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I don’t have all the details about what the actual issue is, but I received the attached question/proposal from a contractor on a design-build project that is currently underway. Apparently the contractor has an issue slip forming KDOT’s typical F-shape barrier through a section where we have stepped pavement (pavement height difference between north and south bound). They are proposing to use a single sloped barrier, similar to MoDOT standard (included in the attached email). I believe Kansas would like to go to a single sloped barrier but we aren’t there yet. Since I don’t have a lot of experience with single sloped barrier and even less with the stepped pavement situation, I would like to get your thoughts.

My biggest question is about the step. Is it better to have the step and the high side doweled in or have a symmetrical barrier with the one side buried?

I appreciate your time and insight.

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I attached the submittal from the design-build team along with the MoDOT detail. We are asking if the proposed single slope stepped barrier is acceptable to use for an approximately 1000’ section of median barrier for the US-69 project? We are hoping to get an answer back to this by next Wednesday 2/1.

Thanks,

There has not been a lot of guidance ore research on grade separated median barriers. There is actually a research project proposed in this years pooled fund to look at this issue in more detail.

In terms your F-shape barrier, I spoke with Tom Rhoads several years ago regarding this barrier. At the time he was asking about MASH compliance of the system based on yield line analysis. Unfortunately, yield line analysis does not provide for calculation of barriers with very little reinforcement like the barrier KDOT uses with only two no. 4 bars or wire rope through the center of the barrier. Thus, we noted that we could not demonstrate MASH compliance through analysis at that time. We did note that there was still potential for the barrier to meet MASH as Ohio had a similar issue and has full-scale crash tested an unreinforced, 42” tall single slope barrier to MASH TL-3.

We have typically recommended that median barriers use reinforcing steel, both longitudinal bars and stirrups, in order to limit barrier damage and prevent cracking and degradation over time due to thermal stresses. Lots of states have moved to reinforced single slope barriers including Iowa and Wisconsin.

For grade separated solutions, the use of reinforcing steel becomes even more important, as the barrier may have increased moment applied during impacts on the upper section of the barrier that must be transferred down to the lower section.

We do not have any calculations or are aware of the design capacity of the MoDOT system. We would agree with the idea that this system should provide equal or greater capacity to the current KDOT standard as it has additional width/section, transverse steel ties, more longitudinal bars, and dowels that are 2” longer and spaced at 2-ft (KS spaces dowels at 4 ft). This would all tend to increase the capacity of the MoDOT design.

Similarly, modifying the current KDOT design to a single-slope as shown would be believed to be as strong or stronger than the current KDOT F-shape barrier due to the increased section of the single slope. What we cannot confirm as easily is whether or not the original F-shape or the proposed single slope meet MASH. I did not see what test level you are looking for. Obviously the concerns for barrier capacity are less if it is TL-3. Higher test levels may be of more concern.

We might suggest that the lower vertical bar be extended upward well past the upper grade and a longitudinal bar be added there like the MODOT system.

We would typically like to see more reinforcing steel in the design. For examples, FLDOT has previously provided details of their grade separated median barrier to us as shown below.

TTI also did a simulation effort with no testing to look at a grade separated median barrier as well. This may serve as an example. - https://www.roadsidepooledfund.org/wp-content/uploads/2011/05/405160-33_35_Version-5.pdf

Let me know if that helps your decision making. We can discuss further if you would like.