A consultant is currently re-evaluating our ground mount sign supports to help us bring our standards into compliance with the latest AASHTO designs standards. One of these post types is our wide flange sign post. As they did their calculations we are running into an issues with sign height limitations within he post capacities. As we talked about options to address this a couple things came up that would affect breakaway performance, if we had latitude to modify these it would significantly improve our situation without introducing new designs and/or hardware…but more than likely it will impact the breakaway performance. I have copied in Tom Lohman, who the lead engineer with Horner and Shifrin on this project so he is in the loop on this discussion.
The first variable which is affecting their calculations is the torque value for the bolts at the slip plate connection of the posts. While we currently have 6 post sizes, it is these breakaway bolts which actually control the post capacity and not the size of the I-Beam, from a structural capacity stand point their calculations indicate we would only need the #2 post with the 5/8” bolt and the #4 post with the ¾” bolt. In the table below we have the current torque requirements for the two sizes of bolts. This torque has a significant limiting impact on the capacity of the post in their structural calculations, if it could be increased the post capacity dramatically increase….but then we are likely negatively impacting the breakaway performance.
Our question, are there any resources, tables, etc. which could be referenced to evaluate higher torque values with respect to breakaway performance or can higher torque values be evaluated in our simulation software or would an actual crash test evaluation be required to truly evaluate any change in these figures? I am not sure crash testing is within our scope, but if it were, would it need to be a full crash test with a sign and a vehicle or some lesser evaluation with maybe just a post and a bogie test vehicle? This is one of the easiest things in the calculations to change to improve sign capacity, but I expect evaluating the impact to safety will be one of the more difficult options.
The second question is related to the weight of the post being impacted. In our current standards we state #3, 4, 5 and 6 posts must be at least 7 feet apart so a vehicle will not impact two post with a weight exceeding 18lbs/ft. Is this weight per foot of post still current or has this standard changed with MASH? We want to update this if it is out of date, but in the off chance this weight per foot has been increased (not likely) we would have more options to install signs on three posts to carry a larger load.
Any guidance you could offer would be greatly appreciated.
We’ll need to look into this and get back to you. One item I wanted to check on, though – you mention:
… so a vehicle will not impact two post with a weight exceeding 18lbs/ft. Is this weight per foot of post still current or has this standard changed with MASH?
Can you clarify the source of the weight per foot limit? Are you referring to a section of MASH, or some other NCHRP or AASHTO document?
We assumed this was something that would be in MASH, this has been in our standards for over 30 years and we assumed it was related to crash standards. Our standards in the Wide Flange Post detail calls out that posts which weighed more than 18 lb/ft have to be spaced at least 7 feet apart. We always assumed this was related to crash standards to help prevent a vehicle from hitting two heavy posts at the same time and impart too much energy into the vehicle.
This was our assumption for the reasoning behind the standard, but we really do not know the origins of that requirement, the reason for our question. Any guidance on the subject would be appreciated.
When you questioned this requirement, I assumed it may not be a crash standard so I looked in the Roadside Design Guide and found it, this statement is from the 2002 edition, but it is the same in the 2011 edition as well. So I think we have the answer we need on the post weight and spacing issue.
While we answered the post spacing question, I was wondering if you had any advice to offer regarding the breakaway torque requirements for the breakaway bolts? As our consultant does their review of our I-Beam posts this is one of the major controlling factors on load capacity beside bolt size.
With regards to the required torque for slip base breakaway releases, there are a few factors to consider.
First, slip bases are designed with a certain amount of preload such that the base effectively anchors the support. However, the torque and preload can affect the release of the slip base. Slip bases function during impact through overcoming the forces that are required to push to bolts out horizontally from the base and release the bolts. This is largely a function of the preload on the slip base bolts and plates. The friction is a function of the preload in the bolts. Because of this, there is a functional limit that one would want to keep the preload below to ensure that the base can effectively release when impacted. That preload is generally set based on whatever torque the preload was set at when the slip base design was evaluated through physical testing.
I should note that while this is what we would generally recommend, the torque to preload relationship is not very exact. There is a lot of variability between the applied torque and the exact preload value that one gets. However, it is the best tool we have.
The other factor that comes into play is the moment that must be resisted by the slip base itself. Thus, the preload in the base bolts of the slip base must be sufficient that the moment in the base from the sign wind loads do not exceed the preload and loosen or unload the slip base.
For your sign design, one would generally want to review the applied torque and associated preload that the slip base was designed for, and then check if that is sufficient to withstand the design wind loads for the sign.