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Type 2 Downstream End Terminal - Illinois DOT

Question
State IL
Description Text  I received the following message from a colleague in my department

"<<0593_001.pdf>> Have you ever placed 2 Type 2's at the end of SPBGR Type D? See the attachment. There is a ramp merging into the mainline which is one way, so the Type D is on the departing end on both sides. Could I use this or use something like a CAT 350, even though it would never be hit head on."
I'm not finding a record of sending this to you before, as I'd promised my colleague in our District 2.


The Type 2 we refer to is our downstream anchor terminal, and we're wondering about applying it on the downstream end of double face guardrail.
It's our Standard 631011-06. http://www.dot.il.gov/desenv/hwystds/rmpdf211.html


We'd welcome any opinion or observations on using this for anchoring the downstream end of guardrails. Per one of our earlier discussions with you or Karla, we do plan to correct the length of the soil tube to 6' from the 7' shown.
Keywords
  • End Treatments & Crash Cushions
Other Keywords none
Date July 13, 2010
Attachment 0593_001.pdf


Response
Response

I am enclosing a pdf file which compares your downstream anchor hardware to that currently used by MwRSF. Within the file, corrections are noted that show which dimensions are actually used within our CAD details. For our general guardrail testing programs, we utilize a standard end anchorage system on both the upstream and downstream ends of our 175-ft long W-beam guardrail installations when terminals are not being evaluated. These anchorages were adapted from the modified BCT, also consider the MGS rail height and cable anchor increased rise, and include a channel strut, two lengthened foundation tubes, and a common cable anchor with bolted attachment plate and bearing plate on each end.

 

In addition, you inquired as to whether the trailing end hardware noted above could be utilized in a double rail or median-type configuration where reverse-direction impacts could not be achieved on the end spoons. In such installations, we believe that this trailing end terminal hardware in combination with the MGS would likely provide sufficient capacity to successfully contain and redirect most passenger vehicles impacting at high speeds and angles. Unfortunately, no full-scale crash testing programs have yet been performed on most trailing end terminal systems.

 

Currently, there have been concerns with many different non-crash tested trailing end terminals that the small car vehicles could become snagged or wedged under the anchor cable on the downstream end if the end post is not fractured or does not release in a timely manner.

 

Two prior crashworthy box beam guardrail end terminals have utilized a post breaker system to ensure post fracture and cable release prior to snagging the small car vehicle. However, the current generation of energy-absorbing and flared W-beam guardrail end terminals do not utilize post breaker features for releasing the cable anchor end located near the groundline of post no. 1. As such, there could be an argument for not utilizing post breakers in trailing end guardrail terminals if similarly configured to current W-beam terminals in terms of anchorage. This opinion would be based on the design, prior crash testing performance, and in-service experience of most crashworthy W-beam guardrail end terminals.

 

Using engineering judgment and in the absence of crash testing, we believe that the downstream trailing end terminal hardware, similar to that used at MwRSF and shown herein, could be utilized in a double rail, median-type configuration. However, full-scale crash testing is the only true way to determine the safety performance of the downstream trailing end terminal system. In addition, it should be noted that future testing may provide a basis for modifying our opinions on this issue.

 

If one were to have significant concerns regarding the potential for small car snag or wedging under the cable anchor, then a slight design change may be considered. First, it may be advantageous to incorporate blockouts with the end posts in foundation tubes, thus allowing a 8-in. lateral shift of the post, strut, and anchor cable. Such a design modification would likely require a longitudinal stagger of the anchor posts combined with a single post installed between the two blockouts. Unfortunately, there are also concerns with this design variation, such as little or no experimental experience, lack of prepared design details, unique loading on anchor posts and foundation tube, and potential for inadequate cable length with the 8-in. lateral shift.

 

At this time, MwRSF has received research funding from the WisDOT to examine, test, and evaluate a standardized downstream anchorage system for the MGS. With this project, I am hopeful that we will be able to provide design guidance for both roadside and median applications, including for double, median-type W-beam guardrail systems.

Date August 24, 2010
Attachment IDOT BCT001.pdf


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