a few bridge questions

[Bigsteel]

hey everyone,i have a few bridge questions for ya'.right now im building a small part of my logging layout and im finishing the trestles and bridges so i can lay track.i have a walthers truss brudge and was wondering,did they mix truss bridges with trestles? i have a small river/waterfall to span and id like to use what i have on hand.and if they did use them,in what manner would they attach the bridge? right now i have the deck just sitting on top of the pilings.and finally,did they ever curve track on a straight bridge?,as in was there an extra wide bridge to accomodate a curve?--josh

[iis612]

Josh, I have seen deck girder sections attached to truss bridges, but never a trestle with a truss bridge. Because I haven't seen it doesn't mean that it wasn't done though.
As far as the curved track on a straight bridge, unless it was a VERY shallow curve, I find it hard to believe that the extra expense of materials would be used to make a straight bridge for a curved section of track. Once again, it is very possible that I am wrong.


Matt

[Bigsteel]

thanks matt,there are truss bridge sections attached to trestles,but i should have stated,mine is steel and not wood per the usual construction.but im wondering if there is a way to modify my bridge to curve....i havent thought of a convincing way of doing it though and is still dont know how the bridge would be attached.but in the end if i cant find any good info ill just wing it and add a little bit more modelers license to my already fantasy inspired world --josh

[Sumpter250]

Having never seen a truss bridge, curved (in the horizontal plane), and giving some thought to how a truss distributes weight and force, I would feel safe stating that the bridge cannot be "curved".
A curve can be carried by several short truss bridges, but I believe deck bridges are more often used, because of side clearance issues.
A steel, or iron truss bridge, supported by stone or concrete abutments, or iron or steel bents or towers, can have wood trestle approaches. I can't remember seeing an iron or steel truss bridge on wood bents. Deck girder? yes, truss no. I believe it has a lot to do with the way trusses distribute weight and force.
The "it aint prototype" reason, does have exceptions, so I can't completely rule out the "no truss bridges on timber bents" though, logic would tend to support it.
Then again, I'm not a structural engineer.
(Edit): In reality, bridges are designed to fit the environment. In model railroading, our environments are designed to fit the bridges. :mrgreen: :mrgreen:

[nkp_174]

I have a degree in Civil Engineering, so I might be able to help a little.

Basics: designs are usually optimized to minimize a valuable resource. Can you guess what is the most valuable today?

Every material has its advantages and disadvantages...rigidity, elasticity, tensile strength, compressive strength, shear strength, price, and ability to be joined. Steel is wonderful in tension, and ok in compression. Steel also has a valuable property known as strain hardening...it gets stronger AND lets you know before it fails. Concrete and masonry are wonderful in compression but worthless in tension and bending. I've seen a half a brick pulverized in a massive press...it is incredible...and yet I can snap it with my bare hands under a bending moment.

Bridges are not as stationary as you think. They usually are pinned at one end and have rollers elsewhere. When traveling across a highway bridge, notice the teeth at one end for an expansion joint? There are rockers or some other type of roller device underneath. This isn't just for temperature changes, but also for deflection under loads.

All bridge spans are compatible with one another...but workers whom deal with steel usually don't know much about wood, concrete, brick, etc. That is a major reason why bridges are usually built to use as few different materials and construction techniques as possible. Note how, typically, all joints on a steel bridge are either riveted, bolted, or welded?

Piers typically only need to resist compression. Therefore, masonry or concrete is an excellent material...especially if the pier is going to be subject to moisture. Steel, on the other hand, is excellent for a truss. Masonry and concrete bridges require arching, and so they usually are far more expensive to build.

Wood, on the other hand, is usually too expensive for both maintenance and labor. It is a bad choice in modern times.

As sumpter has pointed out, curved bridges generally don't exist. Think about the amount of effort required to bend a piece of lumber for such a bridge...worth it? No way! Same thing with steel. And as for casting? That would require special forms. It has been done before, usually in concrete or brick, but they are unnecessarily expensive to build. I would also play havoc on the mechanics of a truss if the parts weren't straight. Trestles and trusses usually fail when deflection under load causes parts to curve. Curved wooden trestles only exist on model railroads. Real "curved" trestles are actually a series of straight bridge spans with the stringers only touching 3 trestle bents. The neighboring stringer ends every other bent, allowing a constant piece above each bent allowing the stringers which end to be pinned through it.

Next time you see a photo of a (edit: damaged) trestle (a bridge consisting of more than one span), notice how only the affected spans are down. These is because each span is structurally independent of the others under normal conditions.

As a footnote, railroad bridges are rated according to the Cooper Bridge Rating system. It was designed in the late 19th century and assumes a 2-8-0 to be the locomotive. The Cooper bridge rating will vary for every span. For shorter spans, shear is the enemy. For longer spans, bending is the enemy. For an NKP berk, the worst Cooper ratings aren't when the entire locomotive is on the span...I know this from having seen the ratings on one of them. The speed is also a factor. Today, those SD70s pulling autoracks are allowed to cross a bridge dependent upon a rating formula which uses a 2-8-0 as the model.

This has gotten a touch long, so I've probably missed some stuff.

Michael

EDIT: wooden bents are probably taboo because they require quite a bit of maintenance. They also would be built to shorter spans than would be necessary for steel. I'm feeling too lazy to go consult my handbook on the compressive strength of wood. If it is worthwhile to replace the deck or truss, it is probably worth it to replace the bents underneath.

[tomustang]

Hope this helps, right in my own town

[tomustang]

Also in my town. the track curves utilizing the piers for turning on the straight portions of bridges

[Bigsteel]

ok guys thanks for the explanations (nkp,man you can type ) this has all helped alot.and i think im just going to go with a straight bridge and curved trstle spans while making the whole turn itself more shallow.the whole point to this is to break up the consistency i have in my bridges.so far there all wood,but ill atleast use a different design on each one.but now i dont know which design to use.i've done the complicated deck bridge for the mainline. http://www.blackbearcc.com/DBK_HO.htm but i wanted something simpler for the branch above it (there in the same canyon) something like what shamus (god rest his soul) had on his layout here:http://www.all-model-railroading.co.uk/cats/aa20.htm the one in the foreground,but mine would be the span on a portion of a trestle and be built with steel strap and bolts not rope.or even a much smaller version of the bridge supporting his trestle her http://www.badger-creek.co.uk/ill post pics as soon as i can--josh

[Bob C]

Tom - Excellent example !!! Those two left-most bridges were carefully engineered for that specific location. The trusses are asymetric to fit the skewed (angled) piers while still aligning the verticle members to carry the floor beams perpendicular between the side trusses. The track rides on straight stringers attatched atop or between the floor beams following the required line for the slightly curved track. Thanks for digging that up, Tom - Do you have a closer (more detailed) photo ? Bob C.

Boy, do I type (think) SLOOOOOWWW!!! BC

[tomustang]

Tom - Excellent example !!! Those two left-most bridges were carefully engineered for that specific location. The trusses are asymetric to fit the skewed (angled) piers while still aligning the verticle members to carry the floor beams perpendicular between the side trusses. The track rides on straight stringers attatched atop or between the floor beams following the required line for the slightly curved track. Thanks for digging that up, Tom - Do you have a closer (more detailed) photo ? Bob C.

Boy, do I type (think) SLOOOOOWWW!!! BC

Thanks

Actually I don't have any good pictures of that bridge since the trees are in its way of taking the pictures in that location, that specific pic is from MSN Live Maps, that I print screened and edited to show

If you go here <!-- m --><a class="postlink" href="http://maps.live.com/default.aspx?v=2&FORM=LMLTCP&cp=40.688953~-75.203691&style=h&lvl=15&tilt=-90&dir=0&alt=-1000&scene=18204596&phx=0&phy=0&phscl=1&encType=1">http://maps.live.com/default.aspx?v=2&F ... &encType=1</a><!-- m --> and go to birds eye view youll see where the delaware and lehigh rever meet, it is right there.

Some day I'll go back and take some better pics when the park finally opens

[jim currie]

as you can see the SP had no qualms about mixing steel and wood this bridge /trestle in near holtville Ca. they beefed up the steel with wood pilings in affect making it a trestle of sorts. there is another trussed bridge in the phoenix main that has a trestle approach on one end will get a photo of it and post later.
Jim

[nkp_174]

as you can see the SP had no qualms about mixing steel and wood this bridge /trestle in near holtville Ca. they beefed up the steel with wood pilings in affect making it a trestle of sorts. there is another trussed bridge in the phoenix main that has a trestle approach on one end will get a photo of it and post later.
Jim

What is under the trestle? Are the pilings always on dry ground? It is an arid region, so one of the great disadvantages to wood does not apply...rot. Thanks for sharing a nice example of an unusual combo.

[Russ Bellinis]

A good book to see some interesting trestles in is "Railroads Through The Coeur D'Alenes by John Wood. I checked a copy out of my local library a few years ago, and that entire area was amazing. They had a "S" curve trestle and a partial "U" curved trestle crossing back and forth over a gorge to gain elevation. If I remember correctly, the trestle bents were installed at angles to each other, and the stringers were straight but making a small turn at the end of each one. The track was curved on top of the trestle. They had one photo taken about 1910 of a small 2-6-6-2 that looked like the prototype for the Mantua Mallet logger with tender. It was pulling 8 cars and a caboose, and the caption said that was the maximum load it could handle in that area without a helper due to the extreme grades! I have often thought that if I had a spare bedroom to use as a dedicated railroad room, I would like to get that book and use those railroads as prototypes for my model. Here is a link to it on Amazon:

<!-- m --><a class="postlink" href="http://www.amazon.com/s/ref=nb_ss_b?url=search-alias%3Dstripbooks&field-keywords=Railroads+Of+The+Coeur+D%27Alenes&x=9&y=15">http://www.amazon.com/s/ref=nb_ss_b?url ... s&x=9&y=15</a><!-- m -->

[Bigsteel]

alright,as i said i ditched the metal truss in favor of a more appopriate logging bridge made of simple 2' dia timbers. i guess this would be a girder? i dont know exactly what kind of bridge it would be classified as but if its good enough for shamus's layout,its good enough for mine .anyways,here it is.the FP7 is just for comparison.but the engine is fargin heavy so it shows how strong the bridge really is on the side with only minimal bracing is ehre it will be buried half way up as its going to be part way onto a cliff.so much bracing asnt neccesary on that side.check out my aloyut thread to see it in the canyon.--josh
ok nevermind,my complete POS computer is not wanting to copy and paste,so guess what? it will ahve to wait until tomorrow

[Bigsteel]

alright,im using my work computer now so i can post my bridge pics





as you can probably see iuts not totally done,i still have to add stringers for the ties then add some strap around the main logs and finally buy some NBW castins to top it off.--josh