Bridge Overpass: Need Opinions

[peteski]

I see the straight steel beams now, obviously your friend did his homework before taking on the task. The results are convincing.

Yeah, Ernie is a pretty good modeler.  Here is a thread about his layout.

[cv_acr]

Great example of a curved concrete bridge but how do we know it's a railroad bridge? Light rail lines do not count as their load rating is similar to highway standards.

Oh it's not a railroad bridge, it's part of a freeway interchange in Toronto.

This topic started out with a model concrete road overpass over the railway.

[glakedylan]

Like this curved N scale beauty?  The missing support is elsewhere, but still, it is just laughable on so many levles.

I give the modeler an A+ for creative use of cauliflower and broccoli for scenery
will be visiting the produce section to pick up some mountains!

in jest,
Gary

[jimmo]

Oh it's not a railroad bridge, it's part of a freeway interchange in Toronto.

This topic started out with a model concrete road overpass over the railway.

My apologies, I asked a question about the railroad bridge that Peteski showed and someone showed me the one you posted.

[nkalanaga]

BNSF's (BN) Latah Creek Bridge in Spokane, WA has curved box-girder spans.  There are a lot of pictures online, but most are of trains in the middle of the bridge, where it's straight. 


http://www.railroadforums.com/forum/showthread.php?41385-Latah-Creek-Railroad-Bridge-Spokane-WA


http://www.railroadforums.com/photos/showphoto.php/photo/20464/title/latah-creek-bridge/cat/504

Unlike most steel bridges, the spans really are curved, and as far as I know, they're also welded together, making the entire bridge one long steel tube. 

The problem with curved bridges is keeping the piers under the center of gravity.  If the weight is too far outside the line between the piers the bridge will tip over. 

If I was building a bridge in the location shown in the first picture my thoughts would be to
1:     Blast out a ledge on the hill, build a straight bridge, and put the curve on the ground at the start of the bridge.  Or, if that was impossible,

2:     Build a straight deck girder bridge across the tracks, with a double-height girder on the side the road approaches from, and attach a second girder span to that extra-height girder.  Then the deck can be curved over the intersection of the two bridges.  I've never seen one like that, but structurally it should work.  I have seen railroad bridges supported on a girder between two piers, usually when a single pier would be in the middle of a street or another track.  Deck girder spans have also been supported on girders across the end of deck truss bridges to avoid stepped piers or extra steel supports for the smaller girder spans.

[jimmo]

BNSF's (BN) Latah Creek Bridge in Spokane, WA has curved box-girder spans.  There are a lot of pictures online, but most are of trains in the middle of the bridge, where it's straight. 


http://www.railroadforums.com/forum/showthread.php?41385-Latah-Creek-Railroad-Bridge-Spokane-WA


http://www.railroadforums.com/photos/showphoto.php/photo/20464/title/latah-creek-bridge/cat/504

Unlike most steel bridges, the spans really are curved, and as far as I know, they're also welded together, making the entire bridge one long steel tube. 

The problem with curved bridges is keeping the piers under the center of gravity.  If the weight is too far outside the line between the piers the bridge will tip over. 

If I was building a bridge in the location shown in the first picture my thoughts would be to
1:     Blast out a ledge on the hill, build a straight bridge, and put the curve on the ground at the start of the bridge.  Or, if that was impossible,

2:     Build a straight deck girder bridge across the tracks, with a double-height girder on the side the road approaches from, and attach a second girder span to that extra-height girder.  Then the deck can be curved over the intersection of the two bridges.  I've never seen one like that, but structurally it should work.  I have seen railroad bridges supported on a girder between two piers, usually when a single pier would be in the middle of a street or another track.  Deck girder spans have also been supported on girders across the end of deck truss bridges to avoid stepped piers or extra steel supports for the smaller girder spans.

I went on Google Maps, street level and checked it out--sure enough the steel spans are gently curved and appear to be welded in a continuous span with what appear to be expansion joints in the steel structure between the piers. Definitely a first for me. Thanks nkalanaga.

[Baronjutter]

Also remember what looks like a decently curved bridge in prototype would end up being something like a 50-100" radius curve in N scale.  Often even the tightest curve the bridge engineering would allow for would still be very hard to model in most of our spaces

[DKS]

Also remember what looks like a decently curved bridge in prototype would end up being something like a 50-100" radius curve in N scale.  Often even the tightest curve the bridge engineering would allow for would still be very hard to model in most of our spaces

This. There are a precious few curved railroad bridges around the world, and the one thing they have in common is that the curve is very broad. Unfortunately, I fear the temptation is to do some "selective compression" and make the curves much sharper; this would, in effect, result in an unbuildable bridge.

[jimmo]

This. There are a precious few curved railroad bridges around the world, and the one thing they have in common is that the curve is very broad. Unfortunately, I fear the temptation is to do some "selective compression" and make the curves much sharper; this would, in effect, result in an unbuildable bridge.

To bring this thread full circle, similar rules apply to curved highway bridges. Obviously the curves can be much tighter than RR bridges, just not as tight as the model originally shown in this thread.

The tightest curves I have seen where concrete roadways are used is on freeway onramps and off ramps, and those are still fairly broad. I tried to research the minimum radius of such on and off ramps, but I found myself mired in endless engineering specifications of all aspects of roadway construction. Anybody out there know off the top of their heads?

It appeared (to me) that the curve radius was largely based on the design speed of the road.

[glakedylan]

given the topic, I was reminded of the WM's Keystone Viaduct:

https://fbcdn-sphotos-c-a.akamaihd.net/hphotos-ak-prn2/t1.0-9/10009808_10153997109895637_491616948_n.jpg

most certainly a work of great architecture and art!

sincerely
Gary

[Ian MacMillan]

For your pleasure
Frankenstein trestle - MEC Mountain Division -  4 degree curve.

[nkalanaga]

Jimmo:  I've seen the Latah Creek bridge numerous times.  It was built while I lived in Washington, and we've been back out there every couple years since moving.  Surprisingly, it's easy to miss, driving on the freeway, because it's above the road.  Someone watching traffic isn't likely to notice the piers or to look up for the bridge.

As far as I know, what looks like the "missing leg" of the wye never had track.  It was just an access road during construction.  None of the railroads crossed the creek at this spot.  The new bridge was part of a rail relocation project for the 1974 World's Fair, which put all of the tracks on the ex-NP grade-separated line through downtown Spokane.

Trivia:  The piers were waxed when the bridge was built.  Spokane insisted that there be no rust stains to make the bridge "ugly", so while the steel rusted, the piers were waxed to keep them from staining.  After the rust coat was stabilized the wax was removed.  Don't ask me how because I never heard that part!

Trivia 2:  The box girders are physically locked into the tops of the piers, not just sitting on them.  Supposedly the piers were bent slightly while the girder sections were installed, then allowed to return to their proper position.  Again, I have no idea how they did it, if they really did, but reinforced concrete will bend slightly.  Maybe they just tied a cable to the top and pulled...

Trivia 3:  There is no such thing as Latah Creek.  There's a Latah county, and the canyon is Latah Creek Canyon, but the creek is officially Hangman Creek.  People in the area keep trying to change the name back to the original Native American "Latah", but they can't get the government mapping folks to agree to the change.  Nearly everyone calls it Latah Creek anyway.

[wm3798]

The Keystone Viaduct, like 99.9% of all "curved" bridges, is (or was) comprised of a series of straight spans.  When the state road was straightened and a pier was removed, a faux curved span was installed, but it only has to carry bicycles.

Lee

[Chris333]

You guys scared the OP away 

[basementcalling]

BNSF's (BN) Latah Creek Bridge in Spokane, WA has curved box-girder spans.  There are a lot of pictures online, but most are of trains in the middle of the bridge, where it's straight. 


http://www.railroadforums.com/forum/showthread.php?41385-Latah-Creek-Railroad-Bridge-Spokane-WA

That bridge is one reason I didn't model a prototype based layout. Fitting a reasonable model into my basement space was too perplexing if I wanted it to look realistic verses representational.