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Our district staff would like to install a modified version of the South Dakota snow gate (See attached "SD Snow Gate.pdf" and "TRP-03-44-94.pdf"). However, they are proposing some modifications to the base:

1.         They would like to use a screw-in Chance Lighting Foundation (See attached "11242ng4 Model (1).pdf").

2.         They would like to use the Transpo Pole-Safe " Model 4100 frangible bolts. (See attached "Pole-Safe 4100 Details.pdf" and "Pole-Safe Skirt Details.pdf").

They have some other thoughts regarding signing and lighting attachments as well as an alternate method of storing the gate in its road open condition. I will pass along information for those items as I receive it, but they would like to begin installing the foundation next Monday.

Would you please review the attached "SD_SnowGate_Mod.pdf" and verify that these modifications will not affect the crashworthiness of the system? Please comment also on the grading as shown. (Sorry for the short notice).

Let me know if you have any questions.

We received the below response from Transpo. Do you have any comments on the information they've provided?

I have reviewed your proposed changes to the snow gate design. The changes you propose create two main issues with respect to the performance of the snow gate.

1. Coupler equivalency
   - You are proposing to change from the Pole-Safer 201 couplers that were used in the tested design to the Pole-Safe 4100 design. In order to replace the couplers from the original tested design, we need to verify that the Pole-Safe 4100 couplers perform in a similar manner and would break away without increasing the occupant risk limits of the design. I have looked up the approval letters for both coupler designs, but they do not contain sufficient information to evaluate the couplers. As such, we would recommend that you contact Transpo for guidance on the replacement of the coupler. If they can show that the coupler design would work similar to the Pole-Safe 201 for a short, heavy break away device, then use of the Pole-Safe 4100 would be acceptable.

It appears that Transpo has stated that the Polesafe 4100 and 201 models have equivalent breakaway loads. Thus, it would be acceptable to use the 4100 model in your modified design. The only way to further evaluate this would be to compare the velocity change for tests of short, heavy poles from testing of both the 201 and 4100 couplers.

2. Foundation design " Your new design also replaces the original 2' diameter x 3' deep concrete footing with a 8.6" diameter x 5' long Chance Lighting foundation. The concern here is that the small diameter foundation may rotate in the soil prior to developing sufficient loads to cause the couplers to break. The original foundation was significantly wider and thus would be expected to develop the soil forces more quickly. There is some concern that movement of the foundation would affect the coupler performance and potentially impede the proper breakaway mechanism. In order to alleviate these concerns, we would recommend a foundation analysis (LPILE or some other method) to determine the relative stiffness of the two foundation alternatives. This should provide enough confidence to use the new foundation if the results are similar.

The use of the 10:1 grading shown is not a cause for serious concern in our eyes.