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Bridge Railing Modification

Question
State IN
Description Text

Attached to this question are two PDFs, Bridge Railing Modification Detail 1 and Detail 2.  In the PDF Bridge Railing Modification Detail 1, can the concrete parapet wall section be reduced to 18”  and retain the single steel
tube at a height of 36"?  There is a similar railing (PDF Bridge Railing Modification Detail 2) that uses an 18” concrete
parapet, but has 2 steel tubes, top tube height of 42". 



 



Would reducing the height of the concrete parapet and retaining the single steel tube at 36" on the BR27C Flush Mounted Bridge Railing (attached Detail 1) likely
have performance implications?  If possible can you comment specifically
related to test levels 4 and 2?



Keywords
  • Bridge Rails
Other Keywords BR27C Flush Mounted Bridge Railing
Date August 3, 2017
Attachment Bridge Railing Modification Detail 1.pdf
Attachment Bridge Railing Modification Detail 2.pdf


Response
Response The proposed modification show for the BR27C combination bridge rail does pose some concern if the parapet is shortened 6" as shown.

The original BR27C was tested to AASHTO Guide Specification for Bridge Railings standards and has yet to be evaluated under NCHRP Report 350 or MASH. The design was grandfathered in under NCHRP 350 in an FHWA memorandum. In the previous testing of the BR@&C with the small car vehicle there was evidence of the vehicle contacting and snagging on the support posts of the system with the front corner and fender of the small car vehicle. These tests were conducted at a lower 20 degree impact angle than current test standards. 

There is concern that lowering of the parapet could increase vehicle snag on the combination rail support posts and create occupant risk concerns. 

In terms of TL-4 performance, the passenger vehicle concerns noted above would exists. Additionally, the railing has not been evaluated to the more recent MASH TL-4 impact conditions with the 10000S vehicle. A 36 in. rail height has shown the ability to safely redirect the 10000S vehicle. However, the effect of the additional vehicle mass and speed on the overall capacity of the BR27C is not known. Only moderate damage was observed in the SUT testing under the AASHTO specifications, so there is potential that the capacity may be sufficient. Further analysis would likely be warranted to determine this.

The performance of the railing under TL-2 poses much less concern due to the reduced impact speeds. However, the potential for snag on the post would exist to a lesser degree.





Date August 17, 2017


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