Logged in as: Public User

SD Concrete End Block and multiple transitions to High Tension Cable Barrier

Question
State SD
Description Text

See
attachments. I included a .dgn file as the .pdf is not very good.

Since I was instructed that South Dakota MUST utilize a transition from MGS to High
Tension Cable Barrier and in the interim time period when there is NO MASH
alternative for a transition from MGS to High Tension Cable Barrier, I drew a
long transition from the SD concrete bridge end block to Nested Thrie Beam to
MGS to W Beam to High Tension Cable Barrier.

Pleaselook at what I have and provide any recommendations as there is probably
something that could be made better. We need a transition from the SD concrete
end block and this may eliminate one of the 7’ long 6”x8” wood posts. We did
not want to use the larger (10”x10”?) wood post transition. We are interested
in only using wood posts and blockouts. We did not want to have a separation
from the MGS to cable barrier as we may have traffic in the opposite direction
at times and want the cable attached to the W Beam. The W Beam to high tension
cable barrier transition is drawn using the Trinity transition from W Beam to 4
Cable High Tension Barrier.

I can’t wait until the pooled fund gets a MGS to Midwest High Tension Cable
Barrier Transition MASH tested and approved. Much will need to be done prior to
the transition testing though.

On another note, I noticed multiple places in the crash test reports that the wood
blockout is dimensioned as 6”x12”x14 ¼” as used in the MGS. I see many states
use 6”x12”x14” blockouts. Is it proper to use the 14 ¼” dimension?


Thanks for looking at the suggested long transition,

Keywords
  • Approach Guardrail Transitions
  • Guardrail
Other Keywords High-Tension Cable
Date June 21, 2016
Attachment InterimBridgeEndTransitiontoCable.pdf
Attachment SDBridgeEndBlock.pdf
Attachment InterimBridgeEndTransitiontoCable.zip


Response
Response

First, I looked at the cable to W-beam transition that you are proposing to use based on the Trinity design. The detail you sent doesn’t show all of the small details of the transition, but I have a few thoughts.

1.       We have had concerns about the currently accepted high-tension cable barrier transition designs and have noted them in the past. There may be some advantage to a high tension cable to W-beam transition in that these cable barriers have a lower deflection than typical cable barriers. This creates a less dramatic stiffness change in the transition and would also reduce the potential for interaction with the terminal end. But we also have concerns.

a.       First, the adequacy of the anchorage of the downstream ends of the cable barriers in these systems is largely unknown. Recall that the testing of the South Dakota cable to W-beam transition displayed two instances where cable anchorage was partially lost and reduced. Expectation for the high tension cable to W-beam transition design would have to be even higher anchor loads, yet these anchorages have not been tested.

b.      Second, the increased tension in the cables could increase the potential for vehicle snag at the point in the transition where the cable and W-beam barriers come together.

c.       Next, cable heights and for the high tension cable barriers are generally significantly higher than the 27” top cable height used on the previously tested transitions.

d.      Finally, the hardware pieces used to transition the cables to the W-beam vary greatly and have not been evaluated.

2.       If you are basing the design off of the Trinity system that has an FHWA approval letter, then I would recommend that you follow as closely to the accepted design as you can. I have attached the letter. Note that the Trinity system uses 10 gauge W-beam and is for a three cable system in the letter. I cannot see the rail type or the anchorages that you are using, but I would follow these guidelines as they are what is in the letter. I don’t have any details or approval for the 4-cable transition.

 

With respect to the AGT details.

1.       What you are showing seems consistent with the MASH upstream stiffness transition we have previously developed and tested with the wood post version of the Iowa AGT. This should be fine. However, if I understand correctly, you wish to omit post 1 due to the shape of your parapet. This may be problematic due to lack of support for the thrie beam and the potential for the omitted post to increase the potential for vehicle snag on the parapet. The original, tested Iowa transition connected the thrie beam to the parapet at approximately 20” from the end of the parapet which left around 11.5” between the center of post no. 1 and the parapet.

2.       Your parapet attaches the thrie beam farther back which does not appear to allow for the placement of post no. 1. As such, there are a couple of potential options:

a.       Move the thrie beam end show connection closer to the end of the parapet to allow for the placement of post no. 1. This would allow for installation of the transition as tested. However, you may have reasons for not doing this currently.

b.      One could omit post no. 1 as you have shown. In order for this to have potential to perform safely, we would recommend that the offset from post no. 2 to the parapet be less than or equal to the 11.5” noted above. This should help reduce snag. There is some concern that omitting post no. 1 will affect the overall deflection and stiffness of the system and may lead to increased snag. However, this can’t be easily investigated without further effort. For this installation, we would also recommend that a wood or steel spacer be placed under the thrie beam in the 18” long, flared back portion of the parapet to provide support for the thrie beam. This should help reduce the snag potential and aid in the transition performing closer to the tested design. The rail can be bolted through the spacer and the parapet to keep things in place.


With  respect to the blockout dimensions. Our details tend to show the 14 ¼” number, but we don’t believe that a 14” blockout height is an issue if you have them in that size.

 

Thanks 

Date July 11, 2016
Attachment SD Transition.jpg


Contact Us:
130 Whittier Research Center
2200 Vine Street
Lincoln, NE 68583-0853
(402) 472-0965
Email: mwrsf@unl.edu
Disclaimer:
The information contained on the Midwest Roadside Safety Facility (MwRSF) website is subject to change without prior notice. The University of Nebraska and the MwRSF is not responsible or liable, directly or indirectly, for any damage or loss caused or alleged to be caused by or in connection with the use or misuse of or reliance upon any such content, goods, or services available on this site.