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MGS Steep Slope

Question
State IA
Description Text

Due to right-of-way restrictions, we have a steep slope situation on a bridge replacement project where we will need to install steel beam guardrail. Design speed of the roadway is 60 mph and the traffic volumes are in the 1500 vpd range.

In your opinion, would it be satisfactory to install the MGS on a 10:1 pad such that a 1.5:1 foreslope begins 24 inches behind the face of rail? Also, could this same cross section be used throughout our approach guardrail transition (see attached BA-201 drawing)?


Thanks
Keywords
  • Guardrail
Other Keywords none
Date January 11, 2012
Attachment eba201.pdf


Response
Response

See my comments below!


Due to right-of-way restrictions, we have a steep slope situation on a bridge replacement project where we will need to install steel beam guardrail. Design speed of the roadway is 60 mph and the traffic volumes are in the 1500 vpd range.

**For steep slope hazards, MwRSF previously developed two W-beam guardrail systems " one for metric-height rail and one for the MGS. In both scenarios, the steel post was centered at the SBP.

In your opinion, would it be satisfactory to install the MGS on a 10:1 pad such that a 1.5:1 foreslope begins 24 inches behind the face of rail?

**As noted above, the MGS option is being considered where a 10:1 roadside slope is followed by a steep 1.5:1 fill slope. For this configuration, the MGS could be installed with as little as 2¾ in. of mostly level terrain behind the steel post in advance of the SBP. This scenario would likely provide similar post-soil behavior to that of a steel post installed at SBP of 2:1 fill slope. Thus, it would be recommended to utilize the MGS System for 2:1 Fill Slopes for your guardrail system used in the application presented above.

Also, could this same cross section be used throughout our approach guardrail transition (see attached BA-201 drawing)?

**At this time, we do not have a design solution for approach guardrail transitions placed with the steel/wood posts located at or near steep slopes. For these scenarios, our first choice would be to modify the fill behind the posts in order to provide 24 in. of generally flat terrain behind the posts. If that cannot be provided, then we would need to investigate whether another surrogate post (larger and/or longer) could provide comparable post-soil behavior to the original transition post founded in level terrain. Although the later could be done, it would certainly require additional analysis and possibly some additional bogie tests. Please let us know whether you desire MwRSF to further explore the second option. Thanks!

P.S. " On another note, the CAD details provided in the attached pdf file depict the use of the wedged-shape drainage curb below the thrie beam rail. In the original testing program, the curb ended at the midpoint of the symmetrical W-beam to thrie beam transition section and started the taper to the ground at the thrie beam end of the section. All crash testing was performed near the bridge end, and no testing was performed near the start of the W-beam to thrie beam transition section. Later, the MGS stiffness transition was developed for use in combination to a thrie beam transition with half-post spacing but without a curb. This stiffness transition was adapted to other common AGTs. Your detail depicts the curb to end at the start of the asymmetrical transition section. Due to concerns for the small car to wedge under the rail, the concrete curb should preferably end at the thrie beam end.



Date January 11, 2012


Response
Response

See my comments in blue below!

Due to right-of-way restrictions, we have a steep slope situation on a bridge replacement project where we will need to install steel beam guardrail.  Design speed of the roadway is 60 mph and the traffic volumes are in the 1500 vpd range.

**For steep slope hazards, MwRSF previously developed two W-beam guardrail systems " one for metric-height rail and one for the MGS. In both scenarios, the steel post was centered at the SBP.

****OK

 

In your opinion, would it be satisfactory to install the MGS on a 10:1 pad such that a 1.5:1 foreslope begins 24 inches behind the face of rail?

**As noted above, the MGS option is being considered where a 10:1 roadside slope is followed by a steep 1.5:1 fill slope. For this configuration, the MGS could be installed with as little as 2¾ in. of mostly level terrain behind the steel post in advance of the SBP. This scenario would likely provide similar post-soil behavior to that of a steel post installed at SBP of 2:1 fill slope. Thus, it would be recommended to utilize the MGS System for 2:1 Fill Slopes for your guardrail system used in the application presented above.

****OK

 

Also, could this same cross section be used throughout our approach guardrail transition (see attached BA-201 drawing)?

**At this time, we do not have a design solution for approach guardrail transitions placed with the steel/wood posts located at or near steep slopes. For these scenarios, our first choice would be to modify the fill behind the posts in order to provide 24 in. of generally flat terrain behind the posts. If that cannot be provided, then we would need to investigate whether another surrogate post (larger and/or longer) could provide comparable post-soil behavior to the original transition post founded in level terrain. Although the later could be done, it would certainly require additional analysis and possibly some additional bogie tests. Please let us know whether you desire MwRSF to further explore the second option. Thanks!

****Our ROW is so restricted on this project that we are unable to provide 24 inches of flat terrain behind the posts throughout the AGT.  Having you conduct some additional analysis and/or testing would be desirable, but I'm doubtful our project timeline would allow for that.  Would you be able to provide a ballpark estimate of how much time such an analysis might take?  

P.S. " On another note, the CAD details provided in the attached pdf file depict the use of the wedged-shape drainage curb below the thrie beam rail. In the original testing program, the curb ended at the midpoint of the symmetrical W-beam to thrie beam transition section and started the taper to the ground at the thrie beam end of the section. All crash testing was performed near the bridge end, and no testing was performed near the start of the W-beam to thrie beam transition section. Later, the MGS stiffness transition was developed for use in combination to a thrie beam transition with half-post spacing but without a curb. This stiffness transition was adapted to other common AGTs. Your detail depicts the curb to end at the start of the asymmetrical transition section. Due to concerns for the small car to wedge under the rail, the concrete curb should preferably end at the thrie beam end.

****Thank you for pointing this out.  We should modify our standard to show the curb ending under the thrie beam.  Note, however, that extending the curb through and beyond the asymmetrical transition section is unavoidable in some cases due to drainage requirements.  When a curb is required in this region, it has been a long-standing practice of ours to limit the height of the curb to 4 inches.  Obviously, the slope at the bottom of the rail is more pronounced on the new asymmetrical transition compared to the old symmetrical one, but I'm not aware of any issues coming up regarding the wedging of small cars under the old transition (or the new one, for that matter). Of course, it might still be an issue. Maybe this is something we could investigate further (with pooled fund money perhaps). Seems like it would fit in well with a proposal to study the necessity of the 4-inch curb at the guardrail/bridge rail interface...


 

Date January 12, 2012


Response
Response

See comment's below in green.

Due to right-of-way restrictions, we have a steep slope situation on a bridge replacement project where we will need to install steel beam guardrail.  Design speed of the roadway is 60 mph and the traffic volumes are in the 1500 vpd range.

**For steep slope hazards, MwRSF previously developed two W-beam guardrail systems " one for metric-height rail and one for the MGS. In both scenarios, the steel post was centered at the SBP.

****OK

In your opinion, would it be satisfactory to install the MGS on a 10:1 pad such that a 1.5:1 foreslope begins 24 inches behind the face of rail?

**As noted above, the MGS option is being considered where a 10:1 roadside slope is followed by a steep 1.5:1 fill slope. For this configuration, the MGS could be installed with as little as 2¾ in. of mostly level terrain behind the steel post in advance of the SBP. This scenario would likely provide similar post-soil behavior to that of a steel post installed at SBP of 2:1 fill slope. Thus, it would be recommended to utilize the MGS System for 2:1 Fill Slopes for your guardrail system used in the application presented above.

****OK

 

Also, could this same cross section be used throughout our approach guardrail transition (see attached BA-201 drawing)?

**At this time, we do not have a design solution for approach guardrail transitions placed with the steel/wood posts located at or near steep slopes. For these scenarios, our first choice would be to modify the fill behind the posts in order to provide 24 in. of generally flat terrain behind the posts. If that cannot be provided, then we would need to investigate whether another surrogate post (larger and/or longer) could provide comparable post-soil behavior to the original transition post founded in level terrain. Although the later could be done, it would certainly require additional analysis and possibly some additional bogie tests. Please let us know whether you desire MwRSF to further explore the second option. Thanks!

****Our ROW is so restricted on this project that we are unable to provide 24 inches of flat terrain behind the posts throughout the AGT.  Having you conduct some additional analysis and/or testing would be desirable, but I'm doubtful our project timeline would allow for that.  Would you be able to provide a ballpark estimate of how much time such an analysis might take? 

**I suspect 1-2 days would be adequate to acquire and review prior bogie testing data and perform simple hand calculations. However, if we cannot find sufficient information and results from prior bogie tests, then a bogie testing program would be needed. At this point, staff could look into this issue later this month.

 

P.S. " On another note, the CAD details provided in the attached pdf file depict the use of the wedged-shape drainage curb below the thrie beam rail. In the original testing program, the curb ended at the midpoint of the symmetrical W-beam to thrie beam transition section and started the taper to the ground at the thrie beam end of the section. All crash testing was performed near the bridge end, and no testing was performed near the start of the W-beam to thrie beam transition section. Later, the MGS stiffness transition was developed for use in combination to a thrie beam transition with half-post spacing but without a curb. This stiffness transition was adapted to other common AGTs. Your detail depicts the curb to end at the start of the asymmetrical transition section. Due to concerns for the small car to wedge under the rail, the concrete curb should preferably end at the thrie beam end.

****Thank you for pointing this out.  We should modify our standard to show the curb ending under the thrie beam.  Note, however, that extending the curb through and beyond the asymmetrical transition section is unavoidable in some cases due to drainage requirements.  When a curb is required in this region, it has been a long-standing practice of ours to limit the height of the curb to 4 inches.  Obviously, the slope at the bottom of the rail is more pronounced on the new asymmetrical transition compared to the old symmetrical one, but I'm not aware of any issues coming up regarding the wedging of small cars under the old transition (or the new one, for that matter). Of course, it might still be an issue. Maybe this is something we could investigate further (with pooled fund money perhaps). Seems like it would fit in well with a proposal to study the necessity of the 4-inch curb at the guardrail/bridge rail interface...

**Small car wedging started to occur with 1100C vehicle on stiffness transition project. However, the test results were satisfactory. Now, if we add a lower curb, it is our opinion that performance could be potentially degraded as wedging and snag could be accentuated. Also, the 2270P vehicle would contact MGS slightly higher while reaching stiffer region. We believe 2 tests would be needed to evaluate curb in advance of asymmetrical part. Also, it would be beneficial to test Iowa transition near bridge end but without curb. Future research would be extremely helpful to investigate whether or not these potential concerns are real.

Date January 13, 2012


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