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We are looking into changing our concrete barrier end section.  Would you be willing to review the new design and provide comments?

Thank you,

I reviewed the attached document and compared it to the MASH-tested buttress we developed here at MwRSF (https://mwrsf.unl.edu/researchhub/files/Report415/TRP-03-369-20.pdf )

There are a few differences:

• Based on the height of the attachment bolt holes, the thrie beam would be installed with a top height of 32”.  (middle bolt hole is at 22” instead of 21”).  Most AGT’s are installed with a nominal guardrail height of 31”. Was this intentional?  Based on the MASH evaluation of NDOT’s 34” tall AGT, 32” should perform fine, I just didn’t know if 32” was IaDOT’s standard. 
• The height of the lower chamfer for the IA design is 16” compared to a height of 14” in the MwRSF design.  Though, if the guardrail is to be installed at 32”, then the height of the lower taper should be 15” nominally. 
• The height at the upstream end of the IA design is 32 7/8” compared to a height of 32” in the MwRSF design.  If the thrie beam is at a height of 32”, then this is a non-issue.  If the guardrail is at 31”, this extra inch of concrete above the thrie beam may create some vehicle snag. 
• The shape transition from vertical to a single slope includes a 6.5” increase in lateral width over a 48” longitudinal distance.  MwRSF has recommended a maximum slope of 10:1 for lateral shape transitions to limit vehicle loading and instability issues. A 10:1 slope would increase the longitudinal distance of the shape transition to 65”.   Note, MwRSF’s 10:1 recommendation is based off LS-DYNA simulation and has been implemented in many other barrier transitions over the last decade.  TTI’s shape transition project successfully MASH tested a 15:1 lateral slope, but they did not crash test a steeper slope. 
• The 10.5” width of the IaDOT design is narrower  than the 12” width used in the MwRSF design.  That may not be an issue if the reinforcement was designed to handle MASH impact loads (70 kips for MASH TL-3).   
• Related to the above comment and focusing on the vertical anchorage/attachment steel bars between the parapet and the pavement:  The MwRSF-tested design used #4 bars @ 6” and included 1.5” of additional width (designed for MASH TL-4, 80 kips), while IaDOT design uses a #5 bar at 12”.  I am guessing this anchorage is carrier over from the bridge rail, which has 6.5” of additional base width.  I didn’t calculate any strengths, but if the width is decreased by 38%, I would think the spacing of the bars needs to be reduced. 
• Also related to the above reinforcement/strength comments, there is no vertical anchorage steel on the front side of the parapet over the first 20” at the upstream end.  There is vertical steel on the back side, but none on the front side to resist overturning moment.  This significantly weakens the upstream end of the parapet. 

Thank you again.

Thank you for looking at this.  I will review your comments and see what can be done to address your comment.  See below

_________________________________________________________________________________________________________

I reviewed the attached document and compared it to the MASH-tested buttress we developed here at MwRSF (https://mwrsf.unl.edu/researchhub/files/Report415/TRP-03-369-20.pdf )

There are a few differences:

• Based on the height of the attachment bolt holes, the thrie beam would be installed with a top height of 32”.  (middle bolt hole is at 22” instead of 21”).  Most AGT’s are installed with a nominal guardrail height of 31”. Was this intentional?  Based on the MASH evaluation of NDOT’s 34” tall AGT, 32” should perform fine, I just didn’t know if 32” was IaDOT’s standard. Iowa has mounted the transition at 32” for several years.  This is to address future 1.5” overlays.
• The height of the lower chamfer for the IA design is 16” compared to a height of 14” in the MwRSF design.  Though, if the guardrail is to be installed at 32”, then the height of the lower taper should be 15” nominally. ok
• The height at the upstream end of the IA design is 32 7/8” compared to a height of 32” in the MwRSF design.  If the thrie beam is at a height of 32”, then this is a non-issue.  If the guardrail is at 31”, this extra inch of concrete above the thrie beam may create some vehicle snag. The intent is for the top of rail to be at 32”.
• The shape transition from vertical to a single slope includes a 6.5” increase in lateral width over a 48” longitudinal distance.  MwRSF has recommended a maximum slope of 10:1 for lateral shape transitions to limit vehicle loading and instability issues. A 10:1 slope would increase the longitudinal distance of the shape transition to 65”.   Note, MwRSF’s 10:1 recommendation is based off LS-DYNA simulation and has been implemented in many other barrier transitions over the last decade.  TTI’s shape transition project successfully MASH tested a 15:1 lateral slope, but they did not crash test a steeper slope. ok
• The 10.5” width of the IaDOT design is narrower  than the 12” width used in the MwRSF design.  That may not be an issue if the reinforcement was designed to handle MASH impact loads (70 kips for MASH TL-3).   I will look into this.
• Related to the above comment and focusing on the vertical anchorage/attachment steel bars between the parapet and the pavement:  The MwRSF-tested design used #4 bars @ 6” and included 1.5” of additional width (designed for MASH TL-4, 80 kips), while IaDOT design uses a #5 bar at 12”.  I am guessing this anchorage is carrier over from the bridge rail, which has 6.5” of additional base width.  I didn’t calculate any strengths, but if the width is decreased by 38%, I would think the spacing of the bars needs to be reduced. ok
• Also related to the above reinforcement/strength comments, there is no vertical anchorage steel on the front side of the parapet over the first 20” at the upstream end.  There is vertical steel on the back side, but none on the front side to resist overturning moment.  This significantly weakens the upstream end of the parapet. I agree, I thought of this, it was a space issue.  If other comments are addressed, it may provide more space for this vertical bar on the front side.

Thank you again.