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MnDOT has used cantilevered, swing-away mailbox support designs since at least the 1990s. About a decade ago we started working with TTI to develop a MASH version. That project was completed three years ago and the resulting design is officially our standard mailbox support. However, we are encountering challenges rolling it out, ranging from the large amount of roadside space it takes up to the high cost of fabrication due to all the pipe bends and welds. There are situations where it just seems overengineered for the setting, such as low-to-intermediate-speed urban streets, but we have a policy of using MASH hardware on all state highways, regardless of design speed or NHS route status.

We are considering piloting the use of NDOT’s PSST mailbox support you developed as an alternative option. We like that it is consistent with our sign support hardware. I have a few questions related to part availability and consistency with our sign support practices:

1. Could we substitute this 48”, two-layer PSST base footing with welded soil fins instead of the single-layer 36” 12-gauge base post? We are also considering a base option without soil fins, which would match our three-wall base, but with the innermost tube omitted.


2. Could we use a 3/8” stainless steel straight through-bolt instead of the corner bolt? Note that instead of bolting the riser post to the base, our sign base designs use a through bolt as a post stop. The top of the base is flush with ground level and the post stop is 4-1/2” blow ground level. The riser post inserts about 4” into the base and rests on top of the post stop bolt. The idea is that the base can remain in place to be reused any time the riser post gets hit. There is no bolt connection; the riser post can be lifted in or out of the base. Does this pose a significant risk of adversely affecting the crash response of the PSST mailbox support system?


3. Could you explain why the single-mailbox mounting bracket you developed for NDOT bolts through holes drilled in the bottom of the mailbox instead of bolting to the prefabricated holes in the sides?
   
   
   
   1. If the goal is to accommodate a range of mailbox sizes, why not use extension brackets?
   
   
   2. The MASH swing-away mailbox support that TTI developed for us is designed to use an extension bracket (TxDOT/DHT #148938) in combination with the mailbox bracket (TxDOT/DHT #148939) to accommodate a variety of mailbox sizes without needing to drill holes. See sheet 3 of our standard plate. It would be nice if we could unify the brackets across the two systems, so I am trying to see if the two could be harmonized somehow to gain economies of scale for fabrication.
   
   
   
   


4. Are any commercially available mounting brackets compatible with the system for a single mailbox?
   
   
   
   1. I see that NDOT’s Special Plan 3072 includes a TAPCO mounting system for a single mailbox. It looks like it is the same connection hardware as was used in the surrogate test for the double mailbox support. Do you have any concerns with using this?
   
   
   2. Low priority to address: [Could Shur-Tite’s single mailbox bracket (DHT 161443) be connected, for example, using a short length of 1-3/4” PSST post nested inside and bolted to both the circular pipe part of the bracket and the top of the 2”x2” PSST support post? I’m not sure how snug the connection would be between the square tube inside the round pipe.
   
   
   3. (Thinking through what a homeowner might come up with that we need to be wary of) I am assuming a pine board with two or four L-brackets would have too great a risk of the mailbox detaching and impacting the windshield, right? Our old swing-away design used a pine board strapped to the cantilevered horizontal pipe arm, so unfortunately metal mailbox brackets are not yet widely sold in MN hardware stores.]
   
   
   
   

Finally, I am wondering if you can comment in any greater detail than in the test report on the question of whether further testing beyond the surrogate testing conducted in Phase I is needed to deem this a MASH-tested system. My understanding is that states have flexibility in evaluation, but I am wondering if you have any particular concerns that you would want us to be aware of that might warrant full MASH testing.

I will try and address your comments and questions as best as possible and then we can do follow questions as needed.

In terms of using the NDOT system that we worked on, I have to note that the system evaluation was not completed during that effort. We ran preliminary testing with a non-compliant vehicle. The results were positive, but I wanted to be clear that we did not complete the test matrix and that the tests conducted were not technically MASH compliant. There were also additional configurations of the mailbox support that may have been worthwhile to investigate, but the scope of the project did not allow for that.

1. For the base post, your 4’ long base tube is the same size and gauge as the tested mailbox tube base. We would not anticipate that increasing the embedment and the addition of fins to the structure as shown in your detail should not adversely affect performance.
2. Changing from a corner bolt to a vertical support bolt at the base of the post may affect system performance. However, it is difficult to assess if it would be adverse. In the two impacts performed in the NDOT study, the corner bolt was loaded sufficiently to fracture prior to releasing the upper section of the support in the single mailbox impact. In the impact with the dual mailbox support, we observed that the upper section post fractured at the corner bolt location. This would suggest that there is significant loading of that bolt and removal of it may affect the release or performance of the mailbox support. One of the more critical factors in this type of system is if and when the mailbox can impact the windshield. Thus, removal of the corner bolt or changing that constraint may affect that aspect of the impact performance. Previous mailbox testing suggests that ductile supports that do not release quickly were more likely to meet crash test criteria.
3. The single mail box support bracket as designed with mounting holes for both bolting through the bottom or the sides of the mailbox. For the single mailbox test, the mailbox used in the test was selected to reflect the largest weight that NDOT wished to allow for mailboxes used with the mailbox support. The mailbox selected only had base bolts and did not have flange mounted option. Thus, the mounting bolts location was dictated by the mailbox chosen. The dual mailbox mount was tested with lighter mailboxes that had a side mounting option.
   1. We did not use extension brackets based on input from NDOT. They wanted a simple, non-proprietary, single-plate mount.
   2. The mailbox mount design selected by NDOT was also influenced by a desire for the mounting bracket to have sufficient strength to retain the mailbox as much as possible to the support during impact. Previous testing had shown that mailbox detachment often led to windshield and occupant compartment penetration. Heavier mailboxes require a more robust mount to accomplish that. As such, a thin plate mount with extensions as you have shown may work with lighter mailboxes, but for mailboxes over 10 lbs like we tested in the single mailbox test, the mounting plate may not be sufficient to retain the mailbox and mitigate windshield contact.
4. We planned to do testing of the Tapco single mail box mount in the project, but we did not get to that point. We noted this in page 92 of the report. As such, the performance of that mailbox mount is unknown for single mailboxes. However, the dual Tapco mount worked acceptably with the 4.6 lb mailboxes in our testing. This would suggest that the single Tapco mount may be acceptable when used with mailboxes lighter or equal to 4.6 lbs. Other proprietary mounted would have similar concerns, so limiting there use to lighter mailboxes would be preferable. We would not recommend the use of wooden brackets based on the concerns for mailbox release.

In terms of the MASH evaluation, the tests we ran were not technically compliant. However, test 3-61 is the most critical test and it is unlikely a slightly newer year small car model would have made a difference in the test results. The biggest hurdles that remained for evaluation were.

1. We did not evaluate all of the potential critical impact angles. This has never been fully investigated in MASH mailbox testing, and evaluation at different impact orientations may alter the results to some degree.
2. As noted above, we did not get a chance to evaluate the Tapco single mount.
3. We had noted that we did not get a chance to evaluate the non-proprietary single mailbox mount with a heavy mailbox with side mount bolts.

Thanks!