I would agree Kurt but then too, some artistic license as well as selective compression is often involved in the design and building of model dioramas. For greater visual appeal I would think the truss bridge would be generally preferable to the more simplistic plate girder bridge.

I tend to be most concerned when someone makes a bridge span that is unrealistically too long rather than too short. (and then places a Sherman or worse yet a Tiger in the middle of that bridge.)

Well Kurt now you have me thinking . . . . I started this project just as an exercise. I had all the parts on hand so I thought I would give it a try.

But now! . . . I DO have multiple bridges and plenty of track. I COULD lengthen it!. As you indicated, right now it is only long enough to display one car. But what length is too long?

I can fairly easily expand the bridge by two frames or I could even double its' length. The images here are just Photoshopped but don't you think the longest one then looks just a little too lite weight to do the job? (In real life that is.) I would be more than a little concerned if I were a yard master getting ready to run a train full of Tigers across that long bridge! - Der Führer ist kein sehr verzeihender Mann!


LONG (Actually NOT so long!)

LONGER

LONGEST

It looks skinny since the truss/girder isn't high enough.




This is an interesting bridge, it has a suspended plattform below acting as a ferry. It crosses, or crossed, the Kiel-canal at Rendsburg.



/ Robin

The thru truss versus deck truss question depends totally on the site location, construction budget and whether the river/stream below needs to be navigable and by what sorts of vessels: canoes, canal boats, ocean cargo vessels??? Also by how high the water might rise during flood season.

A deck truss bridge is cheaper and requires less material to build than a thru truss bridge. All other factors being equal, a deck truss bridge of the same load rating will look smaller and more lite weight than it's thru truss equivalent.

I can immediately think of at least two deck truss steel bridges in easy driving distance to me. In this case these are both highway bridges. One is over a lake who's level is controlled by a damn. That one only need accommodate small pleasure boats. The other is over a unnavigable tributary that feeds into a sizable local river. This second one is closed seasonally due to flooding as the structure is only a few feet above the river.

_______________________________

WOW Robin, that last photo you found is a truly amazing if not a rather strange construction! It appears to carry the load across on a "flying" platform rather than floating it in any way as a cable tethered ferry boat would.

According to Wiki it is a high railroad bridge that also carries a "flying" transporter platform below it. This entire structure must be able to clear large ocean going vessels using the canal. This really doesn't apply to our discussion here but it is impressive!

There are a number of photos available online of this structure.

Really nice ideas

- dutik

I was looking for a truss girder, above or below, which had a similar height/length ratio to what you are building.
I think the firt image got reasonably close and the flying ferry
was just a bonus

I agree with you there - that first image is a perfect comparison.

Inverted/underslung truss bridges :









http://www.steel-bridges.com/under-truss-bridge.html

H.P.

From the link provided by Frenchy:
"Depth is typically 12% of the span, so the roadway has to be higher if the bridge is to have sufficient clearance over water, railways etc."

Railroad bridge in Lockport, NY

/ Robin

Thanks Frenchy and Robin for that 12% ratio figure. Great to have that info.

Those are cool photos! Another potential use for those RR scale girder/truss panels would be as the interior framing of the ceiling/roof of a factory/shop, warehouse, or aircraft hangars & Quonset huts. You could hang chains or pulleys from them as well. It'd be simpler than using a pile of Evergreen I and L-beam pieces.

Jack that is an excellent point.

Growing up in the Louisville area there was an old trolley car barn two blocks from our house. It had a huge uninterrupted interior space like a giant airplane hanger. It was built with a nearly flat roof and several of these type trusses mounted outside, ABOVE the flat roof and then cable braced to the roof, to hold it up!

Yes, when it rained there were plenty of puddles inside but apparently this did not matter to the trolley cars!

Using the 12% figure as an absolute; if I were to double the length of my span (four additional frames) I would be a bit light at a 10.34% ratio of bridge length to depth of span.

However if I were to extend the span only 3 frames I would be just about right at almost exactly a 12% ratio!

From a practical standpoint this slightly shorter bridge would be more work but would actually be more correct.

The "height above waterway" really doesn't concern me that much. This could be a bridge passing over a roadway, or a narrow but deep ravine or an unnavigable waterway. I was planning on leaving that up to the imagination of the viewer.

I envisioned this simply as an extraordinary display base for whatever military/railroad subject I wished to pose on it.

If height above the "water" were to become a real concern I could always just add flat plate bases below the tapered stone abutments to raise the entire structure several inches such as those shown here below.
(Sorry for the poor quality of the drawing.)

Some inspiration for how it can look:


This is a very short canal built between a lake where there is a royal castle and the salt water on the outside so that smaller ships could travel betwee the main royal castle and Haga Slott in what used to be the countryside.
The nearest bridge is a two lane bridge for a narrow gauge railroad (Roslagsbanan)

Google zoomed in

Big G Zoomed out
Bridge at the upper edge, Gamla Stan at the lower edge, ships route goes around to the east where the ferry lines are marked.
/ Robin

Robin I see what you are up to. ;-) That is a very nice scene you are showing us of that narrow waterway. My existing bridge would look very good in that scenario. (As a point of interest I have to mention all those very necessary bumpers/fenders on the front of that boat operating in that tight little waterway.)

But you guys have already convinced me to lengthen the span so in a few days my short little bridge isn't going to exist anymore. Also I may raise the entire assembly to a higher level, again at your urging.

As regards surrounding scenery it has always been my intent to exhibit it just as you see here with the image of the surrounding environment being left to the viewer's imagination.

You guys keep showing me photos of all these flat water canals - maybe I just sit the whole thing on a piece of ripple glass and be done with it? In several of the photos you posted here the water looks like rippled glass anyway.

Another of my many (too many) works in progress, a converted CCKW "locomotive" was to be the first subject exhibited using this bridge. However I right away considered showing the locomotive pushing a boxcar and realized the display was too short for that so I was already thinking about making the bridge longer.





I just wanted the bridge to act as a set piece (a backdrop) for the model. I did not want that backdrop to become the main event and overshadow the models themselves.

I was planning on keeping the colors of the bridge rather subdued hopping that the uniqueness of the locomotive plus a bit of color offered by either a weathered red or dark green boxcar would draw the viewer's eye to where I wanted it to go.

Extending my bridge truss. The math on this 7 frame bridge works out to an almost perfect 12% ratio of length to depth of span.

Construction on my extended 7 frame bridge almost complete: (Still missing a few minor details underneath.)



What about a color choice? Would a WWII era European bridge be any other color than black with some rust highlights??? How about red lead primer???

F.Y.I. - According to my Army Corps of Engineers niece with 18+ years experience, the 12% directive is a guideline for general construction. Actual bridge truss height to length ratios can run anywhere from 10% to a high of more than 35% depending on the expected maximum load to be carried.

Simple definitions and structure of typical bridge types:

http://pghbridges.com/basics.htm

It had concerned me that on my deck girder bridge I was laying the ties (sleepers) directly on top of the the main trusses. It seemed to me in most bridge designs there would be an intermediate set of I-beams that mount between the trusses and the track itself. This would also raise the track even further above the abutments. (Something I DID NOT want!)

I was relieved to find this photograph. It would seem that deck truss bridges can be the exception to this rule.




Photos by Erik Rasmussen

p.s. Note what appears to be the unusually low road clearance (still 13+ feet) under this bridge.