View Q&A

At TRB I mentioned that we would like to revise our criteria
concerning the minimum length of guardrail for a free-standing run.

This is an excerpt from the IL Tollway Traffic Barrier
Guidelines:

The 168.75’ minimum length of a “free-standing” run of
guardrail is based on the system length that has been crash tested.  If
using a Type T1 (Special) Terminal on the upstream end, 34.38’ can be applied
toward the 168.75’ requirement.  For example, a typical free-standing
installation usually includes a Type T1 (Special) Terminal on the upstream end
and a Type T2 Terminal on the downstream end.  For this example, the
minimum length of guardrail required between these two terminals is 137.5’.

I would love to reduce the minimum length shown in
yellow.  What is the minimum that you are comfortable with?  Can it
be 75’?

Thanks again.

Previously, MwRSF has done two research studies that relate to this issue. The first was a study for the MGS system that investigated potential minimum system lengths for the system under MASH TL-3 impact conditions (http://mwrsf.unl.edu/researchhub/files/Report281/TRP-03-276-13.pdf). The second was a study that investigated the crashworthiness and redirective length of the downstream anchorage that is typically used with the MGS (http://mwrsf.unl.edu/researchhub/files/Report279/TRP-03-279-13.pdf).

In the minimum length study, computer simulation and full-scale testing indicated that a 75’ long MGS system would be capable of redirecting a 2270P vehicle under the MASH TL-3 impact conditions. Test no. MGSMIN-1, was performed on the 75-ft long MGS with a top rail mounting height of 31 in. A 4,956-lb pickup truck impacted the barrier system at a speed of 63.1 mph and at an angle of 24.9 degrees. The test results met all of the MASH safety requirements for test designation no. 3-11. The tested system had a total of 13 posts as shown below.

A performance comparison was conducted between 75-ft MGS (test no. MGSMIN-1) and 175-ft MGS. The dynamic deflection for the 175-ft (53.3-m) MGS was slightly higher than observed for the shortened system, but this difference could be due to variations in soil compaction between tests. The working width was nearly indistinguishable. In general, the 75-ft MGS in test no. MGSMIN-1 performed as desired and closely resembled the standard 175-ft MGS.

The second study regarding downstream anchoring of the MGS found that the MGS would successfully redirect 2270P vehicles impacting at 6 posts or more upstream of the end of the system for a MASH TL-3 impact on a 175-ft long MGS system.  

Based on previous testing and the results of test no. MGSMIN-1, MASH TL-3 vehicles impacting between post nos. 3 and 8 of the 75-ft long system should be redirected. Vehicles impacting downstream of post no. 8 may be redirected, but the system would also be expected to gate based on the downstream anchor research.

Although the 75-ft (22.9-m) MGS performed successfully, several factors, including Lateral Extent of the Area of Concern and the Guardrail Runout Length, must be considered when determining the overall barrier length for shielding a roadside hazard. Only a few roadside hazards can be properly shielded by short guardrail installations. Thus, longer guardrail installations are still required for shielding many hazards.

In order to estimate the actual redirective lengths of the shortened system, it was assumed that the shorter 75’ system would potentially continue to redirect errant vehicles impacting at 6 posts or more upstream of the end of the system similar to the 175-ft long system. This has not been proven through testing, but we believe that the performance should be similar. In addition, the beginning of the length of need is typically identified as post no. 3, or 12.5 ft from the upstream end for most terminals. Thus, redirection was assumed for MASH TL-3 vehicles impacting between post nos. 3 and 8 of the 75-ft long system for a length of 31.25 ft. This limits the use of the system to relatively narrow, discrete hazards where proper runout length and length of need can be achieved.

Similar analysis was done for 62.5-ft and 50-ft long systems, as noted in the reports. However, that worked is based only on simulation and has not been tested.

The scope of the research did not include evaluation of the performance of end terminals on a reduced-length guardrail system. Further study may be needed to evaluate reduced system length in conjunction with guardrail end terminals in redirective impacts as well as end-on terminal impacts. Guardrail end terminals may have weaker post sections and/or anchorage than what was utilized in test no. MGSMIN-1. Thus, shorter guardrail lengths may not have the same redirection envelope found in this study and the posts may not resist the rail forces in end-on impacts. Since guardrail end terminals are mostly proprietary, they were not evaluated in this study.

To the best of our knowledge, the shortest installation lengths for compression based terminal testing was conducted on 131.25-ft long system. We believe that this length could be shortened some based on our current knowledge of guardrail compression forces. We have used a reduction in longitudinal rail force of approximately 1-1.2 kips at each post in a guardrail due to the connection between the post and the rail. Current terminal designs tend to have impact head compressive forces that average about 15 kips. This would mean that a minimum of 12-13 posts would be needed to develop the compression load. Of course the end terminal takes out some posts during its compression. However, most of the velocity drop occurs in the first 25-31.25 feet of the compression. Thus, we can assume that if we allow for 31.25 ft of compression and 13 posts to develop the compressive load, an estimated minimum system length for the development of the end terminal compressive loads would be 112.5 ft (13*6.25+31.25).

Because we did not have additional funds or terminal testing and evaluation in the above research, we would recommend minimum system lengths of 112.5 ft in order to be conservative. This would extend the redirective length for the system using the assumptions above to 68.75 ft.

One last factor to consider with the use of terminals on these short systems is the deflection of the terminal when impacted on the end relative to the hazard. As noted above, we believe that the system will redirect the vehicle beginning at post no. 3 in the system. However, in an end on impact of the terminal, the vehicle may deflect down the rail between 37.5 ft – 50 ft. Thus, hazards near the back of the guardrail may still be impacted by end terminal impacts even when they are in the redirective area of the guardrail system. As such, you have to consider both the deflection of the terminal, the redirective region of the LON, and the runout length considerations when designing the placement of short guardrail system.

Let me know if that answers your questions.