Plate Girder Bridges - Need To Know

[dehusman]

Wow thanks! Yes, pretty much resembles what I’m doing (except for the columnal piers). Did you take the photos? If so, where is it and what railroad uses the bridge?

Omaha, NE, 42nd St just south of I-80 and UP main. Its former CNW, now UP.

I did take the shots for a fellow in England on another forum who was asking about US bridge construction.

Dave H.

[markpierce]

Thanks. But do they have them in N-scale?

Uh, hadn’t realized you were in N scale. [D)] Oh well, at least some of the Atlas bridges in N have interior wall detail. Check out item 150-2080 at the Walthers site for an example. But are we back to the start?

Mark

[Railway_Man]

Here is something like what you are building:

And while most of the time they use two girders, sometimes they don’t. Here’s a detail of the center pier support:

Wow, what an ugly bridge from an engineering perspective. From the vegetation and apparent lack of seismic considerations it appears to be in the Midwest. Design details indicate it was built in the 1955-1965 time period, and that it comes out of a highway department design bureau.

It appears to be a continuous span. Settlement of the central pier would be a major problem if it occurred (it doesn’t seem to have happened) but from the point of view of the builder, which is likely to be a highway department, that would be the railway’s problem if it happened after they gave them the completed bridge.

The ugly part is that it is a skew bridge as well as a continuous span. The moments are different for the two girders for a point load and there is uplift on the central pier as well. It would be complex to do the calculations for this bridge.

Appropo nothing, but of interest to me, is you can see the mill mark (Inland Steel) on one of the angles in the detail photo.

RWM

[Metro_Red_Line]

Thanks. But do they have them in N-scale?

Uh, hadn’t realized you were in N scale. Oh well, at least some of the Atlas bridges in N have interior wall detail. Check out item 150-2080 at the Walthers site for an example. But are we back to the start?

Mark

Yes, I actually have that one, and the double-track version elsewhere on my layout…I guess I’d want to have a little variety and not the exact same bridges all over my layout I’ve also been contemplating going the truss route (I learned that Central Valley has an N scale 150’ truss bridge kit so I’m thinking about the price, dimensions and building time.

[Paul_D_North_Jr]

Good discussion and great photos of the bridges here, esp. the bearings - both the ‘pin’ and ‘roller’ or ‘slide’ types, and to illustrate the differences between the continuous and non-continuous spans.

What ‘continuous’ means in this context for non-structural engineers is that the bridge is solid and stiff from end to end, with no ‘break points’ or ‘hinges’ / ‘pins’ in the middle that would allow it to flex more there than in the rest of its length. Think of a single solid board that extends all the way across between the 2 ends and the center pier, instead of 2 short boards that are just between and resting on each end abutment and the center pier, unconnected in any way - and you’ve got the picture. The latter 2-board arrangement is a ‘simple’ or non-continuous span configuration and has many favorable characteristics for railroad bridges.

One of those favorable characteristics is that simple spans are ‘determinate’ - both externally and internally. That’s a structural engineer’s way of saying that once you know the design loads = train weights that the bridge has to be able to carry, you can easily figure out the principal reaction forces at the ends, and the ‘shear’-type forces and ‘bending moments’ anyplace in between, simply from the geometry of the bridge - here, its length - without knowing anything at all about the size of the steel members, etc.

But to analyze ‘continuous’ spans, you first have to know or choose the principal characteristics of those members, and then analyze the internal forces and stresses. Unlike simple spans, in continuous spans those internal forces and stresses are often calculated based mainly on the deflections/ motions that would occur at the ends of each span and at any intermediate piers, except that those same piers should be rigid enough to normally prevent such motion from occurring - that’s why contin

[Paul_D_North_Jr]

Here are some links to fair-quality photos of a 'similar ‘recently’ constructed railroad bridge over the Pennsylvania Route 309 Expressway, just south of Fort Washington, PA in Montgomery County, about 10 miles north of Philadelphia, also just south of its interchange with the Pennsylvania Turnpike I-476 there. The rail line is the former PRR’s ‘‘Trenton Cut-Off’’ - I believe NS now calls it the Morrisville Line or Secondary, or similar.

Note that if you look carefully at the middle, you can see the very narrow gap between the vertical ends of the 2 girders, as others have mentioned above. That, and the presence of 2 ‘shoes’ or bearings at the bottom of those girders on the center pier - tells us that theese are 2 ‘simple’ spans end-to-end, and are not ‘continuous’.

http://www.309online.com/photos/4-07_309RR_5501.JPG

http://www.309online.com/photos/3-22-07_RRBr1.jpg

http://www.309online.com/photos/11-29-06%20Girders_3.jpg

EDIT: See also the main photo gallery pages for this project as below, though you’ll have to scroll through many pages to get to approx. photo nos. 91 - 108, Nov. 2006 - May 2007:

[Paul_D_North_Jr]

Here are links to other photos from the Philly NRHS site - though almost all of these are from up above, and don’t show the sides real well. See if you can find the set with ‘what went wrong’ with one of the early trains over the temporary bridge - ‘Murphy’s Law’ is still in effect, so it’s a good thing that someone at NS was thinking ‘Safety First’ to have the bridge inspected after that . . .

New track and bridge on NS Morrisville Line Fort Washington PA March 20 2006 - Monday March 20 2006 was cut-over day for a major project on the Morrisville Line:

http://www.trainweb.org/phillynrhs/RPOTD060324.html

Second day operations for bridge on NS Morrisville Line Fort Washington PA March 21 2006 - By the next day, a full complement of trains were running. In between, NS continued to work on the new track, ballasting and tamping in an effort to get the track speed up from 10 MPH to 25 MPH:

http://www.trainweb.org/phillynrhs/RPOTW060326.html

NS 67Z and bridge problems on NS Morrisville Line Fort Washington PA April 13 2006 - The temporary bridge over Route 309 was put into service a few weeks ago. While initially the speed limit was 10 MPH, this was later increased to 25 MPH, not quite

[gerhard_k]

Perhaps an exception that proves the rule - A very interesting modern continuous long-span bridge is BN’s early 1970s post-merger connection project Latah Creek bridge on the western side of Spokane, Washington. I’ll see if I can find a link with some good photos of it.

EDIT: This’ll do - http://www.flickr.com/photos/80651083@N00/104439916/

• Paul North.

Paul - Great explanation, even if I wonder how many people actually read it all the way through - one old college buddy used to say “my mind glazed over”.

There’s a small gotcha in the photo that you cite - looking at the first pier, it sure looks like there is, in fact, a break in the girder right at the pier! And as I was about to post about that, I looked at the other picture in that pair, which clearly shows the continuous nature of the beam…

Anyway - good stuff. Thanks.

• Gerhard

[Railway_Man]

Paul – Adding to Latah Creek, the most famous continuous bridge for a railroad I would think would be Gustav Lindenthal’s bridge at Sciotoville. Also worth noting is that swing bridges become continuous bridges in the closed position.

RWM