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Guidance on guardrail placed beyond 12' offset from curb

Question
State	SC
Description Text	I wanted to ask you about a topic that’s come up with one of our projects. There are a few locations along the project where they are wanting to put guardrail located behind a 6” curb. They wanted to place it beyond the 12’ maximum distance recommended in the MASH reports. Likewise, they have a situation where they have a bridge tying into the road with a curb, a green space, and a shared use path, and are wanting to place the thrie beam bridge connector and asymmetrical transition piece several feet behind the curb (I am guessing the distance would be around 15’ to 16’, but it’s hard to tell from the sketch; I know this would be beyond 12’ as well). We were trying to figure out if there was any significance to this 12’ maximum and if we should allow this guardrail and the thrie beam mentioned to be placed beyond this 12’ offset from the curb. Any suggestions or guidance you would have on this issue would be appreciated. Also, we hope you guys are doing well up there and wanted to wish our best for Ron while he is away. Thanks

Question

State

Description Text

I wanted to ask you about a topic that’s come up with one of our projects. There are a few locations along the project where they are wanting to put guardrail located behind a 6” curb. They wanted to place it beyond the 12’ maximum distance recommended in the MASH reports. Likewise, they have a situation where they have a bridge tying into the road with a curb, a green space, and a shared use path, and are wanting to place the thrie beam bridge connector and asymmetrical transition piece several feet behind the curb (I am guessing the distance would be around 15’ to 16’, but it’s hard to tell from the sketch; I know this would be beyond 12’ as well).

We were trying to figure out if there was any significance to this 12’ maximum and if we should allow this guardrail and the thrie beam mentioned to be placed beyond this 12’ offset from the curb. Any suggestions or guidance you would have on this issue would be appreciated.

Also, we hope you guys are doing well up there and wanted to wish our best for Ron while he is away.

Thanks

System Performance Evaluation

System Types
Road Closure Gates
Thrie Beam Guardrails

Applications

System Features

Date June 19, 2018
Previous Views (181) Favorites (0)

Response
Response (active)	We did look at MGS placement at large offsets behind curbs under both MASH TL-2 and TL-3 impact conditions. https://mwrsf.unl.edu/researchhub/files/Report72/TRP-03-205-09.pdf https://mwrsf.unl.edu/researchhub/files/Report56/TRP-03-221-09.pdf https://mwrsf.unl.edu/researchhub/files/Report47/TRP-03-237-10.pdf As part of that analysis we provided guidance on envelopes for placement of the MGS at large offsets behind curbs depending whether or not the barrier was installed at 31” with respect to the roadway, or with respect to the top of the curb (37” tall with respect to the roadway. We then tested the MGS installed at 31” with respect to the top of the curb at an 8’ offset to evaluate the boundary of that guidance. This test failed. Subsequent to that analysis, we performed a simulation analysis to evaluate similar offsets under MASH TL-2 conditions. From that analysis, we determined that a 6 ft offset was critical. We full-scale crash tested the 6 ft offset and got acceptable performance under TL-2. We noted at the time that no relevant TL-2 crash tests were available for 1100C and/or 2270P vehicles striking a curb without other barriers located behind the curb. Thus, a maximum 12-ft lateral offset is recommended; since, prior TL-3 curb testing indicated potential vehicle problems beyond a 12-ft lateral offset. Computer simulation had indicated that a 4’ minimum offset was acceptable for TL-2. The study described above was not truly sufficient in terms of funding and scope to fully investigate the variations in lateral offset and barrier heights for the MGS with curb and further study is definitely needed. These ranges could be fully developed and validated. Currently, we don’t have enough data to determine the performance outside of the conservative ranges chosen in the above study. In terms of using large curb offsets with AGTs, that effort has never been undertaken to the best of my knowledge. Thus, the best current guidance would likely be to use similar offsets to what we have recommended for the MGS. We have answered related questions in the past. See below. https://mwrsf-qa.unl.edu/view.php?id=1074 thanks

Response

Response
(active)

We did look at MGS placement at large offsets behind curbs under both MASH TL-2 and TL-3 impact conditions.

https://mwrsf.unl.edu/researchhub/files/Report72/TRP-03-205-09.pdf

https://mwrsf.unl.edu/researchhub/files/Report56/TRP-03-221-09.pdf

https://mwrsf.unl.edu/researchhub/files/Report47/TRP-03-237-10.pdf

As part of that analysis we provided guidance on envelopes for placement of the MGS at large offsets behind curbs depending whether or not the barrier was installed at 31” with respect to the roadway, or with respect to the top of the curb (37” tall with respect to the roadway. We then tested the MGS installed at 31” with respect to the top of the curb at an 8’ offset to evaluate the boundary of that guidance. This test failed.

Subsequent to that analysis, we performed a simulation analysis to evaluate similar offsets under MASH TL-2 conditions. From that analysis, we determined that a 6 ft offset was critical. We full-scale crash tested the 6 ft offset and got acceptable performance under TL-2. We noted at the time that no relevant TL-2 crash tests were available for 1100C and/or 2270P vehicles striking a curb without other barriers located behind the curb. Thus, a maximum 12-ft lateral offset is recommended; since, prior TL-3 curb testing indicated potential vehicle problems beyond a 12-ft lateral offset. Computer simulation had indicated that a 4’ minimum offset was acceptable for TL-2.

The study described above was not truly sufficient in terms of funding and scope to fully investigate the variations in lateral offset and barrier heights for the MGS with curb and further study is definitely needed. These ranges could be fully developed and validated. Currently, we don’t have enough data to determine the performance outside of the conservative ranges chosen in the above study.

In terms of using large curb offsets with AGTs, that effort has never been undertaken to the best of my knowledge. Thus, the best current guidance would likely be to use similar offsets to what we have recommended for the MGS.

We have answered related questions in the past. See below.

https://mwrsf-qa.unl.edu/view.php?id=1074

thanks

Date June 21, 2018
Previous Views (181) Favorites (0)