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Approach Transition Posts Obstructed by Inlet

State NE
Description Text

What options to move posts do we have in this location?



On the #1095Y Peru Spur project we have a conflict with a flume inlet and the new guard rail installation. I’m attaching a plan with an approx. area where the flume inlet is going to go in relation to guard rail post installation. I spoke with Curt M. & Tyler Chicoine regarding this issue yesterday. I wanted to run this issue by you to see what resolution would be availabble to make this work and still be compliant with the guard rail installation?

  • Approach Guardrail Transitions
Other Keywords none
Date October 15, 2015
Attachment #1095X-Flume Inlet - Guard Rail Conflict.pdf


We looked over the detail you sent. There are not a lot of good options with this setup, but we can give our best modification with the caveat that this has not been tested and represents only what we believe you can do to best fit the situation you have. If this installation has not been put in, the best option would be to shift the inlet to the region with 6’-3” post spacing where the inlet can be installed without concerns for the barrier performance.


The concern with the inlet as shown is that the obstruction of the two posts will change the AGT stiffness transition considerably and would likely lead to pocketing and poor safety performance. The best alternative installation we can consider is to offset the two obstructed posts longitudinally such that both posts are at least 6” from the sides of the inlet. This should allow for placement of the posts within 1’ or so of their intended position. This will likely have the least drastic effect on barrier performance of any alternative we could devise. Relocation of these posts will require field fabrication and spray galvanizing of new holes. We would recommend that we make sure that the new holes land outside of the rail splice overlap.


We would also recommend adding an additional post on the upstream end of the transition. During the design of the AGT, we developed two designs for the transition. The design you have shown is the design we tested, Design K, which was more aggressive. We also had a Design L for the transition that had one additional post at ½ post spacing on the upstream end of the transition. This improved performance of the transition in our simulation models, but we chose the more aggressive version for testing. Because we are modifying the post spacing in this transition, we would recommend adding that additional post back into the system to make the design slightly more conservative. Additionally, the added post may stiffen the upstream end of the transition somewhat and help mitigate the effects of the increased post spacing near the inlet.

So, the options are as follows.

1.       The best option would be to move the inlet to a less critical location.

2.       A second option would be the modified post spacing and the addition of an additional upstream post as noted above. Again, this is not a tested or evaluated modification, but it represents our best current alternative for an inlet in that location. There are some concerns that the offsetting of the posts longitudinally may affect the stiffness of that region of the transition which may degrade system performance. However, the degree of the effect and the overall performance of the system with the modification is not something we can gauge without further study.



Date October 19, 2015
Attachment Trans l and k.jpg


The inlet is built, the contractor is building the guardrail.

If we extend the thrie beam or nested thrie beam past the inlet … What post spacing would be needed to allow this inlet to stay in place?


Would using w8x15 posts thru this area help or spacing/ gap at the inlet?
Date October 20, 2015


If the inlet is in place, then the best option is the one noted below. This was to offset the two obstructed post so that they land just to each side of the inlet and add the additional post upstream from the Design L option.


Using W6x15 posts or extending the nested rail would likely stiffen the downstream end of the transition and increase the effect of the offset posts. Thus, it would not be preferred over the option below.



Date October 21, 2015

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