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Bullnose modification

Question
State MN
Description Text

We are updating our bullnose standard (to the USBP design)
and just have a few questions to run by you.



MnDOT’s current standard for a
widened median (see attachment, Standard Plan 611, sheet 3 of 3) shows
guardrail tapers on both the approach and the opposing sides of the
median.  We haven’t been able to find our background information to
support these tapper designs yet and though we should just ask you. 

We are specifically questioning the
angle turn locations at the posts (#5 & #11 on the approach side and #5
& #8 on the opposing side). We are wondering what the appropriate angles
are and how best to get back to a parallel alignment with the bridge rails?

Also, the MwRSF Drawing MDN01,
Sheet 6 of 6 (see attached) shows 2 designs for wider bullnose systems. 
How would these designs connect/transition into bridge rails?  What are
the guidelines for laying out these designs in widened medians when attaching
to a bridge rail?



Your
help is appreciated.



Bullnose Systems

Date May 1, 2015
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Attachment WideBullnose.pdf
Response
Response
(active) 		Hello Michael,

The design you provided appears to be consistent with guidance provided by FHWA in approval letter CC68, which may be found at http://safety.fhwa.dot.gov/roadway_dept/policy_guide/road_hardware/barriers/term_cush.cfm. Both wider-bullnose FHWA-approved designs utilized a curved nose piece, a curved transition piece, and a length of straight rail on the departure side of the bullnose that could be flared at a high angle. In-line with straight guardrails can produce instability and rollover, as observed during MwRSF short-radius testing. Thus it is recommended to avoid long, straight segments of guardrail extending from bullnose and short-radius systems which could be subjected to head-on impacts. The Minnesota design has two different flares on the back side of the system, which should increase the probability that an impacting vehicle will deflect the rail laterally and not subject it to potential instability and rollover. MwRSF concurs that the design is therefore consistent with FHWA-approved modifications.

Other bullnose designs provided by MwRSF are preferred in general, in part because of the concerns raised above. Preferred bullnose designs are shown in MwRSF research report on page 149 of no. TRP-03-95-00, found at http://mwrsf.unl.edu/researchhub/files/Report120/TRP-03-95-00.pdf. The designs utilize a larger, broader nose piece and variable tapers on the front and back side rails. It is believed that this design will function better when an impacting vehicle strikes parallel with the guardrail on the opposite-side rail.

The guardrails may connect to bridge rails via an approved thrie beam approach guardrail transition (AGT) after the third rail segment on each side (excluding the nose). The bullnose may be transitioned to other barrier types for connection to bridge ends using approved transitions. The guardrail on the front side may be flared at 15:1 starting at post no. 5, and back side guardrail flare rates may be larger. Note that the maximum back-side flare rate approved by FHWA is approximately 3.5:1, for FHWA Approved Wider Bullnose System (15.6-degree flare). Slopes in front of and up to 40 ft behind the barrier must not be steeper than 10:1. Design details for constructing the systems are shown in the FHWA approval letter and MwRSF report.

Let me know if you have any additional questions.
Date May 1, 2015
Previous Views (133) Favorites (0)