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MoDOT Encapsulated Median Barrier

Question
State MO
Description Text
Here is a copy of our 32” Type B unreinforced median barrier curb (Standard Plan 617.10) rehabilitated by encapsulating in new reinforced concrete shell. The shell is shown not embedded but I think it is supposed. New and old concrete is 4,000 psi.

I talked to Scott about this a couple of days ago and he turned me onto using shear-fracture and flexural resistance calculation along failure lines; however, the possibilities are endless and how do you know the ‘right’ strength.

For comparison with a crash-tested median barrier (which is why we asked you for the report), I was just going to show that flexural resistance about centroid is increased.

However, by observation one can see that the new barrier composite is stronger than original.
Keywords
  • Permanent Concrete Barriers
Other Keywords none
Date February 26, 2015


Response
Response

Previously, Bob and I noted that the prior testing on the base system in the 1976/1977 SwRI Research Report. I believe that he or I  may have sent the reference and page range to you a week or so ago.

 

With regard to capacity, I definitely agree that the capacity of the composite system is much, much stronger than the original system. You are right that the number of ways to make comparisons may seem endless. The capacity of the new composite system could be estimated and compared a few different ways.

 

For shear capacity of the beam/wall section, one could look at punching shear at top of parapet where a triangular wedge section is potentially removed by punching backward. Maybe we assume a 6, 8, and 10 ft long segments that propagates down to midpoint, say 2/3 of height from top. Of course, coming up with an actual critical length may take some significant effort so making assumptions may be best for now. Two concrete surfaces have shear capacity, while the vertical steel pins at 24 in. centers have shear resistance on the front and back sides of parapet and over the number placed within an assumed length (6 ft, 8 ft, 10 ft, etc.). The base configuration has no vertical pins. One might simply estimate the punching shear capacity of the barrier/wall both before and after the retrofit. Some may debate as to whether it is appropriate to consider any longitudinal bars which pass through this assumed failure surfaces and which may resist punching shear. Regardless of an approach, the retrofit option is much stronger than the original configuration.

 

For flexural capacity, one could make a few different comparisons. First, one could simply make a comparison of the moment of inertias of the steel reinforcement layout in each case – one bar in middle of section versus six bars spaced off of vertical midpoint plus one bar on middle of section. Further, one could estimate overall flexural capacity of barrier/wall with one bar and for seven bars.

 

Without vertical reinforcement tied into a foundation, it is nearly impossible to attempt to determine the cantilevered capacity of the section at the base in both scenarios. In other words, there is no vertical steel effectively tieing either section to the ground. Thus, this term or capacity does not factor into a yield line analysis. There is a shear capacity for the barrier at the base, as provided by the original keyway. Finally, one could assume a contact or load length along the wall and also compare shear strength for three surfaces – two vertical and base at keyway. The base strength would be the same for both cases, but the vertical planes would be larger/stronger in the retrofit as compared to base condition.

 

In the end, there is may not be a perfect way to make such comparisons, as you noted below. One just has to select various assumed failure planes/mechanisms and make approximations to compare capacities as best as possible. Let us know if you want additional clarifications or desire for checks on calcs. Scott can further assist is desired.

 

Date March 6, 2015


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