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MGS Thrie Beam Median Barrier

State WI
Description Text WisDOT wants to install a double faced MGS with thrie beam.

I have grabbed the cadd for the MGS double faced drawings from the web.

I know that the blocks need different geometry for thrie beam.

I was wondering how to split the double face into separate into two single faced thrie beams.

I was also trying to figure out how to bolt the rails to the blocks and posts.

  • Guardrail
Other Keywords none
Date May 19, 2013



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?


Date May 19, 2013


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.
Thrie Beam Blockouts
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State 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 • Guardrail
Other Keywords none
Date May 10, 2006

Response 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 Guidance
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State IA
Description Text Are you aware of any minimum and/or maximum allowable height guidance for thrie-beam guardrail?
Keywords • Guardrail
Other Keywords none
Date September 21, 2010

Response 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                                     Result
404211-5a                                               Modified Thrie Beam w/ 81” Steel Post & Tapered Block                                     34”                                        Passed 8000S TL-4 test
404211-11                                               Strong-Post Thrie Beam w/ 81” Wood Post & Wood Block                                  31.65”                                Passed 2000P TL-3 test
404211-10                                               Thrie Beam w/ 81” Steel Post and Routed Wood Block                                         31.65”                                Passed 2000P TL-3 test
471470-31                                               Thrie Beam (G9) w/ 78” Steel Post and Steel Block                                                   32”                                        Failed 2000P TL-3 test
471470-30                                               Modified Thrie Beam w/ 81” Steel Post and Tapered Block                                33.6”                                   Passed 2000P TL-3 test
Recent 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 9
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State 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 Radius
Other Keywords none
Date February 10, 2012

Response 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.



1. Standard Thrie Beam (G9) system did not pass 350 – 6.5 ft W6x9 posts with 21.5” long W6x9 blockouts w/ 32”rail height
2. Modified Thrie Beam = 6’-9” long W6x9 posts with 49.5”embedment and 18” deep M14x18 blockouts w/ 33.6” rail height
a. Tested to TL-3 and Tl-4
3. 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 height
4. 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 height

Short answer, 81” long posts for steel and 78” long wood have passed.
Date February 11, 2012

Date May 21, 2013


What is the working width of the double faced thrie beam?

Date May 22, 2013


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


Date May 23, 2013

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