Replacing an old truss bridge?

[cx500]

CNW 6000

Must be bridge season...or the engineers like playing cards with each other...

CN has started the process to replace the swing bridge over the Fox River in Oshkosh, WI.  It was erected in 1899 and is structurally strong...but other bridge related machanical failures.  Currently the bridge is a truss bridge: three sections with the center pivoting to allow marine traffic clearance.  The replacement seems likely to be a TPG in four sections with #3 being a bascule, pivoting up to 60 degrees.  Any idea why the RR is likely to change from one type to the other here?

The railroad is not the only party involved, and it may have been the choice of the navigation folks (Coast Guard, Army Corps of Engineers?).  There are lots of variables that go into the decision-making process.

John

[CNW 6000]

Paul,
You're correct on the supposition of two equal openings.  If the information I have is correct there will be about 6' of clearance under the non-moving spans.  The height at the peak of the bascule section is just short of 70' when fully opened.  It should be built alongside the existing bridge and then cut over when ready, if the information I have is correct. 

cx500,
That makes sense.  I will be curious to see what actually happens.

[7j43k]

Paul,

A couple of comments about "my" bridges:

The center two panels of these bridges each contain an "X" pattern--hard to see are the tension rods (very roughly an inch in diameter) that start from the top center of the truss and go diagonally downward across the middle two panels (an inverted "V" form).

Also, on these bridges, the bottom chords are pin connected tension bars for the two or four (in this case) inner panels.  So your comments about tension only for these members is spot on.  And that is the trickiest part about converting the Central Valley bridge--scale size tension bars are going to make the model very delicate when it's not in position.  There are also other members that I'll have to build.  I've visited these bridges a couple of times and have taken a lot of pix--though there always seems to be the one(s) that I SHOULD have also done.  But I'm not trying to do an exact model, just one that looks right--that's hard enough.  

 

Ed

[Paul_D_North_Jr]

Dan / CNW 6000 - Sounds interesting.   (  Personally, I'd prefer a Scherzer Rolling Lift Bridge* instead - but that's just me . . . )  See if you can document its progress with photos as it goes on, a la K.P. Harrier.  Know anybody with a boat ?    

- Paul North. 

*EDIT: For those of you who don't know what this is, see:

 http://www.rideau-info.com/canal/images/places/img-scherzer.html and

http://en.wikipedia.org/wiki/File:MovableBridge_roll.gif  - PDN. 

[Paul_D_North_Jr]

Ed/ 7j43k - Thanks for that additional information.  When I have a little more time I'll play around with what the load distribution would be with and without those tension rod 'counters' - I don't have that kind of thing 'imprinted' into my mind well enough anymore to recall it with any confidence.

I'm still troubled about why the much thicker/ larger/ heavier bottom chord members at the ends of that bridge - that makes no sense when considering the vertical/ weight loads on the bridge, which would likewise cause only tension forces there, and in smaller values at that.  However, those members may be designed to also resist longitudinal train forces such as pulling or more likely braking, which would have been a known load 'back in the day', too.  Those train action forces could be large enough to overcome and dwarf the primary tension forces caused in those bottom end members from weight alone, and place them into compression instead. 

[Warning: Detailed explanation and some math follows !]  For instance, if a train of approximately uniform weight per linear foot was on the far half of the bridge only, the center of gravity of the portion of its weight on the bridge would be at 1/4 of the distance out from the far abutment, and so only 1/4 of that weight would be carried by the near abutment (if you're not familiar with engineering Statics or a related subject, just trust me on this).  For a bridge of these proportions - a height of about 20' for a 150' span - the tensionin the bottom chords  from that weight at the 2nd "panel point" out from the near abutment = at 1/4 of the way across, which is where the larger chords first appear - would be about 150 / (4 x 4 x 20) = 44% of that weight.  However, a "full service" or emergency brake application could easily generate longitudinal compression forces of from 25% to 33% (based on the same coefficient of friction of steel wheels on steel rail as for adhesion, etc.) of that same weight - not quite enough to overcome the tension in the bottom chord at that 2nd panel point, but getting close.  And a similar calculation for the 1st of the 8 panel points out from the near abutment would have a similar 25 to 33% compressive train force against a tensile force of 150 / (8 x 4 x 20) or about 23.5% of the train's weight, so a compression condition is quite likely there.  Considering all of that, I believe that's why those 1st 2 chords are so heavy - the 1st one because it needs to be for this reason, and the 2nd one becaus it's 'borderline' in that regard and the designers rightfully had and used an abundance of caution.  Maybe more on this later . . . As you can maybe tell, iI'm a little intrigued by this opportunity to analyze and think about this venerable structure. 

- Paul North.

[CNW 6000]

Paul,

I planned on that already.  I have a few pictures of the boring (rock, that is) work that was done last fall and yes, I own a boat. 

[7j43k]

I've added a shot from the side of one of the smaller SP&S truss bridges:

 

It's a big picture, but then you can see the details that way.

 

 

Ed