Truss Bridge Design

[Big Cat]

While visiting Montana last autumn, we drove out to Rainbow Falls outside of Great Falls.

 

There is a railroad bridge near the dam which has a design that I haven't seen before.

 

http://www.rrpicturearchives.net/showPicture.aspx?id=377698

 

It is common to see through or deck truss bridges.  It looks like a through truss bridge with the deck on top.

 

What is the design advantage to doing it that way.  Does that style bridge have a unique name.  I thought that I might have seen another bridge like that in the Trains magazine issue where the Big Boy is show crossing a river in TX with an old SP bridge in the background.

[MidlandMike]

I would still call it a deck truss bridge.  The stone pier construction is kept to a minimum.  Deck bridges don't have the clearance problems of a thru truss.

[tree68]

As a simple comparison, consider the covered bridge.  Most people think of them as through structures, but a good many (chiefly railroad) were built with the traffic travelling across the top.

In each case, the cover protects the elements of the truss bridge.

[Leo_Ames]

tree68

As a simple comparison, consider the covered bridge.  Most people think of them as through structures, but a good many (chiefly railroad) were built with the traffic travelling across the top.

Interesting. 

I've seen a picture of such a covered bridge on the East Tennessee & Western North Carolina Railway, but thought it was one of a kind. 

Edit: Couldn't find a photo online of it, but here's a model.

 

[timz]

Big Cat

http://www.rrpicturearchives.net/showPicture.aspx?id=377698

I assume that's a typical deck truss. What is a deck truss, if not that?

[ORNHOO]

A couple of replies to this thread seem to have been posted to the one about Maps and Railroads. Anyway... One bridge that has always been an eyecatcher (and headscratcher) for me is the Eagle Creek bridge of the (long defunct) Portland Traction Co. line to Estacada OR. The bridge is a *hanging* truss design, essentially a Warren Truss flipped upside-down, with the roadbed on top. My guess is that this reverses the loads on the individual bridge elements, an element that would be under tension in the upright version would be under compression in the hanging version ( but then, I have never stayed at a Holiday Inn Express ).

Edit:Bridgehunter.com | PEPCO - Eagle Creek Trestle

[Overmod]

timz

What is a deck truss, if not that?

Has the "deck" on the lower chord of the "truss", turned upside down.. Inverted Warren or bowstring shouldn't matter.  The point is that the deck is supported the whole length of the 'tension' lower chords in a through truss, and in the deck-truss structure tha is the through truss turned upside down with tension and compression reversed, that is also the case.

If ballasting the deck of such a truss, little additional structure is required; that is NOT the case for the plate girder 'between truss spans' in the design pictured.

The bridge in question appears to have a complete plate girder deck bridge arranged along the crossmembers of the tops of the truss spans.  I have not found a 'name' for that type yet.

[selector]

Sub-divided Warren trusses on which deck girders are set.  The two closest elements are sub-divided Warren truss piers.  I'd like a better view to be sure, though.

[Murphy Siding]

Big Cat

While visiting Montana last autumn, we drove out to Rainbow Falls outside of Great Falls.

 

There is a railroad bridge near the dam which has a design that I haven't seen before.

 

http://www.rrpicturearchives.net/showPicture.aspx?id=377698

 

It is common to see through or deck truss bridges.  It looks like a through truss bridge with the deck on top.

 

What is the design advantage to doing it that way.  Does that style bridge have a unique name.  I thought that I might have seen another bridge like that in the Trains magazine issue where the Big Boy is show crossing a river in TX with an old SP bridge in the background.

 

  I wonder if having the structure below the deck instead of above it reduces the amount of wind loading that has to be built into the design?

[cx500]

I'm wondering if the truss portion of the bridge was designed for possible future double track, since it is obviously wider than the trestle approach.  Above the top chord of the truss I can see floorbeams sticking out, and presumably there will be stringers bewteen the floor beams to support the bridge ties.

John

[rdamon]

More info:

 

https://bridgehunter.com/mt/cascade/bnsf---rainbow-falls/

[selector]

Those are Pratt trusses.  If it's part of the same bridge, then I was incorrect earlier.

[Overmod]

selector

Those are Pratt trusses.  If it's part of the same bridge, then I was incorrect earlier.

The technical differences between Pratt and Warren trusses for those who wonder.

[rdamon]

Overmod

 

 

selector

Those are Pratt trusses.  If it's part of the same bridge, then I was incorrect earlier.

 

 

The technical differences between Pratt and Warren trusses for those who wonder.

 

 

It's all about minding your N's and V's :)

[Overmod]

There's an online 'cloud' service to calculate the stresses, and it features this handy analysis.

[selector]

This is my go-to site for quick references.  Not definitive, of course, but it's pretty handy.  Quick 'n dirty, as we say in the armed forces.

Bridge Basics - A Spotter's Guide to Bridge Design (pghbridges.com)

[timz]

You've heard of through plate girder bridges? That seems to be the usual term, but that site says it's wrong -- should be pony plate girder.

[Overmod]

timz

should be pony plate girder.

That's a little silly of them, isn't it?  "Pony" (which is properly seen in relation to 'truss' in my experience so far) means that it has no cross-reinforcing elements above the deck, and I don't recall ever seeing a through plate-girder bridge that had cross-reinforcement (catenary or wire supports don't count! ).  So the correct term is still 'through plate girder' for any practical purpose.

There are much better reasons to refer to 'pony trusses' as most bridges of 'through truss' construction have their sideframes cross-tied for better structural strength.