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Thrie Beam Height Guidance

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


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

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