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NH 5-3(103)129; Karrow to Mountainside; CN 2017001 - Two Tube Bridge rail to parapet detail

Question
State WY
Description Text Would you have any suggestions for this gentleman (Chris) from TDH engineering? He would like to construct a concrete parapet at the end of Wyoming's TL-4 Twin Steel Tube Railing for a project in Montana. It posses an interesting question since our rail cantilevers beyond the post. I thought I did see another state that used some kind of parapet at the end of the steel tube railing, but I don't know if it was secured to the railing. I am interested in the response as well.

Details at: http://www.dot.state.wy.us/home/engineering_technical_programs/bridge/standard_details.html

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I am working on the above MDT project where we are using the 2 tube TL-4 rail (MDT calls it W-830 rail) on a structure, and the road designers are calling for an impact attenuator instead of a bridge approach section due to space constraints off the end of the structure. My thought was that since attenuators can be backed up on concrete parapets, if there is a way to tie the 2 tube rail to a short concrete parapet, that might be a solution in this case. If you have any information about situations where 2 tube rail has been transitioned to concrete rail or parapet I would appreciate it if you could pass those on so we can review. If you have any other experience about utilizing an impact attenuator in conjunction with 2 tube rail (without an intervening bridge approach rail section) that would be great to hear about also. Thanks for any input you can provide.
Keywords
  • Bridge Rails
Other Keywords none
Date September 25, 2014


Response
Response We looked at a somewhat similar problem a while back for Iowa regarding transitioning of the BR27C bridge rail to a concrete parapet. See the discussion and solution at the link below.

http://mwrsf-qa.unl.edu/view.php?id=691

For the Wyoming TL-4 rail you have shown, we would propose a similar solution that attaches the rail to a flared parapet be cutting the tube to match the flare. Because the Wyoming bridge rail uses the tube rails to provide the majority of the redirective capacity of the barrier, unlike the Iowa BR27C rail which has a concrete parapet, we would recommend that base plates be attached to the flare cut tubes at the parapet to allow for anchoring of the tubes to the parapet. You may also want to consider keeping a bridge rail post relatively close to the parapet to limit the loading of the tubes and anchorage where they attach to the parapet. Note that the parapet design would need to consider impact loading from a vehicle as well as sufficient capacity to anchor the tube railing.

Let me know if that gets you going in the right direction or if you need more guidance. 
Date September 25, 2014


Response
Response I am not quite sure I understand what to do with the bottom railing.

I think they were thinking more in terms of a parapet shown in the following bridge rail end, although in this case, the concrete parapet would be downstream.

http://guides.roadsafellc.com/bridgeRailGuide/index.php?action=view&railing=129 
Date September 25, 2014


Response
Response The example I sent was for a combination bridge rail with only one tube and a concrete base, but the concept would be the same for a two tube design. Essentially, we recommend overlapping the parapet with the tubes. We recommend tapering the parapet and then cutting the tubes to match the taper. This is done to ensure that the impacting vehicle doesn't snag on the parapet end.

We have not recommended systems like the one in the link below for the downstream end due to concerns for vehicle snag on the end if the parapet. For upstream ends, the design shown may work because the vehicle is stepping down for the parapet to the tubes. However, on a downstream end, the vehicle would have a tendency to redirect along the front face of the tubes with some components of the vehicle protruding past the front face of the tubes. This could creat vehicle snag as it reaches the parapet end.

Let me know if that clears things up.
Date September 25, 2014


Response
Response

I was thinking of something like this.

Date September 26, 2014
Attachment WY Tube Rail.jpg


Response
Response

I like this idea very much!

Thanks for your help!

Date September 26, 2014


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