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Recently, Ed Demming of the CFLHD in the Denver area asked MwRSF to evaluate or estimate the height tolerances for some existing barrier systems covered in their standards plans. I recall that the Buried-in-Backslope Terminals were in that group and maybe the TL-2 MELT. When I get to that review, I will make sure to copy you on the results as I assume that this inquiry has potentially arisen from the same FHWA directive.

 In any event, I am currently unaware of an existing, 31” tall generic guardrail end terminal. 

Gentlemen, what guidance should we give the states when they ask for generic 31” end treatments?  Are we asking them to go to 31” while not offering a crashworthy end treatment? 

Hamilton Duncan, P.E., P.G.

Safety Engineer - FHWA West Virginia

There are many options available:

1) The state may specify that a crashworthy 31-inch end terminal be used. Proprietary designs will be provided.

2) The state may wait for MWRSF to offer guidance on existing generic terminals. The Division Office may allow them to wait.

3) The state may pay for development and crash testing of a 31-inch generic terminal.

4) The state may join a pooled-fund group and vote for generic 31-inch terminals to be developed and tested.

5) The state may use 27 ¾ inch high generic terminals and transition to 31-inch rail height.

Nick

Nicholas Artimovich, II

Highway Engineer, Office of Safety Design

Last month, I was copied on a discussion or inquiry regarding whether existing non-proprietary guardrail end terminals could possibly be adapted for use with 31-in. high, W-beam guardrail systems. The noted terminals included: (1) the buried-in-backslope terminal [FHWA acceptance letters CC-53 and CC-53A] and (2) the CST or cut slope terminal [no FHWA acceptance letter].

BURIED-IN-BACKSLOPE

Testing and Evaluation of W-Beam Guardrails Buried-In-Backslope – November 1996 – Arnold, Buth, and Menges

Testing and Evaluation of W-Beam Guardrails Buried-In-Backslope – January 1999 – Arnold, Buth, and Menges

Two TL-3 pickup truck crash tests were successfully performed on the guardrail terminal according to test designation no. 3-35 using the NCHRP Report No. 350 safety performance criteria. For these tests, the terminal attached to metric-height, W-beam guardrail located downstream of the terminal region. The first test occurred on a barrier system placed in combination with a 1:10 approach slope as well as a flat bottom ditch. The second test occurred on a barrier system placed in combination with a 1:10 approach slope, a V-ditch, and a drop inlet. Both barrier systems were anchored into an upstream 1:2 backslope using a reinforced concrete anchor block. A W-beam rubrail was utilized across the ditch sections.

Crash Test of the G4 W-Beam Guardrail with Terminal Buried-In-Backslope – March 1998 – Buth, Menges, and Arnold

One TL-3 pickup truck crash test was successfully performed on the guardrail terminal according to test designation no. 3-35 using the NCHRP Report No. 350 safety performance criteria. For this test, the terminal attached to metric-height, W-beam guardrail located downstream of the terminal region. The test occurred on a barrier system placed in combination with a 1:6 approach slope as well as a V-ditch. The barrier system was anchored into an upstream 1:4 backslope using a post end anchor system. Longer post lengths (i.e., 8 ft) were used throughout the ditch region. A W-beam rubrail was utilized across the ditch section as well.

NCHRP Report 350 Assessment of Existing Roadside Safety Hardware – November 2000 – Buth, Menges, and Schoeneman

The test results reported in the March 1998 document noted above were also contained herein.

Another TL-3 pickup truck crash test was successfully performed on the guardrail terminal according to test designation no. 3-35 using the NCHRP Report No. 350 safety performance criteria. For this test, the terminal attached to metric-height, W-beam guardrail located downstream of the terminal region. The test occurred on a barrier system placed in combination with a 1:4 approach slope as well as a V-ditch. The barrier system was anchored into an upstream 1:2 backslope using a post end anchor system. Longer post lengths (i.e., 8 ft) were used throughout the ditch region. A W-beam rubrail was utilized across the ditch section as well.

Discussion

Within the FHWA-TTI crash testing programs noted above, only test designation no. 3-35 was performed. No 2000P crash tests were performed closer to the upstream end but instead were run downstream from post no. 8. In addition, no small car crash tests were conducted along the terminal length placed with a V-ditch.

As such, it is difficult to form a complete opinion regarding the adaptation of the Buried-In-Backslope Terminal to 31-in. tall guardrail in the absence of tests that were not performed or impact locations which were not evaluated. Based on the results from the prior 2000P tests and using identical impact locations, it would seem reasonable that a slightly taller barrier would remain capable of redirecting the pickup truck (3-35). The upstream tension anchorage systems for the W-beam rail may require slight modifications to account for the raised rail height, reduced embedment, and decreased post-soil forces. In addition, some consideration may be necessary to evaluate whether the rubrail should remain at its current height or whether it is preferred to incorporate a similar height increase of 3.25 in. to mitigate any potential concerns for small car underride and/or post snag in ditch section. I suspect that a 3D plan of a modified terminal system and roadside geometry may help to alleviate any of these concerns.

Alternatively, the current Buried-In-Backslope design could be left alone. Instead, the metric-height W-beam guardrail located downstream and tangent to the roadway could be vertically transitioned in height to match up with 31-in. tall guardrail. The height transition could easily occur over a 50 ft segment length of barrier.

CST – CUT SLOPE TERMINAL

At this time, I am unable to find any crash testing programs corresponding to the CST system. From a WvDOT CAD detail, it is apparent that the CST system does not utilize a rubrail nor provides similar anchorage to that utilized in the systems described above. As such, there are concerns with the existing design that would not be mitigated by raising the rail height to match that of 31-in. tall guardrail. If by chance the CST design has demonstrated a satisfactory safety performance, then the tangent region downstream of the flared terminal section could also be adjusted using a height transition similar to that noted above.

If you have any questions regarding the information noted above, please feel free to contact me at your earliest convenience. Thanks!

P.S. – If new information does become available, it may be necessary to revise or restate my opinions noted above.

Thanks for your discussion of the buried in backslope. Until someone conducts crash testing on a 31-inch version we will recommend that the crash-tested version be installed, and then the height of rail be transitioned up to the LON height as you have noted.

I am copying Will Longstreet on this as he will be at the Pooled Fund meeting next month. I will suggest that he bring up the subject of generic terminals for 31 inch barriers (unless someone else beats him to it.)

Nick

Nicholas Artimovich, II

Highway Engineer, Office of Safety Technologies

The Texas A&M Transportation Institute conducted MASH crash testing of a 31-in. tall, buried-in-backslope, non-proprietary W-beam end termination. Performance of the buried-in-backslope terminal was determined to be successful for the two impact points selected according to MASH TL-3 impact conditions. Details about the system are shown here:

https://www.roadsidepooledfund.org/buried-in-backslope-terminal-variations-in-foreslope-backslope-and-ditch-configurations/

https://www.roadsidepooledfund.org/608431/

Hawaii DOT sponsored a research study starting in 2021 to evaluate a buried-in-backslope design with more aggressive and comprehensive evaluation of impacts near the end anchorage and in reverse-direction impacts. Results of the study are still ongoing, but will be published on the MwRSF Research Hub website when completed.