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6" vs 8" unreinforced concrete median barriers

Question
State MN
Description Text As you probably know we are trying to increase the thickness of our median concrete barriers from 6" to 8" and add min. amount of reinforcement to match AASHTO's design. However, we are facing some resistance from the designers, they are telling me that we do not have any problem with the non-reinforced 6" F-shape design now why change? On the other hand our bridge department did some strength requirement calculations and they are telling me that our 6" non-reinforced barrier does not have the required strength under NCHRP 350 (~ 54-60 Kips). I appreciate your comments and guidance in this matter.
Keywords
  • Permanent Concrete Barriers
Other Keywords none
Date June 24, 2004


Response
Response With regard to the issue of concrete median barriers, I am aware that much discussion has taken place within MnDOT over the last 6 to 12 months. In my opinion, non-reinforced concrete barriers are not my first choice, in that over time, significant cracking can occur due to environmental changes. If impacts later occur in regions where these cracks or even large gaps are located, then vehicle penetrations or snag may occur. Yes, it is possible to design thick, non-reinforced barriers to resist the vehicle loads imparted into them when the barriers have continuity. However, when discontinuities exist, it is uncertain whether vehicle containment and a safe redirection will result.

In my opinion, the issue of barrier width is mostly a non-issue as long as adequate steel reinforcement is provided. A steel reinforced barrier (half-section bridge railing), configured with a 6-in. width, has been shown to meet both the TL-3 and TL-4 safety performance requirements of NCHRP 350. Thus, as long as adequate longitudinal and vertical steel is provided, then narrower widths can be designed to meet the safety standards.

Over the last several years, MwRSF researchers have conducted significant research on an 8-in. wide (top end), steel reinforced, F-shape concrete barrier that could be used in both temporary, free-standing applications as well as in two alternative tied-down applications. This research was all conducted according to the TL-3 safety standards. From this effort, the importance of the steel reinforcement and 8-in. barrier width was shown on more than one occasion, actually 3 to 4 occasions. Had this barrier design been narrower, say 6-in. at the top, and had the same quantity of steel but just placed inward due to the 2-in. decrease in barrier width, we likely would not have safely redirected the pickup truck in multiple crash tests.

In summary, several concrete barrier designs can be optimized to use either the 6 or 8-in. top widths as long as adequate steel reinforcement is provided. However, non-reinforced concrete barriers should not be used unless it can be shown that they will perform in an acceptable manner after being exposed to significant environmental conditions and contain the cracks or gaps within them at the time of testing (i.e., loss of rail continuity).

If you have any questions regarding the information contained herein, please feel free to contact me at your earliest convenience.
Date June 28, 2004


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