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PCB Overlap Offset

Question
State OH
Description Text ODOT rates the deflection of our unanchored 32" portable concrete barrier at 5.5' from the face of barrier. We know it is not a good idea to install unanchored PCB against rigid concrete Jersey or SS barrier. Our direction has been to leave 3.5' of space between the PCB and the rigid barrier to allow for deflection and to prevent the PCB from rotating. My understanding is, if unanchored PBC is placed against rigid concrete wall it is like pinning only the back side of PCB. What if we install PCB against guardrail? Since guardrail is semi-rigid will it deflect and not allow the PCB to rotate. Another scenario would be when one run of unanchored PCB is installed against another run of unanchored PCB. Let me know if these scenario's have been tested, if not your opinions would be helpful.
Keywords
  • Temporary Barriers
Other Keywords none
Date August 22, 2011


Response
Response

In the past, we have generally recommended using a 2' offset between overlapping barriers when using PCB in front of another barrier system. Historically, we have recommended the overlapping method in situations where TCBs are to be placed in front of a rigid end of a concrete parapet. This recommendation was given prior to the development of several in-line attachments between freestanding and permanent concrete barriers. For the overlapped option, we stated to use 8 barrier sections beyond the end of the permanent barrier with a 2-ft gap between the freestanding and permanent barriers in order to reduce the propensity for vehicle pocketing and snag on the upstream barrier end. For overlapping TCBs, it would seem reasonable to use an overlap of at least 8 or 9 barrier segments for each run " front and back. However, I believe that 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.

 

I don't believe that we have ever given recommendations regarding TCB placed directly in front of guardrail. Based on the rail geometry, the guardrail would act to resist the rotation of the barrier on the sloped face of the section prior to the toe of the barrier butting up against the base of the post. This would suggest that it is allowable to place the PCB directly against the guardrail. However, doing so will stiffen the deflection of the PCB significantly. Thus, if the PCB mounted in from of the guardrail has free-standing PCB on the upstream end, you will need to use an approach stiffness transition to prevent pocketing and instability.
Date August 23, 2011


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