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|Description Text||Please review questions in the PDF. I have a region that does not like having water flow over the top of the box culvert. They are wondering if it is possible to do something like what I have attached.
What I have is a situation where there is a significant elevation difference between the shoulder by the beam guard long span and where the culvert headwall is located. The region staff wants to keep the headwall to prevent water from flowing over the head wall (I don’t know why.).
The truck or car can only drop so much during an impact into a long span. For the sake of illustration, I’ll pick an imaginary number of 2’ of vehicle drop during a crash test. If the culverts headwall is 3’ below the shoulder surface, the vehicle cannot interact with the headwall during an impact. It should be O.K. for the head wall to remain in place.
The question becomes, What is the truck or car’s drop during an impact into a long-span. I pick the imaginary number of 2’ out of a hat for the sake of illustration. But you smart people have the slow motion cameras, computer models, and other sorts of wizardry and probably could actually come up with a more accurate, scientific guess than I could.
|Date||December 23, 2015|
If I understand correctly, you are wanting to install the long span at an offset from the head wall larger than what we tested, but with a slope and drop to the head wall behind it.
I don't see this as much of an issue as the long span was tested with a vertical drop off approximately 34" behind the face of the rail. I believe that we require the long span to have 24" behind the posts prior to a 2:1 slope. Assuming that you have continue that grading behind the unsupported span prior to the slope and head wall, I don't see this installation as more severe than what was tested. I believe that the slope to the head wall would only provide additional vertical support to the vehicle and improve stability.
If you plan to start the slope closer to the rail face we may need to discuss it further.
|Date||January 4, 2016|
2’ of grading behind the post is not a problem. But where the span is plan to be the 2:1 starts close to the back of rail.
|Date||January 5, 2016|
I don’t believe we can recommend the system as shown due to concerns with the drop of the slope relative to the rail position in the unsupported span.
In the long span testing, the posts were aligned with the back of the post flush with the face of the headwall. This placed the face of the guardrail in the unsupported span area approximately 34” in front of the headwall. Thus, during an impact in the unsupported span, the ground was supporting the wheels of the vehicle for approximately 3’ prior to the vehicle dropping behind the headwall.
In the installation you have shown below, the slope begins directly behind the face of the guardrail. This would allow wheel drop much earlier than the tested system. Beginning of the wheel drop sooner may allow the vehicle to fall farther as it extends over the culvert which could compromise vehicle capture, vehicle stability, and cause issues with the vehicle climbing back over the headwall as the vehicle is redirected.
As such, we cannot recommend the installation shown below.
|Date||January 6, 2016|
130 Whittier Research Center
2200 Vine Street
Lincoln, NE 68583-0853
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