View Q&A

We have information about how to build line posts for MGS when the steep slope begins at the back of post.  However, a designer in one of our Districts is dealing with a location where the slope continues in a similar manner all the way to a bridge.  Are there any recommendations about adding to the length of posts for the transition from MGS to a bridge parapet?

Our particular transition to the parapet, given adequate support behind the posts is given in our Standard 630001.

http://www.dot.il.gov/desenv/hwystds/rev211/Web%20PDFs/

At this time, we do not have any available design information for modifying the length of the transition posts when located on a steep slope near the bridge end. However, MwRSF does have a research project with the Wisconsin DOT to provide recommendations for addressing various transition issues. One of the noted issues will be to provide design guidance for situations when steep slopes are found behind the posts. It is expected that this effort will be initiated within 2 months.

The Wisconsin-funded study included some recommendations for posts installed near the SBP of a 2:1 and 4:1 slope using timber posts. The report is located here:

https://mwrsf.unl.edu/reportResult.php?reportId=5

An asphalt overlay was included over the soil behind the post and near the SBP. The asphalt overlay measured 2 in. thick. Component testing of the post, soil, and overlay were observed to increase average forces compared to the same posts without an asphalt overlay. A conclusion in the report stated: "...In fact, the 2-in. (51-mm) thick layer of asphalt increased the maximum resistive force and average resistive force at 10 in. (254 mm) of deflection by approximately 60 percent and 57 percent, respectively." Although there are opportunities for slopes located in conjunction with AGTs, this caution was noted: "For wood posts positioned on a 2H:1V fill slope, a 2-in. (51-mm) thick asphalt confinement was not shown to negatively affect post behavior. Thus, wood transition posts subjected to such confinements would only have modest increased risk for post fracture. However, the forces observed in test nos. WIA-1 and WIA-2 did not reach the design force used for Wisconsin approach guardrail transition systems. Therefore, any wood transition post positioned on a 2H:1V fill slope and surrounded with 2-in. (51-mm) thick asphalt pavement should be supplemented with an additional steel post as per the design recommendations..."