre-stressed concrete bridges are favored as they are said to allow a smoother ride at high speeds because of less deflections under load. But the steel bridges have still their place when the spans are to long for concrete.

Regards, Volker