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MGS to low tension cable guardrail transition

Question
State NE
Description Text

Can I use the steel pile anchor as used in the low-tension
cable barrier testing for the cable to MGS transition?

Test from reports TRP-03-155-05 & TRP-03-192-08

Or, should this be the concrete anchor block as used in the transition testing?

If we can use the steel I beam anchorge, where should it be placed?

Should I use the anchor in-line (under the w-beam) or should we place it a couple feet off set behind the w-beam like the transition testing used the concrete anchor?


Keywords
  • Guardrail
Other Keywords Cable Barrier
Date March 29, 2012


Response
Response

I looked into your questions regarding adapting the low-tension cable barrier to W-beam transition over to the MGS and the type of anchorage to use. Looking at the details of the original cable to W-beam transition several issues arose in addition to the cable end anchorage you noted below.

1.       The cable heights for the original system used a 27" top cable height with 3" cable spacing. This cable height and spacing correlated well with the W-beam barrier height used in the design and allowed the top cable to be run along the top of the W-beam and the bottom two cables to be run along the bottom of the W-beam as they were transitioned from one system to another. Connection of this same cable system to the MGS poses a greater challenge as the cable heights will have to be modified more to be moved behind the barrier using the existing design. The change in cable heights to match the MGS height could cause potential concerns for vehicles becoming snagged in the cable transition and may require changes to the cable bracket design.

2.       The increased height of the cables in the transition as they meet and transition behind the w-beam will likely affect anchorage performance. The increased height on the cable anchorage will increase the vertical loading on the bracket which may act to pullout the anchorage, especially the driven pile anchor you referenced in your email. In addition, the change in cable anchor may require revision of the cable anchor bracket for attachment of the cables as well.

3.       The driven pile anchorage you refer to below is capable of anchoring a cable barrier system. However, impacts near the end of the system would increase the loading of the anchor. As this anchor is designed to deflect in the soil as it develops load, the driven pile anchor may allow increased cable deflection as compare to the large concrete anchor used in the testing. This may be an issue that would need to be further investigated.

4.       The original concrete anchor used in the cable to W-beam transition was allowed for use on sloped terrain behind the system. The driven pile anchorage would need to be re-evaluated if it was to be installed on slopes.

5.       The original cable to W-bean transition was tested with both a BCT end terminal and a FLEAT end terminal. The BCT end terminal has not been tested with 31" high guardrail and would not be recommended for this application.

Based on these factors, we believe that further study is required to allow the cable to w-beam transition to be adapted for use with the MGS. It is also possible that the required changes to the design to adapt the transition to the MGS would require full-scale testing.

 

Thanks

 

Date March 30, 2012


Response
Response

Clarification: I am attaching 30" low tension cable to 31" MGS.

I see benefits to an increase in height of this system that should improve the system; as with the cable height raised from 27" to 30" & W-beam raised from 27 5/8" up to 31".  

Please help me decide whether to use Concrete anchor as tested offset behind the w-beam or Steel.

Date March 30, 2012


Response
Response

I agree that there is much less concern for cable transitioning to the MGS when the cable height is 30".

 

However, the concern still exists that impacts near the end of the system would increase the loading of the anchor. As this anchor is designed to deflect in the soil as it develops load, the driven pile anchor may allow increased cable deflection as compare to the large concrete anchor used in the testing. As such, we would recommend that the tested concrete anchorage be used due to its larger size and increased capacity. .

 

We would also recommend that the anchorage be offset in the same manner as the test. The offset helps prevent  the vehicle becoming wedged under the cables at the point where they attach to the ground anchor.

 

Thanks 

Date March 30, 2012


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