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Bridge Rail Retrofit

State WI
Description Text

WisDOT has a project where an existing thrie beam bridge rail was installed too low.  Regional staff has asked if:

1.         The existing longitudinal channel on top of the bridge rail could be removed.

2.         A small box beam or steel tube could be bolted to the existing post (i.e. to get the correct rail height)

3.         Existing longitudinal channel is reinstalled.


From what I understand, the existing deck bolts and nuts are very rusty and difficult to remove.  This will make it difficult to remove the existing post and replace with new taller posts.  In addition the taller post are more expensive to fabricate than the smaller box beam/steel tubes.


An example of the retro fit is attached (W Rail Retrofit.pdf)


An example of our current  thrie beam retro fit  is also attached (3002.pdf).


The rail used on the existing bridge is also  attached.

  • Bridge Rails
Other Keywords none
Date July 28, 2010
Attachment 3002.pdf
Attachment struct_plan B-52-822&823.pdf
Attachment W Rail retrofit.pdf


The Wisconsin design is similar to the Missouri thrie beam and channel bridge rail tested by TTI.  However, the bridge rail plans  show the system as 4 inches shorter than it was tested at previously.  Thus, the addition of the 4 in. tall spacer block to the top of the shorter post allows the rail to be installed at the correct height.  The use of four 5/8 in. diameter bolts to connect the block to the post should provide more than enough strength to prevent shear failure during an impact.


The anchorage for the Wisconsin bridge rail seems to be a modification of the tested system as well.  Tested used three 1 in. diameter A307 bolts, while the current drawings show 4 7/8 in. diameter A449 bolts.  Noting that A449 provides a 20-50% increase in strength (depending on grade), the Wisconsin bridge rail design should provide equal or greater anchorage strength.


Therefore, the proposed bridge rail design appears to be of comparable strength and geometry to that of the tested Missouri thrie beam and channel system.

Date August 2, 2010

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