Nebraska LTAP


Date of this Version


Document Type



Rosenbaugh, S.K., Fallet, W.G., Faller, R.K., Bielenberg, R.W., and Schmidt, J.D. (2019.) "34-in. Tall Thrie Beam Transition to Concrete Buttress" NDOT Research Report #TRP-03-367-19.


Roadway resurfacing and overlay projects effectively reduce the height of roadside barriers placed adjacent to the roadway, which can negatively affect their crashworthiness. More recently, bridge rails and concrete barriers have been installed with slightly increased heights to account for future overlays. However, adjacent guardrails and approach transitions have not yet been modified to account for overlays. The objective of this project was to develop an increased-height, approach guardrail transition (AGT) to be crashworthy both before and after roadway overlays of up to 3 in. (76 mm).

A 34-in. (864-mm) tall, thrie beam transition was designed such that the system would be at its nominal 31-in. (787-mm) height following a 3-in. (76-mm) roadway overlay. Additionally, the upstream end of the AGT incorporated a symmetric W-to-thrie transition segment, which would be replaced by an asymmetric transition segment after an overlay in order to keep the W-beam guardrail upstream from the transition at its nominal 31-in. (787-mm) height. The 34-in. (864-mm) tall AGT was connected to a modified version of the standardized buttress to mitigate the risk of vehicle snag below the rail.

The barrier system was evaluated through two full-scale crash tests in accordance with Test Level 3 (TL-3) of the American Association of State Highway Transportation Officials’ (AASHTO) Manual for Assessing Safety Hardware (MASH). Both MASH test nos. 3-21 and 3-20 were conducted near the upstream end of the rigid buttress and satisfied all safety performance criteria. Thus, the 34-in. (864-mm) tall AGT with modified transition buttress was determined to be crashworthy to MASH TL-3 standards. Finally, implementation guidance was provided for the increased height AGT and its crashworthy variations.