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Iowa Transition and Curbs - Blockout Length and Depth Inquiry

State IA
Description Text

I've got a follow-up question regarding steel-post versions of the Iowa transition:  Are we limited to the version shown in the ITNJ report (6.5-foot W6x9s with tapered tube blockouts)?  Or is there an allowable substitution we can make in order to use the 19-inch wood blockouts with steel posts?

  • Approach Guardrail Transitions
  • Guardrail
Other Keywords none
Date June 5, 2011


The Iowa approach guardrail transitions were developed and crash tested with steel and wood post options. The systems were configured as follows:


Steel Post " Test ITNJ-2


6.5-ft long by W6x9 Steel Post w/ 49" embedment depth

17.4-in. long by TS 7"x4"x3/16" steel tapered blockout w/ 3.2 in. extending below lower thrie beam bolt


Wood Post " Test ITNJ-4


7-ft long by 6"x8" Wood Post w/ 52" embedment depth

18-in. long by 6"x8"wood blockout w/ 3.1 in. extending below lower thrie beam bolt


Several years ago, MwRSF developed and crash tested transition designs which utilized 19-in. long wood blockouts. In this case, two tests (STTR-3 and 4) were successfully conducted with a 4-in. extension below the lower thrie beam bolt when used in combination with a half-post spacing AGT.


More recently, MwRSF also developed and crash tested standard and simplified designs which utilized 19-in. long wood blockouts when adapting the MGS to a thrie beam transition which utilized a half-post spacing AGT. For these tests, a 4-1/8 in. extension was used below the lower thrie beam bolt.


Currently, the Iowa DOT is specifying a 19-in. long blockout with a 4¼-in. extension below the lower thrie beam bolt. The IADOT has inquired as to whether: (1) the 19-in. long wood blockout with a 4¼-in. lower extension is a acceptable alternative to the slightly shorter blocks having slightly smaller extensions and (2) it would be expected to provide satisfactory safety performance.


Based on my review of the prior systems and satisfactory crash testing results, I believe that a 19-in. long wood blockout with a 4¼-in. extension below the lower thrie beam bolt could be used in FHWA-accepted thrie beam approach guardrail transitions configured with either quarter-post or half-post spacings. This block length and lower extension should still allow the lower thrie beam corrugation to push back and/or fold under when impacted by vehicle wheels. In addition, it is my opinion that 7 or 8-in. deep wood blockouts could be substituted with 12-in. deep wood blockouts.

Date June 6, 2011

Response Would your recommendation remain the same if we were to increase the extension below the lower thrie beam bolt by 1/8 in. to 4-3/8 in.? See the attached drawing for proposed blockout dimensions.
Date June 14, 2011
Attachment Blockouts.pdf

Response I do not believe the additional 1/8 in. of blockout length beyond the lower guardrail bolt would cause to change my opinion. As such, you should be fine using the slightly longer block.
Date June 15, 2011

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