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W Beam Cable transition

Question
State IA
Description Text

We have a request from the district to narrow the w beam
terminal section on a cable to w beam transition.  The project falls
within a lager water shed and they are trying to stay within the existing
footprint.  We have our standard BA-206 (4 foot offset) designated for
this transition.  They are asking if they can use the BA-205 (2 foot
offset).  They are mainly trying to minimize the grading foot print. 
I looked at the two terminals we have for the BA-206 and they are the FLEAT-MGS
and the SRT-31.  According to the manual I found the Fleat can go down to
a 2’6” offset and the SRT can go down to 3 foot offset.    To me
it looked like we could easily go down to 3 feet of offset but did not know how
that would affect the interaction of the cable and w beam. 
    If they really wanted to go down we could use the Fleat
only and go go down to 2’6” but again did not kown how that would affect the
interaction between the two.  Any input would be greatly
appreciated. 






http://www.iowadot.gov/design/SRP/IndividualStandards/eba206.pdf



 

Keywords
  • Guardrail
Other Keywords Cable Barrier
Date February 12, 2015


Response
Response

You are correct that we have tested two versions of the cable to W-beam transition system. The first test was conducted using the standard low-tension cable system transitioning to G4(1S) guardrail. The original cable to W-bean transition was tested with both a BCT end terminal and a  the second test used a FLEAT end terminal.   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.

 

http://mwrsf.unl.edu/researchhub/files/Report164/TRP-03-80-98.pdf

 

In both tested systems, the terminals were offset 4’ laterally from the cable barrier. For both systems the testing of the 2000P vehicle showed the potential for vehicle instability when the system was impacted such that the vehicle contacted the terminal end and the cable system simultaneously. Thus, there is concern that moving the terminal ends closer to the cable barrier may increase the vehicle instability further. In addition, the vehicle could deflect the cable system ahead of the terminal end allow the vehicle to get behind the end of the terminal at the shorter offset. This may further degrade the system performance. As such, we have allowed the end terminal systems to be extended to larger offsets in the past, but we have not allowed shorter offsets as we believe that they would potentially adversely affect the transition.

 

Let me know if you need anything else.

 

Thanks 

Date February 13, 2015


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