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omitted post MGS, minimum distance from obstruction to post

Question
State	IN
Description Text	We have assumed that where one post is omitted, the clear distance between the outermost side of the underground obstruction and the first driven post should be 12 inches, similar to the MGS Long Span. Since CRT posts are not used to omit one post, could this clear distance be less than 12 inches for a steel post? In TRP-03-326-16, it appears that post 13 bent at the ground surface. As long as the underground obstruction is buried and the 2" (max.) projected headwall is a minimum of 1.5 feet from the back of post, could the clear distance be reduced to 6 inches? Attached should be a PDF that may help explain our questions. Thank you

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Description Text

We have assumed that where one post is omitted, the clear distance between the outermost side of the underground
obstruction and the first driven post should be 12 inches, similar to the MGS Long Span. Since CRT posts are not
used to omit one post, could this clear distance be less than 12 inches for a steel post? In TRP-03-326-16, it appears
that post 13 bent at the ground surface. As long as the underground obstruction is buried and the 2" (max.)
projected headwall is a minimum of 1.5 feet from the back of post, could the clear distance be reduced to 6 inches?
Attached should be a PDF that may help explain our questions. Thank you

System Performance Evaluation

System Types
Road Closure Gates
Thrie Beam Guardrails

Applications

System Features

Date June 19, 2019
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Attachment Guardrail grading question.pdf

Response
Response (active)	When a guardrail post is placed longitudinally adjacent to an obstruction, such as a culvert or wingwall, it reduces the amount of soil that would be displaced by post rotation. Thus, the soil resistance is reduced, and stiffness of the system is reduced. The quantification of this force reduction has not been fully studied, which has led MwRSF to recommend the 1 ft offset between an in-ground structure and a guardrail post. We have utilized this recommendation within multiple systems, including the MGS-long span and top-mounted MGS for culverts. Although shorter offset distances may still perform adequately, we will continue to recommend a 1 ft offset until further research is conducted. Post 13 of the MGS omitted post testing you are referring to is largely bent downstream instead of backward. This is a result of the vehicle directly contacting and overriding the post. However, the post was providing lateral resistance (with some lateral deflect too) prior to the vehicle contact. Reducing that post’s lateral strength may lead to increased deflections, rail stresses, and pocketing. As far as the lateral offset of 1.5 ft between the back of the post and the headwall, I do not foresee any problems. The long span system was evaluated with the back of the posts even with the inside face of the headwall and top-mounted systems have been evaluated with a 12-in. offset to the headwall. Thus, your 1.5 ft offset should be crashworthy.

Response

Response
(active)

When a guardrail post is placed longitudinally adjacent to an obstruction, such as a culvert or wingwall, it reduces the amount of soil that would be displaced by post rotation. Thus, the soil resistance is reduced, and stiffness of the system is reduced. The quantification of this force reduction has not been fully studied, which has led MwRSF to recommend the 1 ft offset between an in-ground structure and a guardrail post. We have utilized this recommendation within multiple systems, including the MGS-long span and top-mounted MGS for culverts. Although shorter offset distances may still perform adequately, we will continue to recommend a 1 ft offset until further research is conducted.

Post 13 of the MGS omitted post testing you are referring to is largely bent downstream instead of backward. This is a result of the vehicle directly contacting and overriding the post. However, the post was providing lateral resistance (with some lateral deflect too) prior to the vehicle contact. Reducing that post’s lateral strength may lead to increased deflections, rail stresses, and pocketing.

As far as the lateral offset of 1.5 ft between the back of the post and the headwall, I do not foresee any problems. The long span system was evaluated with the back of the posts even with the inside face of the headwall and top-mounted systems have been evaluated with a 12-in. offset to the headwall. Thus, your 1.5 ft offset should be crashworthy.

Date July 26, 2019
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