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Have any other states reported any
Have we been getting any negative feedback on the MGS rail
The first involved barrier rail on the Allen 75
Ted Foster noticed that the bolt head just pulled through
So Ted is telling me that once in a while his crews can
Is there a problem state-wide with the MGS rail or the
|Date||January 23, 2017|
|Attachment||ALL-75-122.9 (1) (2).JPG|
|Attachment||ALL-75-122.9 (2) (1).JPG|
I have several thoughts on the material you sent. We are always glad to get feedback from the states when they observe field impacts on barrier systems.
First, when the MGS was originally designed, one of the areas of improvement targeted for the system was facilitating easier rail release. There was concern that the washers used on older guardrail systems and the double ply of the W-beam at the splice/post connection may have been too strong and could potentially prevent rail release. This could have pulled the rail down during impact and compromised vehicle capture. Thus, the design of the MGS looked at reducing the strength of the post to rail connection. In the final design, it was decided that the repositioning of the splices away from the posts was sufficient to produce the desired rail release. Thus, the design of the MGS was intended to provide a tradeoff between previous rail release forces and easy rail release for safety performance purposes. We do not recommend washers on the rail as that has proven to pull down the rail element and compromise capture. I believe that you noted that in your email as well.
In testing and evaluation of the MGS we have observed disengagement of the rail on some of the most sever impacts and some of the special applications due to increase rail tension loads and reserved bending that pries the rail away from the bolt heads. This has not been a safety issue as it does not affect capture or redirection of the vehicle. Examples of the tests we have observed this in include the long span and omitted post systems among others.
In terms of the examples you sent, they appear to be extreme impacts that may have exacerbated the rail release to some extent.
On the MGS median barrier system impact, the truck and trailer likely represented a high impact severity on the rail due to the increased vehicle mass as compared to the full-scale tests. Of course that would depend on speed and angle. Inspection of the photos shows some flattening of the impact side rail which would indicate is was loaded significantly. Additionally, in order to impact in the region of the guardrail shown a relatively short distance from the median bridge rail, the vehicle would have needed impact at a relatively high angle as well. The impact may have been non-tracking as well based on the position of the vehicle.It appears from the photos that the rail ruptured, which again would suggest a high impact severity. Once the impact side rail ruptures, the vehicle would impact the backside rail and the posts. This would tend to displace the rail laterally away from the remaining posts and down towards the ground as the impacted posts rotated. This would knock the backside rail to the ground and push the rail away from the adjacent guardrail posts. The backside rail is not designed to provide capture for that system, so I don’t believe that the outcome is that surprising given the loading pattern that I can observe in the photos.
For the terminal impacts, it appears that the semi-tractor impacted an energy-absorbing end terminal of some kind. A semi-tractor impact again represents a significantly more severe impact that what the MGS was designed to meet. It also appears that the terminal head was crushed and jammed during the impact. As such, there was potential for increased compressive loading and reverse bending the rail that may have led to the increased disengagement from the rail. We have observed terminal testing on the MGS with approved terminal that have not shown this type of rail disengagement under the MASH and 350 impact conditions. Thus, we think that the level of rail disengagement observed may have occurred due to the extreme nature of the impact.
We have not had specific complaints in the past from state DOT’s about large lengths of rail disengagements, but we can monitor for this and try to determine if it is an ongoing issue. Do you know if you observe this in less severe oblique impacts more typical with our TL-3 testing?
Let me know if you want to discuss this further.
|Date||January 23, 2017|
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