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We are developing a detail to allow an opening in TBR for contractor access and I’m struggling to find guidance on what buffer length we should be using upstream of the opening for both the anchored and unanchored conditions. Attached is the current draft version.
|Date||April 8, 2015|
I have reviewed the detail you sent.
Currently, we have recommended that a minimum of 8 barrier segments be used prior to the beginning of length of need or downstream of the end of the length of need for TCB installations. This recommendation is based on testing of these systems typically being performed on 200’ long (16 barriers) systems. Thus, what you have shown in the detail for free-standing TCB is appropriate based on our current knowledge. We do not have a clear definition in the performance of the TCB when impacted near the ends of the system, so we have been conservative.
We currently have a project with NDOR to evaluate the actual beginning and end of LON lengths for the F-shape TCB, but that work is not yet completed.
In the past we have also recommended that openings in runs of TCB be overlapped. I am not sure if that is possible for your purposes, but this option is a little more well defined, as the overlap of the PCB runs creates less uncertainty near the ends of the installations. For overlapping TCBs, we have recommended an overlap of at least 8 or 9 barrier segments for each run - front and back. The gap between both barrier runs could be reduced to 6 to 12 in. or so due to both barrier systems being freestanding, thus reducing the propensity for vehicle snag/pocketing. If limited space exists at the roadside edge for the overlapped option, one may consider the slight flaring of the rearward (shielded) TCB system in order to save space near the shoulder. In your case, a larger gap could be used to facilitate vehicle access. In addition, overlapping of the TCB systems eliminate the need for the sand barrel array.
Some states have asked about using the steel pin tie-down to anchor the segments and shorten the beginning and end of LON, however, we have not investigated that at this time and it may need to be further investigated prior to our full endorsement of it.
For the downstream end of the system, we have developed an end anchor for the TCB system that could be applied. This anchor was tested to MASH TL-3 and allows for the LON to start at the first barrier segment. See report below.
You refer to this below, but I don’t believe that you would need the 50’ denoted in the detail for the anchored system. If the TCB were unanchored, I would recommend sticking with the 100’ shown.
Let me know if you need anything else.
|Date||April 13, 2015|
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