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(1) On the Steel Thrie Beam length a
(2) Typically, when guardrail comes off the
Some of these
Was all the
|Date||March 20, 2017|
|Attachment||GR8 special detail 4-20-15.pdf|
We are actually in the process of the MASH evaluation of the thrie beam bullnose for the Midwest Pooled Fund. I have some comments below in red.
I also noted that you have a note on your plans that “THE USE OF STEEL POSTS ON THE BULLNOSE IS NOT ALLOWED”. We do have a version of the bullnose with breakaway steel posts if you are interested in seeing it. Let me know.
We are developing plans to install a number of Thrie Beam Bullnoses on twin structures and a few questions have come up I hope You can help me with. Attached is the detail Midwest developed. Our only changes are some drafting and adding a note to clarify the gauge of the Thrie Beam.
(1) On the Steel Thrie Beam length a “STANDARD WOOD BLOCK”, (8”X6”X14” blockout) is called for at Post Nos. 9-12 and beyond Post 12. We are developing a Special Detail (soon to be a Standard) for Modified Thrie Beam. We would like to use the tested Modified Thrie Beam for the run between Post 12 and the Thrie Beam Transition. Do You have any concerns using the tested Modified Thrie Beam for this instead of the “STANDARD WOOD BLOCK”?
I am proposing to use the steel blockout with the clipped web as tested in the development of Modified Thrie Beam in lieu of the 14” deep blockout.
The modified thrie beam blockout could likely be used in that region. We typically have allowed standard thrie beam construction starting at post no. 9 in the system. We tested the system with shortened wood blockouts based on previous experience with thrie beam transitions that suggested that the shortened blockouts perform better than full length blockouts. Modified thrie beam blockouts have a similar shortened profile.
Modified thrie beam blockouts have only been evaluated to NCHRP 350. This is true of the bullnose as well. Thus, they can likely be used adjacent to the bullnose system. New Jersey and CALTRANS are currently looking for partners to evaluate the modified thrie beam system to MASH TL-3 if that is something West Virginia would be interested in.
One important note is that the modified thrie beam blockouts require the use of a backup plate to prevent the potential for stress concentrations and rail rupture when the W-beam folds around the blockout.
(2) Typically, when guardrail comes off the End Wall there is a very short length that is parallel to the roadway and a taper away from the shoulder begins. Is it acceptable to place a taper on the Thrie Beam Transition and length of “Steel Thrie Beam”? Also, where should the centerline of the bullnose be in relation to the centerline of the roadway.
We would not recommend flaring of the approach guardrail transition. These systems have never been evaluated tapered or flared and there are concerns that flaring them would increase the potential for pocketing and snag. We do believe you could flare the thrie beam guardrail once you were a minimum of 12-6” (one rail segment) past the end of the approach guardrail transition. This would mean 12-6” past any reduced posts spacing, non-standard posts, or nested or 10 gauge rail sections. The attached report has a schematic of such an installation. http://mwrsf.unl.edu/researchhub/files/Report120/TRP-03-95-00.pdf
We also developed wide designs for the bullnose. They are in the attached report but they do not have an FHWA eligibility letter.
I am not sure I follow what you mean by the position of the bullnose relative to the centerline?
Some of these proposed bullnose terminals are in a curve. These curves are not extreme curves since it is an arterial roadway, but still there are some alignment issues to deal with as shown below.
Was all the testing performed in tangent sections? Yes.
|Date||April 27, 2017|
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