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Thrie Beam Transition to Standardized Buttress Post Length

Question
State	IN
Description Text	In our previous question ID 41985, we were reference to TRP-03-417-20 for the Thrie Beam Transiton to a Standardized Buttress. Within the report it states the posts may be up to 81" in length. For INDOT Modified Thrie Beam we call for a 7 ft post, I assume it was to use an even foot length. Will it affect the performance of the systems if we use the 7 ft (84") posts in the transition and within the run of modified thrie beam run?

Question

State

Description Text

In our previous question ID 41985, we were reference to TRP-03-417-20 for the Thrie Beam Transiton to a Standardized Buttress. Within the report it states the posts may be up to 81" in length. For INDOT Modified Thrie Beam we call for a 7 ft post, I assume it was to use an even foot length. Will it affect the performance of the systems if we use the 7 ft (84") posts in the transition and within the run of modified thrie beam run?

System Performance Evaluation
MASH

System Types
Approach Guardrail Transitions (AGTs)
Stiffness and Height Transitions
Thrie Beam Guardrails

Applications
Median Barriers

System Features

Date June 8, 2026
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Response
Response (active)	For the modified thrie beam guardrail that we tested to MASH for Caltrans and NJDOT, the line posts were 81” long. Increasing the post length and embedment by 3” will tend to increase the post soil resistive properties. We have used a rule of thumb that embedment depth changes affect the post soil forces by a scale factor based on the ratio of the modified embedment to the original embedment squared. Thus, we would expect post soil forces to increase 1.13 times for the slight change in embedment. While this may affect the system performance to some degree, I don’t believe it would be a significant issue. In general, we would expect that variation in the yield strength of the post steel and the soil strength itself to varying in a wider range than 13%. Thus, the small change in embedment is not believed to be an issue.

Response

Response
(active)

For the modified thrie beam guardrail that we tested to MASH for Caltrans and NJDOT, the line posts were 81” long. Increasing the post length and embedment by 3” will tend to increase the post soil resistive properties. We have used a rule of thumb that embedment depth changes affect the post soil forces by a scale factor based on the ratio of the modified embedment to the original embedment squared. Thus, we would expect post soil forces to increase 1.13 times for the slight change in embedment. While this may affect the system performance to some degree, I don’t believe it would be a significant issue. In general, we would expect that variation in the yield strength of the post steel and the soil strength itself to varying in a wider range than 13%. Thus, the small change in embedment is not believed to be an issue.

Date June 10, 2026
Previous Views (29) Favorites (0)