View Q&A

LRFD Requirements for Guardrail Posts in MSE Walls

Question
State	WY
Description Text	Section 11, page 77 of the AASHTO LRFD requires a post embedment of five feet into an MSE wall, and placed back three feet from the edge. Was this configuration crash tested or was there any research to establish these requirements? Does this adversely affect the performance of the guardrail? Is this something that needs to be added to the Roadside Design Guide? Also, I'm not sure how you can drive the posts without cutting some of the geo-textile reinforcement? Any light you can shed on this subject would be helpful.

Question

State

Description Text

Section 11, page 77 of the AASHTO LRFD requires a post embedment of five feet into an MSE wall, and placed back three feet from the edge. Was this configuration crash tested or was there any research to establish these requirements? Does this adversely affect the performance of the guardrail? Is this something that needs to be added to the Roadside Design Guide? Also, I'm not sure how you can drive the posts without cutting some of the geo-textile reinforcement? Any light you can shed on this subject would be helpful.

System Performance Evaluation

System Types
Road Closure Gates
Thrie Beam Guardrails

Applications

System Features

Date February 13, 2014
Previous Views (94) Favorites (0)

Attachment LRFD Requirements_11_77.pdf

Response
Response (active)	Let me start by recommending you review a recent report that MwRSF put together for FHWA - Central Federal Lands Highway Division. it is report number TRP-03-235-11 and can be obtained from the Research Hub link on MwRSF's website. As part of this project, 2 MASH TL-3 full-scale crash tests were conducted on steel post MGS systems placed on top of MSE walls. The barrier system performed successfully, and the wall sustained no visible damage or displacements. Final recommendations for the minimum lateral offset were to center the post 4'-3" from the face of the MSE wall. This offset (detailed in the report) was selected in part to avoid the larger rock of the MSE wall face in which the posts can not easily be driven through. The large rock typically extends 3' laterally from the face of the MSE wall. I am unsure were these recommendations listed in the AASHTO LRFD specs originated from as they preceded the MwRSF study noted above as well as some work that TTI has been doing with concrete barriers placed on MSE walls. However, the 3' minimum lateral offset may have been specified to avoid the larger rock facing of the MSE wall (since the rock typically extends 3' from the face) - similar to the MwRSF recommendations. The 5' embedment depth specified by AASHTO is interesting. I do not know why this would be specified as the embedment depth should be a function of the barrier post strength. Testing with standard steel guardrail posts with 40" of embedment proved adequate in the MwRSF project for both redirection strength and damage prevention for the MSE wall. Further, the posts bent over in the full-scale crash tests showing the surrounding soil was strong enough to support the posts. Extending the posts deeper into the ground will not have any significant effects on the system or the MSE wall. To answer your question about interference with wall reinforcement, AASHTO section 11.10.10.4 addresses the issue of reinforcement obstructions and provides a few design methods that can be implemented. I would recommend following those. Any additional reinforcement that may need to be added for the wall should be minimal as the post obstructions have a small footprint and would only be placed every 6 ft.

Response

Response
(active)

Let me start by recommending you review a recent report that MwRSF put together for FHWA - Central Federal Lands Highway Division. it is report number TRP-03-235-11 and can be obtained from the Research Hub link on MwRSF's website. As part of this project, 2 MASH TL-3 full-scale crash tests were conducted on steel post MGS systems placed on top of MSE walls. The barrier system performed successfully, and the wall sustained no visible damage or displacements. Final recommendations for the minimum lateral offset were to center the post 4'-3" from the face of the MSE wall. This offset (detailed in the report) was selected in part to avoid the larger rock of the MSE wall face in which the posts can not easily be driven through. The large rock typically extends 3' laterally from the face of the MSE wall.

I am unsure were these recommendations listed in the AASHTO LRFD specs originated from as they preceded the MwRSF study noted above as well as some work that TTI has been doing with concrete barriers placed on MSE walls. However, the 3' minimum lateral offset may have been specified to avoid the larger rock facing of the MSE wall (since the rock typically extends 3' from the face) - similar to the MwRSF recommendations. The 5' embedment depth specified by AASHTO is interesting. I do not know why this would be specified as the embedment depth should be a function of the barrier post strength. Testing with standard steel guardrail posts with 40" of embedment proved adequate in the MwRSF project for both redirection strength and damage prevention for the MSE wall. Further, the posts bent over in the full-scale crash tests showing the surrounding soil was strong enough to support the posts. Extending the posts deeper into the ground will not have any significant effects on the system or the MSE wall.

To answer your question about interference with wall reinforcement, AASHTO section 11.10.10.4 addresses the issue of reinforcement obstructions and provides a few design methods that can be implemented. I would recommend following those. Any additional reinforcement that may need to be added for the wall should be minimal as the post obstructions have a small footprint and would only be placed every 6 ft.

Date February 24, 2014
Previous Views (94) Favorites (0)