To make this simple, I might suggest that you use the modified thrie beam system, which uses a longer steel post and tapered, deep steel blockout. This system is shown in the 2006 RDG, page C-11. If you do not really like this option, then I would adapt from 32” MGS and use shortened wood blockouts (like for W-beam or used down sides of bullnose) on 78” long steel/wood posts. The later would need to be drawn and likely sent to FHWA for comment.
Which path do you prefer?
I have a call in to Nick A. of FHWA to further discuss the MGS Thrie Beam Median Barrier concept, especially in terms of minimum height and blockout configuration. While we wait for that discussion, I scanned through our Pooled Fund Consulting website to review our prior guidance on thrie beam related issues that seem very relevant. As you will see, several items may be critical, including rail height, post type, blockout size/type/length, rail-to-post attachment, etc. These items will certainly affect the overall configuration.Ron**********************************************************Thrie Beam BlockoutsView as AdminQuestionState WI Description Text We would like some guidance on the appropriate blockout to use for a standard thrie beam, both for wood posts and steel posts.WisDOT is working on developing a thrie beam bullnose terminal based on the MwRSF design(report TRP-03-95-00, 6-1-2000) that was approved by FHWA (11-8-2000 letter, HSA-1/HSA-cc68). The blockout for the standard thrie beam construction from posts 9 to 12 is shown as 360 mm (14.17 in) high.Sheet 2 of Minnesota DOT's Standard No. 5-297.611 "Thrie Beam Bullnose Guardrail for Medians" (dated 8-20-2001), which is based on the approved MwSRF design uses a 22-inch high blockout. Keywords • GuardrailOther Keywords noneDate May 10, 2006 ResponseResponse Per your inquiry, I have summarized several of the successful thrie beam transition tests that have been conducted according to the NCHRP Report No.350 requirements. They are as follows:No. 1 - Wood Post - Wood Blockout 550 mm center rail height 804 mm top rail height 150 mm by 200 mm by 554 mm blockouts 150 mm by 200 mm by 1.9 m posts 0.68 m dynamic deflection TL-3 pickup truck test (404211-11)No. 2 - Steel Post - Modified Steel Blockout 610 mm center rail height 864 mm top rail height W360x33 tapered blockouts W150 x14 posts 0.71 m dynamic deflection TL-4 single-unit truck test (404211-5a) 610 mm center rail height 864 mm top rail height M14x18 tapered blockouts W6 x9 posts 1.02 m dynamic deflection TL-3 pickup truck test (471470-30)No. 3 - Steel Posts - Routed Wood Blockout 550 mm center rail height 804 mm top rail height 150 mm by 200 mm by 554 mm routed blockouts W150 x14 posts 0.58 m dynamic deflection TL-3 pickup truck test (404211-10)In summary, you can use either the long wood blocks, shortened wood blocks, or even the tapered steel blocks in combination with the standard thrie beam guardrail systems after post no. 9. Date May 15, 2006 Thrie Beam Height GuidanceView as AdminQuestionState IA Description Text Are you aware of any minimum and/or maximum allowable height guidance for thrie-beam guardrail?Keywords • GuardrailOther Keywords noneDate September 21, 2010 ResponseResponse I am not aware of a documented height tolerance for thrie beam guardrail systems. Initially, one may attempt to argue that the minimum height could be as low at that corresponding to W-beam guardrail systems. However, I would suspect that mounting thrie beam with a top height of 27¾ in. would potentially increase the propensity for vehicle climb, barrier override, and/or rollover upon redirection due to the increase face below normal W-beam rail with same top height.At this time, the roadside safety community has considered the minimum top height for W-beam rail to be approximately 27¾ in., while the maximum top height for the MGS is 32 in. At the minimum W-beam top height, a thrie beam element would extend downward to 7¾ in., thus potentially creating new safety risks. Selected thrie beam guardrail systems have successfully met crash testing guidelines when installed with a top height of 34 in. As such, it is my opinion that the minimum height tolerance for modified thrie beam guardrail may be somewhere around 31 in. for NCHRP 350, while the top height tolerance may be closer to 39 to 40 in. at the TL-3 impact conditions.Thrie beam has been successfully crash tested over the years. Below, I have provided a few of the test results but not those for the T-39 thrie beam guardrail system.Test No. System Description Top Rail Height Result404211-5a Modified Thrie Beam w/ 81” Steel Post & Tapered Block 34” Passed 8000S TL-4 test404211-11 Strong-Post Thrie Beam w/ 81” Wood Post & Wood Block 31.65” Passed 2000P TL-3 test404211-10 Thrie Beam w/ 81” Steel Post and Routed Wood Block 31.65” Passed 2000P TL-3 test471470-31 Thrie Beam (G9) w/ 78” Steel Post and Steel Block 32” Failed 2000P TL-3 test471470-30 Modified Thrie Beam w/ 81” Steel Post and Tapered Block 33.6” Passed 2000P TL-3 testRecent Test Thrie Beam (G9) w/ 78” Steel Post and Full-Depth Wood Block 31.5” Failed 2270P TL-3 test (MASH)Although I have yet to see the results of the recent failed MASH test, it would seem reasonable that improved safety performance could be obtained by using a shortened wood blockout or the modified steel tapered (collapsible) blockout – both of which reduced climb and allow the lower corrugation to fold back.Date December 13, 2010 Thrie Beam Bullnose Post Length after Post 9View as AdminQuestionState KS Description Text What is the allowable post length for the crash tested options for thrie beam guardrail systems? We wlould like to know what post length is acceptable beyond post 9/10 in bullnose. We would like to use 6 ft.Keywords • Bullnose Median Barrier & Short RadiusOther Keywords noneDate February 10, 2012 ResponseResponse We reviewed the prior research and found that three different thrie beam configurations have met the 350 safety standards. They are listed below. Upon inspection, it would seem appropriate to maintain the use of 6.5 ft long posts for the standard wood post thrie beam guardrail.________________________________________________________________Summary1. Standard Thrie Beam (G9) system did not pass 350 – 6.5 ft W6x9 posts with 21.5” long W6x9 blockouts w/ 32”rail height2. Modified Thrie Beam = 6’-9” long W6x9 posts with 49.5”embedment and 18” deep M14x18 blockouts w/ 33.6” rail heighta. Tested to TL-3 and Tl-43. Thrie beam with 6’-9” long W6x9 posts with 49.5”embedment and 6”x8”x21.8” long routed wood blocks passed NCHRP 350 w/ 31.65”rail height4. Thrie beam with 78” long 6”x8” SYP posts and 6”x8”x21.8”long routed wood blocks passed NCHRP 350 w/ 31.65” rail heightShort answer, 81” long posts for steel and 78” long wood have passed.Date February 11, 2012
What is the working width of the double faced thrie beam?
Estimate W.W.:
System width = (3.25”)*2 + 6” + (14”)*2 = 40.5”
Dynamic Deflection for single-face system w/ pickup truck impact = 36.7” (1.02 m)
Est. Dyn. Defl. In RDG for double-face system = 20”
W.W. = 20” + 40.5” = 60.5” which seems somewhat too high
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