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Long Span Guardrail -- Minimum Lengths with Common Terminals

Question
State IL
Description Text

Illinois has developed Highway Standard 630106-1 (attached) to implement the MGS Long Span design for crossing culverts.  In this design we have shown 62.5 feet of Type A guardrail (MGS with 6’-3” post spacing) beyond the three CRT posts before any other pay item may begin.  This means that attachment of a crashworthy end terminal is generally going to add another 37.5 feet of length of need guardrail (according to IDOT pay item definition for these items), plus the gating portion , if any.

 

We note that a previous question on the consulting website (6/18/2007) inquired specifically about the FLEAT terminal and how it could be used with the long span design.  As we read the result, the 37.5 feet of the proprietary FLEAT design plus 25 feet of guardrail item would satisfy the needed 62.5 feet minimum installation beyond the three CRT posts.  This was allowed specially for the FLEAT because crash testing in the flared section showed that the FLEAT works for impacts in that region.

 

We would like to adopt this practice to help make the long span installations more economical and practical, and also wish to explore similar applications for other guardrail terminals to allow for competitive bidding for these terminal applications.

 

The SRT terminal by Trinity is a flared terminal that includes a version accepted for use with the MGS system.  Is it acceptable to use this terminal in a similar layout to that described for the FLEAT system?  It does not appear that the length of need point impact was tested in the acceptance for MGS use, but that was considered and agreed to be waived by FHWA.

 

With respect to “tangent” guardrail terminals, the ET-Plus by Trinity and the SKT by Road Systems are commonly used in Illinois.  Neither of these terminals were tested at the length of need point for the MGS version.  Could the length of these systems be counted against the required 63.5 feet beyond the three CRT posts?  Would the same answer also apply if these terminals were flared at a rate of 1:50 as is the practice to avoid having extruder heads overlap the shoulder area?

 

Lastly, Connecticut submitted a question on July 5, 2012 regarding the long span installation and you responded soon thereafter July 16th,  in which you pointed out MASH testing done on 75’ long version  of the MGS  - we ask, would this also apply to the use of the same flared terminals just discussed above? 

 

 

Keywords
  • End Treatments & Crash Cushions
  • Guardrail
Other Keywords Long Span
Date August 29, 2014
Attachment Std 630106-01 HighwaysStandardsRevision215.pdf


Response
Response

As you note in the email, the MGS long span system was tested to MASH and the implementation of the system was accompanied by recommendations regarding the system length and the amount of guardrail tangent to the long span. Special systems such as the MGS long-span could actually further increase the loading of the barrier system and create higher anchor loads and affect the length of the system and the anchorage. Although it was likely that guardrail lengths shorter than 175 ft could redirect 2270P vehicles impacting at the TL-3 conditions, there was no crash test data to support or recommend the use of shorter lengths at that time. Based on these noted concerns, it was recommended that the minimum installation length of the MGS long-span be set at 175 ft for a long span length of 25 ft. However, if a shorter long span length was used, it was still recommended that the upstream and downstream lengths of the installation including the end anchorage be no less than 62.5 ft beginning at the third CRT post. This length is based on the 175 ft system length that was tested. At that time, there may be a potential to reduce the downstream distance, but this would require further analysis and verification with full-scale crash testing.

Following those recommendations, we were contacted by IaDOT for clarification on the system lengths as well as questions regarding the use of the FLEAT terminal with the MGS long span. At that time we reviewed this question, the FLEAT had been tested near the beginning of length of need with the MGS under the NCHRP Report 350 impact criteria. See attached. Because the FLEAT had been tested under NCHRP 350 in the flared section of the barrier, it was believed that the FLEAT possessed sufficient anchor capacity for the MGS long span even if the flaring of the terminal occurred within the 50 ft of tangent guardrail recommended originally. The overall length of the installation remained 175 ft. Of course, this recommendation was only relevant for NCHRP 350 impact conditions, but it gave IaDOT some additional flexibility in their installations. MASH impacts (with their higher angle and vehicle mass) have not been conducted on the FLEAT or the majority of the other terminal system at the beginning of length of need.

You have requested our thoughts regarding the use of the SRT terminal and its parabolic flare in a similar fashion. There is potential that this configuration would be acceptable.  FHWA acceptance letter HSSD/CC-100, dated August 30, 2007 specific to “NCHRP Report 350 Test 3-35 of the SRT-31” contains details of successful Test 3-35 of the SRT-31. Based on this test, the argument for using the SRT 31 would be very similar to the FLEAT. Thus, it would be acceptable to use the SRT 31 and its associated flare with the MGS long span as long as the minimum system length is still met.

With respect to tangent terminals, the length of the terminal is included in the 62.5 feet required adjacent to the CRT posts in the MGS long span. The SKT system uses a similar anchorage to the FLEAT system that was tested to NCHRP 350 with the MGS. No terminal system has repeated their beginning of LON tests under the MASH criteria. Thus, we have confidence in their ability to anchor systems under NCHRP 350 loading and they potentially can develop the loads for MASH impacts. However, we cannot definitively determine their anchor capacities with respect to MASH loading at this time.

For a tangent terminal with a 50:1 flare, we do not expect to see a large difference in the loading of the terminal anchor. This flare is approximately a 1.15 degree angle and would not have a large effect on anchorage of the MGS long span system.

FHWA has also had discussion regarding the flaring of tangent terminals in a general sense based on NCHRP 350 testing of terminals. They, along with industry and researchers at MwRSF and TTI, determined that 15:1 flares were appropriate for general tangent terminal applications (see attached). However, the use of these more aggressive flares with shorter system lengths or special applications like the long span would likely require further analysis and study.

Finally, with respect to your last question, we have given guidance previously based on reduced system lengths for the MGS long span system with shorter unsupported span lengths. This guidance was based on recent research we had done on the minimum system length of the MGS system, collection of data regarding the capacity of the generic end anchorage we use in our evaluation testing, and the assumption of tangent guardrail. The addition of a flared terminal such as the FLEAT could create potential issues when applied with reduced system lengths. Specifically, there are some concerns that shorter systems   would place the CRT’s and unsupported span significantly closer to the end terminal. Placement of two critical regions of the barrier in such close proximity could have additional consequences and affect the performance of the barrier system. Thus, we believe more study would be required to evaluate the flared terminals with the reduced system lengths that we developed for Connecticut.

 

Date September 13, 2014
Attachment TANGENTIAL TERMINAL FLARE GUARDAIL INSTALLATIONS.pdf


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