RR Bridge Girder

[iv63]

I am designing 75ft span RR bridge inside of the existing plant with very limited space of only 2ft between bottom of bridge and top of rails. Attached sketch shows proposed cross section of the bridge. Since this is my first RR bridge I have a couple of questions:
1. Can rails sit directly on the cross beams?
2. Can distance between stiffeners be used for girder’s unbraced length Lb of the compression flange?
Any other comments/suggestions to the proposed cross section would be greatly appretiated.
iv

 

[MikeHalloran]

A search on "Half through plate girder bridge" taught me more than I knew before, and produced a lot of commentary on specific design features and complete design reports.






Mike Halloran
Pembroke Pines, FL, USA

 

[iv63]

Thanks Mike for the quick response. If you have saved your searches would you mind sharing them (I have googled “Half through plate girder bridge” with no valuable results)?
Regards,
iv

 

[MikeHalloran]

The search worked better when split up this way:
{"Half through" "plate girder bridge"}

http://en.wikipedia.org/wiki/Plate_girder_bridge

http://www.archive.org/details/historybridgeen00tyrrgoog

leads to

http://books.google.com/books?id=l0pDAAAAIAAJ&oe=UTF-8

which also has links to 5 other e-books
I love old books

http://nptel.iitm.ac.in/courses/IIT...tures_II/9_bridges/6_plate_girder_bridges.pdf

http://www.wisegeek.com/what-is-a-plate-girder-bridge.htm

http://www.deldot.gov/archaeology/historic_pres/hist_bridge_survey/pdf/steel.pdf

... that should get you started.



Mike Halloran
Pembroke Pines, FL, USA

 

[DWHA]

About 3 years ago I designed a thru girder RR bridge.

It consisted of W12x??? spanning between the main girders (perp to rails). I do not remember what the weight per for was, but these things were beefy. Then between the W12x??? I had some stringers spanning the gaps. The top of stringers were aligned with the top of the W12x???'s. The W12x??? were spaced at the same spacing as the minimum axle spacing on the cooper E80 design loading.

No, stiffeners are not considered brace points.

The W12x??? had basically a knee brace which went up to the top flange of the main girders, these were the braces for LTB for the main girders.

RR ties were connected to the structural steel using some sort of spring bolt.

This is from memory, but I believe the max gap between the ties was required to be 6", so for a 8" wide tie, the ties were on 14" cts.

 

[DWHA]

Here is a skech. I showed the knee brace on one of the sides.

 

[MikeHalloran]

I notice that it is/was fairly common to make the half/through girder bridge as basically a tray, with regular ballast and ties above that. I thought it odd, until I realized that it solves the problem of what to do about the approaches.

I.e., laying the rail directly on the cross beams seems okay within the confines of the bridge per se, but if the approach rails are not rigidly supported, the rails will be subject to extreme bending stress as the approach rail/tie/ballast dynamically deflect under the weight of the train, and the rail on the cross beams does not/ cannot also move. So rail on cross beams should be okay if the approach rails are embedded in a rigid factory floor, but not if they're ballasted.



Mike Halloran
Pembroke Pines, FL, USA

 

[MichSt]

Mike - with direct fixation of the rails on the bridge steel, and even with a ballasted bridge deck, the RR's typically require a tight tie spacing with extra long ties near the bridge to help reduce differential deflection of the rails (or the bump if you will) at the bridge approach. Track approach slabs buried under the ballast are also used to help with this transition. Though using a ballasted deck gives the RR's more flexibility when it comes to track work, grade adjustments, etc.

 

[MikeHalloran]

Thanks for that; learning something makes the day a success.


Mike Halloran
Pembroke Pines, FL, USA