The National Academies

NCHRP 22-38 [Completed]

Development of MASH TL-3 Deflection Reduction Guidance for 31-inch Guardrail

  Project Data
Funds: $499,429
Research Agency: Texas A&M Transportation Institute
Principal Investigator: Mr. James Kovar
Effective Date: 7/8/2019
Completion Date: 9/15/2023

In the design and installation of guardrail systems, there routinely exists a need to provide a stiffening mechanism to locally reduce the lateral deflection distance where there is an area of concern such as bridge piers, poles, and other rigid objects located near roadway shoulders. Various means of providing reduced deflections have been made with past systems that have included nesting of guardrail and the reduction of guardrail post spacing. With the adoption of the American Association of State Highway and Transportation Officials (AASHTO) 2016 Manual for Assessing Safety Hardware (MASH), there is currently little to no guidance on the use of deflection limiting techniques for MASH compliant guardrail installations and their necessary transitions. Thus, a need exists to provide guidance for the use of stiffening mechanisms that will reduce the deflection distances behind guardrail while maintaining the integrity and safety performance. This research will continue the development of guidance for the design and installation of reduced deflection guardrail systems that are MASH compliant.
Most research on guardrail used the NCHRP Report 350: Recommended Procedures for the Safety Performance Evaluation of Highway Features testing criteria. The AASHTO and Federal Highway Administration’s (FHWA) Joint Agreement for the Implementation of MASH requires the use of MASH compliant hardware. Further research is needed and should be conducted using the most commonly used 31 in. generic guardrail system, the Midwest Guardrail System (MGS).
The objective of this research was to develop guidance for the practical and cost effective use of stiffening mechanisms to locally reduce the deflection for MASH TL-3 compliant 31-in. MGS installations or equivalent guardrail including any needed transition from the standard system to the stiffened system while maintaining its integrity and safety performance.
The guidelines are to consider the effects of dynamic deflections and working width, fill condition behind the system, post spacing, post length and embedment depth, nested rail, block-out depth, barrier offset, and full development length of the stiffening mechanism.  It is anticipated that a combination of previous research, computer simulation modeling, and full-scale crash testing conducted by an ISO 17025 certified crash laboratory will be required to develop and verify the guidance to demonstrate MASH compliance.
STATUS: Research is complete; publication decision is pending. 

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