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Wildlife Crossing Holes in Barriers

State MO
Description Text

Our environmental specialist that handles threatened and endangered species has proposed a small wildlife crossing for single-slope barrier (See attached Figure 1.jpg). He met with a contractor who ensured him the construction of the barrier as shown is entirely feasible, even for slip forming.

Given the diameter of the opening, I suspect this would neither add a snag point nor in any other way significantly decrease the crashworthiness of the barrier. I'm basing that on of the Roadside Design Guide which states that single cross drainage structures with diameters less than or equal to 36 in. are traversable. Of course that is referring to inslopes and not barriers.

The dimensions shown below were those given to me, but I would also like to know if I could extrapolate your opinion to an 18 in., 21 in., or 24 in. CMP. Any diameter larger than that would begin to conflict with the reinforcing steel in the barrier.

Do you concur with my analysis of this situation?

  • Permanent Concrete Barriers
Other Keywords none
Date April 29, 2011
Attachment Figure 1.jpg


It is well known that the structural capacity of a barrier is controlled by its ability to safely contain and redirect the heavy, taller vehicles which impact at high-speeds. Thus, a structural analysis could be performed to modify the barrier reinforcement in regions where drainage holes are desired. Drainage openings in concrete parapets should be acceptable as long as the hole does not interfere with the structural steel reinforcement and the barrier capacity is not weakened in this region. If the 42-in. tall, single-slope concrete barrier section is weakened with a lateral hole, then the longitudinal and/or vertical steel reinforcement surrounding this area may need to be increased in order to provide equivalent or greater barrier capacity.


In general, it would be my first impression that the size of the half hole (i.e., 15 in. diameter by 7.5 in. tall) through the barrier would not significantly degrade barrier performance as long as the vertical steel anchorage could be placed at its normal locations adjacent to the opening, adequate longitudinal steel is provided, and as long as the small car's front wheel does not negatively interact with the downstream side of the opening. Small cars have wheel center heights ranging from 10 to 12 in. As such, it would be necessary to ensure that the wheel/steel rim does not snag on the downstream side of the hole, cause increased vehicle climb up the barrier, or result in barrier override or vehicle instability.


For rigid, vertical shapes, it would seem reasonable to assume that a hole height less than or equal to 50% of small car wheel center height, or 5 to 6 in., would not post too much concern for small car instability, climb, or snag. For rigid, safety shape parapets, the safety risks would be accentuated due to increased lateral snag distance at the downstream side of hole caused by a sloped, front face on a barrier system. As such, the maximum allowable hole height would likely be lower for safety-shape parapets as compared to vertical parapets. Previously, MwRSF provided guidance as to a 3-in. maximum allowable hole height for safety-shape parapets. Single-slope parapets would likely fall somewhere between the two noted above, or between 3 and 6 in. However, it should be noted that no official safety guidance or criteria exists for configuring the size and height a drainage holes in rigid, concrete barriers.


Safe half-hole heights for single-slope parapets are believed to fall somewhere between 3 to 6 in. using my best engineering judgment. It would seem possible for them to work 7.5 in. as well. However, my concerns for small car rim snag on hole edge, wheel climb on hole, instability go up as we increase hole size. If I were picking a recommended upper limit for the single slope barrier, I would say 5-6 in. for half-hole height.


Based on the enclosed information, I am unable to extrapolate the noted guidance to the larger opening sizes shown in Figure 1.jpg.

Date May 2, 2011

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