odd bridge

[jim currie]

found this pic of a what apperes to be a standard tied arch with the addition of five side posts (was wondering if they might be for clearance) on each span . any else got any info on this type of bridge or seen this type.
jim

[Gary S]

Jim, I've never seen a bridge like that. It looks so "spindley". All the bridges down here that I've seen just look so much beefier than others I've seen elsewhere. I wonder if it is because we have no bedrock? Just sand or clay straight down for miles.

[Russ Bellinis]

I tried to replay yesterday, but the Gauge kept timing out. I finally gave up and figured I'd come back today. I'm wondering if the bridge tried to "rack" from side to side, and they found that the angle braces on the outside of the structure were not strong enough to do the job. The way those vertical pieces are cross braced at the top suggests that they are structural.

[Sumpter250]

I tried to replay yesterday, but the Gauge kept timing out. I finally gave up and figured I'd come back today. I'm wondering if the bridge tried to "rack" from side to side, and they found that the angle braces on the outside of the structure were not strong enough to do the job. The way those vertical pieces are cross braced at the top suggests that they are structural.

The arches carry the weight of bridge deck, and bridge traffic, the "bracing" looks to provide lateral support for the arches, and suspend the bridge deck from the arch.
This bridge would make an interesting model, and addition to a layout.

[sgtcarl1]

I noticed the description on the bottom of the photo, itself. Since it's a military bridge, I'm thinkin' that maybe they needed to redo the stucturing to hold more weight. Quite possible that it was built after WW1, but needed to beef it up for use during the second war. Heavier tanks, artillery, etc. ...
Just my un-educated guess. ...

[Charlie B]

here is a picture from another angle. It looks like it is "as built" with the braces, not an addition.
<!-- m --><a class="postlink" href="http://en.wikipedia.org/wiki/File:Fort_Laramie_NHS_Bridge_over_North_Platte_Wyoming.jpg">http://en.wikipedia.org/wiki/File:Fort_ ... yoming.jpg</a><!-- m -->
be sure to check out the King Bridge Company. <!-- m --><a class="postlink" href="http://en.wikipedia.org/wiki/King_Bridge_Company">http://en.wikipedia.org/wiki/King_Bridge_Company</a><!-- m -->

Charlie

[Russ Bellinis]

I think you are right Charlie. That picture you linked to shows that those braces were built as part of the original structure. I'm wondering if they built the bridge with those braces as temporary supports while they built the rest of the bridge, and then built the rest of the bridge around them and left them in place after wards?

[jim currie]

well went to the FT. Laramie historical site and found that it is a Bowstring Truss not a Arch type and there are many still standing today several still in use as a road bridge most as foot traffic or closed to all traffic . the top pices are for wind braces and some like the FT. Laramie one are elevated for clearance the longest one still standing from what i can find is 147 ft span.

[Charlie B]

Jim, you have got to quit finding all this interesting stuff. I don't have time to enjoy it properly at this time
Charlie

[jim currie]

note also that the FT. Laramie bridge had the orignal stone footings replaced with steel legs, don't have a time line on this.
jim

[Sumpter250]

well went to the FT. Laramie historical site and found that it is a Bowstring Truss not a Arch type and there are many still standing today several still in use as a road bridge most as foot traffic or closed to all traffic . the top pices are for wind braces and some like the FT. Laramie one are elevated for clearance the longest one still standing from what i can find is 147 ft span.

I looked up "Bowstring Truss", and the difference between it and an arch bridge, is in the way the forces are resolved. An Arch distributes the force down, and out, at the ends of the arch. A Bowstring Truss distributes the forces straight down, and in tension along the "string". The Bowstring Truss doesn't require the massive abutments that an arch needs.The bow "string", however, is under no small amount of tension, which would limit the size, and the weight that could be carried.
I just might have to use several of these to provide a walkway out to the bridge tender's shack for the rolling lift bridge on the new modules. The total span would be 159'-6" ( three 53' bowstrings ), and would sit about 8' above mean high water ( it would parallel a through girder rail bridge that is also 8' above mean high water ). It would be something you don't see modeled very often, and I think, would look quite interesting, while not blocking the passage of small craft.

[jim currie]

I just might have to use several of these to provide a walkway out to the bridge tender's shack for the rolling lift bridge on the new modules. The total span would be 159'-6" ( three 53' bowstrings ), and would sit about 8' above mean high water ( it would parallel a through girder rail bridge that is also 8' above mean high water ). It would be something you don't see modeled very often, and I think, would look quite interesting, while not blocking the passage of small craft.[/quote]


Pete would love to see that post a photo when done.
jim

[Sumpter250]

I was able to save to disk, the 1.2 MB photo from the wikipedia site / King Bridge Company. The next step is to try to figure out at least one "knowable" dimension, in order to determine whether this bridge, or ( and this will probably end up where I'll go ) a smaller version using the same construction, just in lighter materials. It will, after all, be a walk bridge and not carrying any real heavy loads......................unless, of course, the bridge tender gains a "pound or two" :o

( It's called getting LPB's "in bulk" )

[Sumpter250]

These six shots are "details" of the bridge. I was able to isolate areas, and in some cases, brighten them up to make out some of the finer detail......just in case anyone else might want to model this neat bridge.
It wasn't until I was able to brighten up the last photo, that I realized that the "bowstring" was actually pin connected.
My best guess is that the timber is 2" X....... 2" X 4" decking, 2" X 6" stringers, and 2" X 8" cross members. I will use that as the basis for the rest of the structural dimensions for my project.
The angled braces in the first photo appear to be "T" section, either forged into round for the threaded lower end, or possibly the threaded part was welded. I lean towards that being forged, as this was primarily ironwork.
   
   
   
   
   
   

[jim currie]

Pete the libary of congress has two drawings of this bridge giving most of the dimensions from them you can build a model. thing that puzzles me is the drawings show the steel legs not the stone ones
jim