NCHRP Research Report 1100 presents a guide on use of stiffening mechanisms to reduce the deflection for 31-in. guardrail installations while maintaining system integrity and safety performance. The guide reflects test conditions specified in the American Association of State Highway and Transportation Officials (AASHTO) Manual for Assessing Safety Hardware (MASH) and is based on computer simulations and full-scale crash testing. The guide should be of interest to state departments of transportation and other agencies considering implementation of the stiffening mechanisms evaluated in this research. 

Roadside obstacles are often too close to the roadway to permit the installation of common guardrail designs. When this occurs, the protective capability of a guardrail system is desired, but without large deflections from vehicle impacts. To achieve this, guardrail systems are often modified with a combination of tighter post spacing, nesting of rails, and other guardrail stiffening mechanisms to reduce deflections. Considerable research has been completed on these topics, and the AASHTO Roadside Design Guide (RDG) provides guidelines on this topic. However, much of the available research is based upon test conditions specified in NCHRP Report 350: Recommended Procedures for the Safety Performance Evaluation of Highway Features. Hence, a guide was needed on use of stiffening mechanisms to locally reduce the deflection for 31-in. guardrail installations while maintaining system integrity and safety performance. This guide would reflect test conditions specified in MASH rather than NCHRP Report 350.

Under NCHRP Project 22-38, “Development of MASH TL-3 Deflection Reduction Guidance for 31-inch Guardrail,” Texas A&M University Transportation Institute was asked to develop a guide 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 research team developed recommendations for selecting stiffening mechanisms to further investigate with computer simulation. This simulation effort estimated dynamic deflections and crashworthiness for 26 stiffening mechanisms with a variety of evaluation conditions. From this effort, the research team provided recommendations for selecting stiffening mechanisms to further evaluate through full-scale crash testing, which involved four MASH crash tests: 3-10, 3-11, 3-20, and 3-21. All four crash tests showed the stiffening mechanism and appropriate transition successfully met MASH evaluation criteria. The research team also provided guidelines for implementing this stiffening mechanism on the roadside. Lastly, an updated table summarizing guardrail deflections from recent crash tests and computer simulations from this project was developed. This table is presented for consideration as a possible addition to future editions of the RDG.

In addition to NCHRP Research Report 1100, three deliverables are not included in the published report but are available on the TRB website at trb.org by searching for NCHRP Research Report 1100. The deliverables are as follows: (1) slide presentation summarizing the project and including videos of crash testing; (2) slide presentation without the crash testing videos; and (3) crash testing videos.