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Cable Guardrail Next to Slopes

Question
State WY
Description Text

I know the pooled fund did a crash test and report with low tension cable guardrail placed in four feet in front of a 1V:1 1/2H slope using a four foot post spacing.  Is it possible to correlate your results to a high tension cable guardrail system.  For example, we are using Trinity's TL-3 system with the C-Channel post.  Deflection is reported to be 9 ft. at a standard post spacing of 16'-6".  I was wondering what post spacing would be necessary for it to work similar to MwRSF's test.

I know the reported deflection in the crash test was around 9 feet, but I also remember Dean Sicking saying deflection is greater on a slope than flat ground, therefor presumably the low tension system would have been a bit stiffer than the 9 foot deflection as seen in the slope test.


Our field guys are wanting to use some high tension cable barrier for shoulder applications so we need to develop some criteria.

Keywords
  • Guardrail
Other Keywords none
Date April 15, 2008


Response
Response

I have reviewed your request for guidance on placement of the CASS system adjacent to steep slopes. You stated that you are using the CASS with 16'-6" (5-m) post spacing. Review of the FHWA approval letter for the CASS with that configuration showed that the CASS system had a dynamic deflection of 2.8 m. The CASS uses a top cable height of 750 mm and a C-section post embedded 813-mm into the soil. The system is a high tension system.

 

The cable adjacent to steep slope research at MwRSF was conducted on a low-tension cable system with a top cable height of 813 mm. The system used S3x5.7 posts with 813-mm embedment and a 203-mm wide soil plate. Post spacing for the system was 1.22-m (4'), and the system was offset 1.22-m from the slope breakpoint. Dynamic deflection for this system was approximately 3.16 m.

 

Comparison of the two systems shows that the MwRSF system had a top cable height approximately 63-mm (2.5") higher than that of the CASS and had 1/4 post spacing. While the two systems displayed similar dynamic deflections, the CASS system was tested on a relatively short installation on flat ground, while the MwRSF low-tension system was tested with a 494-ft installation adjacent to a steep slope. Thus, we expect that the deflection for the CASS system would increase if installed under similar conditions.

 

In order for the CASS system to function safely adjacent to a steep slope, the deflection of the system may need to be reduced in order to assume that the the cables effectively interlock with the front and rear corners of the vehicle. Because the CASS has a lower top cable height and we expect the it to have higher deflections in this type of installation, as mentioned above, we would recommend a reduced post spacing for the CASS with a 1.22-m offset from the slope similar to the MwRSF testing. The CASS has been successfully tested with reduced post spacing's of 2-m and 3-m with dynamic deflections of 2.06-m and 2.4-m, respectively. We could recommend that CASS systems adjacent to steep slopes use the 3-m post spacing with a 1.22-m offset from the slope breakpoint. While this recommendation is believed to be conservative, it should account for the effect of the lower cable height on capture and the higher deflections expected for the CASS when installed adjacent to a slope.

Date April 29, 2008


Response
Response

Great analysis! That was the recommendation I was looking for. I would rather be a little on the conservative side without actually crash testing.


One additional question I have. Has there been any evaluation as to what a minimum post spacing would be for the S3x5.7 post (both for cable and other applications)? We use that post for box beam guardrail as well. We have usually regarded 4 ft. as the minimum spacing. Does that sound appropriate? I realize it may depend on the post release mechanism. It appears it worked well on the low tension cable system, but that was the pickup test. I thought I saw one state specify up to a 3 ft. spacing to limit deflections, that seems a little much.

Date April 29, 2008


Response
Response

To my knowledge, there has been no cable testing with post spacing less than 4 ft. I have always recommended this as a minimum spacing to limit the possibility of post interaction. When we tested closely spaced flanged channel u-posts for mail box supports, we saw interaction that produced rollover in a small car. If I recall correctly, this behavior disappeared at a spacing of about 4 ft.


Hence we would recommend a 4 ft minimum spacing.


Any other input is welcome.

Date April 30, 2008


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