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TL-3 Railing for Timber Bridge

Question
State WY
Description Text

We have a very old timber bridge which is not slated for replacement, but would like to upgrade the bridge and approach railing.  It is located on a rural 2-lane highway with a posted speed of 65 mph.  The length of the bridge is 23 feet.  I have attached a few photographs and a very old and hard to read standard plan for which this structure was constructed.


Could you recommend some treatments for bridge railings for this structure?  They do not have to be aesthetic in nature.  We have considered the use of the MGS long span, but as nearly as I can tell, the structure width is only about 25 feet, so the railing needs to be as flush as possible with the end of the deck.  The MGS long span normally requires a post and blockout width (20") plus an assumed wing wall width of around 8 or 9 inches, so it appears the railing would have to be placed around 28" inboard of the end of the deck. 


The MGS Low Cost Bridge Rail might be an option if a mounting could be developed with would greatly reduce the stress normally transmitted to a concrete bridge deck, or in this case, to the wood stringers. 


If you note in the photos, although there is a timber curb, the roadway appears to be built up above the curb (consequently the rail height is very low).  Any help you could provide would be appreciated!

Keywords
  • Bridge Rails
Other Keywords timber bridge
Date November 26, 2013
Attachment Timber Bridge Details.pdf
Attachment 016.JPG
Attachment 025.JPG


Response
Response

After a quick review of your pictures and drawings, we see two options for possible treatment of this timber bridge: (1) a long-span system, or (2) an adaptation of the weak-post, MGS bridge rail. Either system would be TL-3 crashworthy.  Both have benefits and downfalls as described below.

The benefits of a long-span system would be that the rail itself would not be directly attached to the bridge.  Thus, impact loads would not be imparted to the bridge elements.  However, use of a long span system would most likely require some soil fill to be added around the abutments to ensure the adjacent posts were properly installed.  In addition and as you have already mentioned, the offset of the rail and the edge of the bridge deck would result in a loss of roadway width.

Utilization of the weak-post, MGS bridge rail (or similar variations designed for attachment to culvert headwall) would help to maximize your roadway width by attaching to the outside face of the bridge deck / stringers.  However, properly anchoring the sockets which attach the posts to the bridge could prove difficult as soil fill and asphalt overlays may prevent bolting options.  Further, anchorage strength may be limited dependent upon the structural integrity of the timber elements you tie into.

Let us know if you would like us to further investigate the specifics for either of these treatment options.

Date December 10, 2013


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