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Cable to W-beam Transition

Question
State IA
Description Text

I would like your opinion regarding a construction issue with Iowa's cable guardrail to w-beam transition (which is now voided).  The standard drawing for this transition (http://www.iowadot.gov/erl/archives/2009/april/RS/content_eng/re84.pdf) is based on the South Dakota design. 

 

"‹Å“Case A' on the drawing allows one of the transition brackets to be placed on the w-beam end of the w-to-thrie transition piece.  As is clear from the drawing, especially in the plan view on sheet 2, this configuration has proven very difficult to construct; the downstream post and blockout interfere substantially with the path of the cables as they travel from the transition bracket to the end anchor.

 

In your opinion, should we allow "‹Å“kinks' in the cables as they travel around the post and blockout?  If not, would we be able to adjust the location of the transition bracket and the end anchor to provide a straight line of travel for the cables?

Keywords
  • Approach Guardrail Transitions
  • Guardrail
Other Keywords none
Date June 29, 2010


Response
Response

We have looked at your details and have a few comments and responses to your questions. We looked at several options for Case A in your details.

 

A total of four solutions were investigated.  The first solution consisted of allowing the wire rope to bend around the post at the midpoint between posts at standard spacing.  However, analysis of the degree of bending of the wire rope around the posts, in combination with concern that the wire ropes will either lose tension during post deflection or be pulled from the terminal, indicates that this alternative is likely not an acceptable solution without crash testing to prove crashworthiness.

 

The second solution proposed by the Iowa DOT was to shift the downstream transition bracket further upstream which would decrease the effective angle to the anchor bracket and allowing the cables to bypass bend locations around the post.  While this design would help alleviate cable interference with the post, it is not known what the effect of shortening the overlapped cable length would have on the design. Changing the position of the transition bracket would changed the angle of the cables to the ground anchor. One of the concerns in the original design of this system was the potential for snag of the vehicle in the area where the cables angle down towards the ground anchor. Thus, I am leery of changing the transitioning of the cables or the location of the anchorage without further analysis.

 

An additional option proposed was to drill a hole in the blockout of the post which interfered with the cables.  This design option has several advantages, in that the positioning of the bracket and the W-beam do not change relative to each other, minimizing the potential for snagging, pocketing, and loss of cable tension.  However, the required size of the hole required to pass the cable through the blockout would be very large, which could lead to lower compressive strength of the blockout, greater propensity for twisting, and the cables would be subject to post rotation or fracture in the soil.  Damage to the post at the point of cable routing could interfere with the cable's tension and could potentially cause catastrophic release of the cable from the end terminal.  Furthermore, the additional labor required for field drilling holes in the blockout and the potential to cause unexpected damage are high; therefore this is not an optimal solution.

 

The final design option is to add an additional 12-6" of guardrail between the the flared crashworthy end terminal and the approach transition. By introducing an additional span of guardrail, transition bracket interference issues, cable tension concerns, and field operations are maintained.  In addition, this options allows the cable transition to be completed before the approach transition to the bridge rail begins. Though this may be the be slightly more expensive option, it is nonetheless the most crashworthy from a design standpoint, and will most likely result in acceptable performance of the transition design.

 

An additional issue which was brought to my attention was the standard plan design of the cable anchor.  This cable anchor, a 4" x 4" anchor angle, does not have sufficient strength to maintain the loads from the cables during a crash event.  Cable loads on anchors can, in TL-3 crash conditions on low-tension cable guardrail systems, rise as high as 60 kips with peak loads from a single cable as high as 25 kips.  It is conceivable that higher-energy impacts may cause tension increases in excess of this number.  The angle bracket anchor shown in your detail will most likely not be sufficient to maintain these loads without a large degree of deformation, which may compromise the performance of the anchorage. It is recommended that Iowa adopt the design tested in the test report prepared for the South Dakota Department of Transportation entitled, "Crash Testing of South Dakota's Cable Guardrail to W-beam Transition", by Faller, Sicking, Rohde, Holloway, Keller, and Reid, MwRSF Research Report No. TRP-03-80-98.  Anchor bracket design details tested in the report are attached. This design uses a gusseted anchor plate that is significantly stronger.

Date July 8, 2010
Attachment Recommended Cable Anchor Design.pdf
Attachment Cable to W-beam Transition Page_2.jpg
Attachment Cable to W-beam Transition Page_1.jpg


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