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Attaching guardrail to flared bridge wingwalls

State MO
Description Text

I have a design group that is
wanting to flare the end of a bridge rail (at the wing wall) and then attach
the approach rail system on a 24:1 flare in an effort to open up sight distance
for a nearby gravel approach.  The bridge width is around 26 feet wide (11
foot lanes with 2 foot shoulders).  This is a lower volume roadway, with
something around 500 ADT I am told. 


Can you comment on the use of
the entire rail system on a flare using our current designs for bridge rail,
barrier wall attachment, thrie beam stiffness transition, the transition to
W-Beam, a small run of MGS rail and then the energy absorbing end terminal all
being planned to be on a 24:1 flare from the bridge end? 


I have concerns at the bend in
the bridge rail that starts the flare on structure that it might create a
critical point on the bridge.  Are these concerns warranted?  Also,
is there reason for concern about pocketing or other crash concerns on the
flared approach transition to the flared bridge rail? 


Any comments you may have on the
topic or guidance to related research or other state design standards that
might have a flared approach rail system would be appreciated. 

  • Approach Guardrail Transitions
Other Keywords none
Date May 1, 2017
Attachment 004_PP_01_J7S3154_I50.pdf


For installations where a bridge rail with limited space for approach guardrail and an end terminal on the upstream end of an installation, a flared approach guardrail transition may be desired to reduce runout length. Currently, guidance exists for flaring the Midwest Guardrail System (MGS) away from the roadway. However, no research or full-scale crash testing of flared approach guardrail transitions has been conducted under the NCHRP Report No. 350 or MASH evaluation criteria.


Approach guardrail transitions are sensitive systems, as the gradual increase in lateral stiffness along the transition length is critical to its safety performance. Improper designs or abrupt changes in lateral stiffness can result in guardrail pocketing, vehicle instabilities, and vehicle snag on the rigid bridge rail/parapet. Additionally, seemingly small changes to a crashworthy guardrail transition (e.g., the shape of the downstream parapet, the addition or removal of a curb below the guardrail, and/or the removal of a single post within the system) have led to failed crash tests and inadequate system designs. Due to the sensitivity of these systems and the limited knowledge about their performance in flared configurations, current guidelines are to place guardrail transitions tangent to the roadway.


Previous testing of flared guardrail systems and tangent transitions lead to several concerns related to flared transitions. Flaring of the transition would increase the effective impact angle, which would raise the potential for vehicle snag, pocketing, and vehicle instabilities. Increases to the loads imparted to the barrier would also be expected, which could lead to rail rupture.


MwRSF recently developed an upstream stiffness transition for use with previously-approved thrie-beam approach guardrail transitions and the MGS. As part of that research, the use of flared guardrail adjacent to the transition region was addressed. MwRSF recommended a minimum of 25 ft of tangent MGS to be used between the upstream end of the asymmetrical W-beam-to-thrie beam transition section and the start of the flared section (i.e., the bend between flare and tangent sections). No flaring of the actual transition was recommended without further research.


Full-scale crash testing of the MGS upstream stiffness transition with the 1100C vehicle indicated that wedging of the vehicle occurred under the asymmetrical W-to-thrie beam transition section, resulting in vehicle snag on the posts. While decelerations were below critical levels, there may be potential for increased occupant risk values as the flare rate increases for the critical small car impact. Finally, the use of flared transitions may increase the potential for vehicle instability due to the increased impact angle, increased vehicle snag, and the increased potential for pocketing. Therefore, a flared guardrail transition would need to be evaluated for impacts to the upstream W-to-thrie stiffness transition as well as near the downstream end attachment to a rigid buttress.


MwRSF is currently working on the development of a standardized end buttress for guardrail transitions that can connect any crashworthy, thrie-beam guardrail transition to various bridge rail shapes. The goal of the project was to develop a buttress that reduces snag potential and pocketing concerns by flaring the face of the buttress. Because of its increased safety performance, the standardized end buttress may allow for guardrail transitions to be safely flared by alleviating some snag and pocketing concerns near the rigid parapet associated with the increase in impact angle. However, the new buttress design would not alleviate snag and pocketing concerns near the upstream end of the transition.


The MGS has been tested under NCHRP Report No. 350 at flare rates up to 5:1, so thus use of 24:1 flares with the LON portion of the MGS would not be a concern. However, impacts near the end anchorage have not been evaluated at higher flare rates.


FHWA has allowed the installation of tangent, energy-absorbing terminals at flares of 25:1 over 50’ minimum at TL-3. The flare you note here would slightly exceed that.


Thus, we cannot currently recommend flaring of the AGT and guardrail system directly off the bridge end for TL-3 applications based on the concerns above. These concerns may not be as substantial when considering a low-volume road application like you have with the potential for lower speeds and ADT’s. In these applications, impact conditions may be less severe and the concerns noted above would be reduced.


Please let me know if you have further comments and/or questions.



Date May 2, 2017

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