Logged in as: Public User

Low Tension Cable Barrier Adjacent to 2:1 Slope

Question
State NE
Description Text

I have a question about 16' spacing. See from email exchange.  From below "¢â‚¬Å“ it was stated -"However, I know there are states that would install with 16' spacing if the cable was 2' from a 2:1 slope as you show below and I don't believe that is a major concern."

 

I prefer to use the 16' spacing 2' from a 2:1.

 

Would you confirm that this is the current practice and reference any testing to support this?

 

The following email correspondence summarizes the discussion:

 

Roadside Cable Guardrail:

We are updating our cable plans to include the in-line anchorage with 3 cables.

What is the current guidance about slopes behind the cable guardrail?

We have many installations of the cable guardrail on the shoulder of our highway 2' from a 2:1 slope

I have considered using 4 cable as tested for the median 14" to 34", what are the pros / cons of this.

 

MGS 31":

What guidance can you give for implementation of 31" including the Bridge Approach Section?

 

****************************************************

 

Cable barrier " Are you using the generic (low tension) cable barrier where you install an anchor every 2000'?  Most states have changed over to require the high tension systems that don't require the intermediate anchors.

 

In general, placing the cable 2' in front of a 2H:1V slope is considered acceptable.  I know Brifen tested a system at the edge of a vertical drop and it worked and the folks at MwRSF tested a system 4' in front of a 1.5H:1V slope with 4' post spacing.

http://trid.trb.org/view.aspx?type=MO&id=855946

http://www.brifenusa.com/

 

 

I like the systems that are now being tested on 4H:1V slopes.  The MwRSF is testing a generic system and several manufacturers have tested their systems.  In general, these systems have 4 cables that cover a wider range of heights to help catch the smaller vehicle that might try to go under as well as the larger vehicle that might try to go over.  Having a system that was tested on a 4:1 slope gives a better tolerance to the slope issues that we have seen.  The approach being taken at MwRSF is to test the barrier at the worst location and then it should work anywhere in the median.  There are some debates on this but we hope to reach agreement on the number of tests for median testing so that everyone is approaching it consistently (this will be discussed in May).

 

The biggest con of the 4 cable systems tested on 4:1 slopes is the additional cost but I think it will be worth it.

 

31" Guardrail

I believe that states should switch over to the 31" guardrail for new installations. It doesn't cost any more (both Washington and Illinois found no additional cost) and provides much better performance in a number or areas.  In Washington, we adopted it because it allowed us to reduce the height as a result of overlays, and still have a crashworthy system (standard guardrail at 28" height passes MASH testing).

 

For transitions to bridges there are a couple options.  My preference is to use a "stacked" W-Beam.  You maintain the 31" height of the rail but add a w-beam rub rail.  The stacked W Beam was tested under NCHRP 350 for a 28" guardrail and when WSDOT was developing the plans, Dick Powers with FHWA HQ confirmed that we didn't need to retest with a 31" height. Attached is a link to the WSDOT standard plan

http://www.wsdot.wa.gov/publications/fulltext/Standards/english/PDF/c25.18-01_e.pdf

The other option is to use a thrie beam transition that was developed by MwRSF.

http://www.wsdot.wa.gov/publications/fulltext/Standards/english/PDF/c25.20-04_e.pdf

 I attached a couple of photos of these transitions.

 

 *************************************************************

Low tension cable:

I like the low tension cable because of its safety record & for the anchorage in the sandy soils we have in Nebraska.

I agree that the low tension (generic) system is a very good system.  Most states have gone to the high tension to reduce deflection distance, reduce maintenance, and reduce the number or anchors needed. To me the biggest of these was the maintenance because we often had to convince the maintenance folks in order to get it installed.

 

We have implemented this 3 cable low tension system cable w/ heights @ 30" 27" & 24".

Most Low tension systems I am aware of have the bottom cable at 21".  Some states have installed it with the top cable at 33" and with 6" spacing between the cables. For medians, lower cables are being used to address the underride issue. I am not aware of a crash tested system with the lower cable at 24".

 

How close can this be placed to a 2H:1V? And with what post spacing?

The testing of 4' post spacing @ 1' from a 1.5H:1V failure, and 4' post spacing @ 4' from a 1.5H:1V pass has us considering 4' post spacing at 4' from a 2H:1V " this is impossible in some locations with 2H:1V at the edge of shoulder.

I am not sure I have an absolute answer for you on this so I will give you some opinion.

I believe the slope behind the cable is much less of a concern than the slope in front of the cable. The critical concern for cable is having the bumper of the vehicle engage at least one cable.  

I have seen tests of a high tension system in front of a vertical drop-off.  While the tires went over the edge, the cables engaged the body and brought it back.

I am aware that MwRSF tested a system in front of the 1.5:1 slope with ¼ post spacing.  However, I know there are states that would install with 16' spacing if the cable was 2' from a 2:1 slope as you show below and I don't believe that is a major concern.

Our past implementation (See sketch.jpg);

 

This sketch with 16' post spacing has seemed to work very well.

 

Now we are updating our plans to the inline steel I beam end treatment & I'm wondering " is this the best system?

Is this still the generic system but with a proprietary end treatment like what is used on the high tension systems?

 

I want to implement the 4-cable 14" to 34" " the system MwRSF tested & met 350 in the median 4:1 down to 4:1 up section.

This system will improve catching the variety of bumper heights.

Would this be acceptable to implement this system for roadside use?

I think the systems that are being tested on 4:1 slopes would be excellent for roadside use and may change the way we treat slopes and roadside objects.  I was thinking the MwRSF system was higher than 34" and I know they are looking at lowering the bottom cable.  I think the considerations on the roadside make it easier since you wouldn't have a back slope where the vehicle might bottom out.

Let me know if I created new questions or not.

 

******************************************

MwRSF said they are talking 13 inches for the bottom cable and 45 inches for the top cable for a high-tension four-cable system. Their research is showing that early cable-release from the post is critical to prevent the cables from causing serious damage to impacting vehicles. 

 

I suggest you contact UNL/MWRSF for the latest scoop on their recommendations.

 

 

Keywords
  • Guardrail
Other Keywords none
Date April 26, 2011
Attachment sketch.jpg


Response
Response

Based on the low-tension cable barrier research I conducted, I had no instances of testing in the last 30 years of a cable barrier 2 ft from a 2:1 slope.
There was sedan testing conducted in 1965 by New York where a low-tension cable barrier was placed 18" from the break point of a 2:1 slope; however, this was never tested with a truck and the truck performance on this configuration would be necessary for approval according to 350, much less MASH.


Based on my research I would be unable to recommend that configuration.
Unless some research which I have not seen turns up somewhere, this would be considered untested.

Date May 2, 2011


Response
Response We were unable to find any prior pickup truck research/testing demonstrating that the 3-cable, low-tension barrier with 16-ft post spacing is crashworthy when installed 2 ft from the slope break point of a 2:1 fill slope. Please let us know if you locate the supporting research so that we can add it to our literature review.
Date May 2, 2011


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.