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Low Tension Cable Slopes

Question
State NE
Description Text

Low Tension Cable Guardrail

We are implementing the 30" high low tension cable guardrail with 27" & 24" lower cables.

The proposed standard installation for given slopes:

 

- S3X5.7 " 5'3" with 4' post spacing at 4' from a slope less than 1V:1.5H " tested to 25 degrees 62 mph by MwRSF.

- S3X5.7 " 5'3" with 8' post spacing at 3' from a slope 1V:1.5H to 1V:2H " transition between practice & tested.

- S3X5.7 " 5'3" with 16' post spacing at 2' from a 1V:2H or flatter standard practice & TL-2 tested in the 60's & 70's (possibly with 27" high top cable).

 

  The explanation behind this implementation is the failed test which used 16' spacing 1' from a  1V:1.5H, where the front tire did not contact the soil before flipping, the tire is in contact with the soil on a flatter slope.

 

  My thought is that the tested; flat concrete to 1V:1.5H slope, is different from the real world 2% lane, 4% shoulder, 1V:10H surfacing under guardrail to a 1V:2H slope, that this is enough difference to keep the front tire on the slope in testing conditions & that this force on the vehicle will keep it from flipping over the top of the cable.

 

Can the 2' from a 1V:2H be modeled with the slope rounding which Ken Opiela spoke of at TRB?

 

Keywords
  • Guardrail
Other Keywords Cable Barrier
Date February 28, 2012


Response
Response

 

Over the past couple of years, the placement of low-tension cable barriers near steep slopes has garnered much discussion. Some of this discussion has been archived in the following Pooled Fund Consulting Summaries:

 

http://mwrsf-qa.unl.edu/view.php?id=233

http://mwrsf-qa.unl.edu/view.php?id=234

http://mwrsf-qa.unl.edu/view.php?id=328

 

The recent questions found below pertain to the potential transitioning of the crashworthy low-tension, cable barrier system to variations in the configuration when placed near alternatives fill slopes.

 

Previously, a low-tension, cable barrier system with three cables at 30, 27, and 24 in. provided acceptable TL-3 safety performance under NCHRP Report No. 350 when placed 4 ft away from the slope break point of a 1½:1 fill slope. In this test, maximum dynamic barrier deflection reached approximately 125 in. (10.4 ft) with a vehicle traveling on level terrain prior to impacting the barrier system. Using this information, the 2000P vehicle extended approximately 77 in. (6.4 ft) beyond the slope break point. For a 77 in. (6.4 ft) lateral offset on a 1½:1 fill slope, the corresponding vertical drop from a horizontal datum is 51.3 in. (4.3 ft).

 

Previously, TTI researchers crash tested a low-tension, 3-cable barrier on level terrain with a 16-ft post spacing at TL-3 of NCHRP Report No. 350. This 2000P crash test resulted in a 3.4 m (134 in.) maximum dynamic barrier deflection, which was slightly larger than the barrier deflection observed by MwRSF in the ditch test.

 

If a 16-ft post-spacing design were placed 4-ft away from the slope break point of a 1½:1 fill slope, the 2000P vehicle would likely extend 86 in. (7.2 ft) over the slope terrain. For a 86 in. (7.2 ft) lateral offset on a 1½:1 fill slope, the corresponding vertical drop from a horizontal datum is 57.3 in. (4.8 ft). If a 16-ft post-spacing design were placed 4-ft away from the slope break point of a 2:1 fill slope, the 2000P vehicle would likely extend 86 in. (7.2 ft) over the slope terrain. For a 86 in. (7.2 ft) lateral offset on a 2:1 fill slope, the corresponding vertical drop from a horizontal datum is 43 in. (3.6 ft). >From this simple investigation and comparison, one may infer that the 16-ft post spacing design at a 4-ft lateral offset may result in slightly reduced safety performance as compared to the 4-ft post spacing design at a 4-ft lateral offset for 1½:1 fill slopes. For 2:1 fill slopes, one may infer that similar performance would likely be obtained. However, another factor may influence whether or not the 16-ft post-spacing design would adequately perform when placed at a 4-ft lateral offset. Recall that the 4-ft post spacing design has many more cable hook bolts and posts for which to support and contain the three cables. As a result, the 16-ft post spacing design may allow for increased cable drop and vehicle roll motion with reduced cable support from decreased hook bolts and posts. Thus, it still may be difficult to infer whether comparable vehicle and barrier performance would be obtained with the 16-ft post spacing design.

 

If a 16-ft post-spacing design were placed 3-ft away from the slope break point of a 1½:1 fill slope, the 2000P vehicle would likely extend 98 in. (8.2 ft) over the slope terrain. For a 98 in. (8.2 ft) lateral offset on a 1½:1 fill slope, the corresponding vertical drop from a horizontal datum is 65.3 in. (5.4 ft). If a 16-ft post-spacing design were placed 3-ft away from the slope break point of a 2:1 fill slope, the 2000P vehicle would likely extend 98 in. (8.2 ft) over the slope terrain. For a 98 in. (8.2 ft) lateral offset on a 2:1 fill slope, the corresponding vertical drop from a horizontal datum is 49 in. (4.1 ft). >From this simple investigation and comparison, one may infer that the 16-ft post spacing design at a 3-ft lateral offset may result in moderately reduced safety performance as compared to the 4-ft post spacing design at a 3-ft lateral offset for 1½:1 fill slopes. For 2:1 fill slopes, one may infer that similar performance would likely be obtained. Once again, differences in system design may influence whether the 16-ft post-spacing configuration adequately performs when placed at a 3-ft lateral offset. As noted above, the 4-ft post spacing design has many more cable hook bolts and posts for which to support and contain the three cables. Thus, the 16-ft post spacing design may allow for increased cable drop and vehicle roll motion, thus making it difficult for the 16-ft post spacing design to provide similar vehicle and barrier performance.

 

Similar comparison could be made for a 16-ft post spacing design at a 2-ft lateral offset with either slope condition. However, the results noted above would lead one to believe that further degraded performance may be obtained as compared to the 4-ft and 3-ft lateral offsets. In summary, it is difficult to know to the degree at which barrier performance is potentially degraded with increased post spacing and reduced lateral offset from steep fill slopes.

 

As noted above, the physical crash testing programs were performed with impacting vehicles traveling on level terrain prior to striking the cable barrier systems. You noted that in real-world applications, highways are typically constructed with cross slopes, shoulder slopes, slight grades or rounding near barrier and at slope break point in advance of the steel fill slopes. However, these geometric conditions may not exist at the outside of curves which protect steel slopes. In any event, it was theorized that these deviations from level terrain testing may potentially mitigate some of the degrading effects noted above. As a result, you inquired as to whether transitions to an 8-ft post spacing at a 3-ft lateral offset and then a 16-ft post spacing at a 2-ft lateral offset may be considered in advance of steep fill slopes. Based on our review of test results, simple investigation and comparisons above, and engineering judgment, we feel that it is difficult to support such transitions without additional R&D. This R&D could begin with computer simulation with LS-DYNA or other vehicle dynamics simulation codes, but it may eventually require compliance testing to verify that such real-world conditions can mitigate other degrading effects and prevent pickup truck rollovers.

 

Date April 12, 2012


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