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Iowa Approach Guardrail Transition - Steel Post Alternative - Recessed Steel Post, Tapered Steel Block, & Full-Height Wood Block

State IA
Description Text

Chris Poole had a question regarding the length of the posts in the Iowa approach guardrail transition.

  • Approach Guardrail Transitions
  • Guardrail
Other Keywords none
Date June 7, 2011


Your current inquiry pertains to the desire to utilize 7-ft long, W6x9 steel posts in lieu of the 6.5-ft long, W6x9 steel posts in an approach guardrail transition configuration which was based on the use of posts installed on a quarter-post spacing.




In the late 1990's, two Iowa approach guardrail transitions with a quarter-post spacing near the concrete buttress were developed and crash tested under NCHRP 350 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

top of steel post recessed approximately 2.44" below top of tapered tubular steel blockout

Maximum (visible) dynamic deflection was found to be approximately 5.2 in.


Wood Post " Test ITNJ-4


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

top of post flush with top of wood blockout

Maximum (visible) dynamic deflection was found to be approximately 3.9 in.


Recent Testing


Later and under NCHRP Project 22-14(2), the original steel-post AGT was retested under the proposed MASH guidelines using the new 2270P pickup truck. During this test, the maximum dynamic deflection was found to be 11.4 in., which was significantly higher than the magnitude of the visible dynamic deflections found in the prior NCHRP-350 crash testing programs.




The two transition designs noted above utilized slightly different post embedment depths " 49 versus 52 in. Although two depths were used, one may be able to argue for the standardization of this parameter.


For the steel-post, steel-blockout AGT, a tapered steel blockout was successfully used in combination with a slight recessed post. The tapered steel block and recessed post were used to reduce concerns for the pickup truck to extend over the thrie beam rail and snag on the metal elements as well as decrease the potential for vehicle instabilities. These features were originally implemented as design improvements to a thrie beam AGT for attachment to Missouri's single slope concrete median barrier. For the both the successful AGTs for both the Iowa and Missouri systems, the steel post was recessed in combination with a tapered steel blockout.


For the wood-post, wood-blockout AGT, a full-height post and blockout was successfully used in the crash testing program. In this test, the vehicle extended over the rail and contacted the wood components but was believed to more easily gouge the upper regions, thus likely reducing the resistance imparted to the pickup truck and potentially reducing concerns for vehicular instabilities.


Based on a review of the prior crash testing programs noted above, it would seem reasonable that a consistent embedment depth of 52 in. could be utilized in the steel post AGT. Unfortunately and in the absence of supporting test results, it would also seem appropriate to maintain the use of a recessed top of steel post in the region of quarter-post spacing as well as a tapered steel tubular blockout. However, it would seem appropriate to allow the use of a full-height wood blockout in combination with a recessed steel post.

Date June 7, 2011


What are your thoughts about using wood blockouts that were designed for use with wood posts (post bolt holes centered horizontally within the blockout), but using them with steel posts (post bolt holes off-center due to the flange)?

Date June 7, 2011

Response I would maintain the use of a centered wood blockout installed to a steel post. Thus, an off-centered bolt hole should be used in a wood blockout if attached to a steel wide-flanged post. A routed section in a wood block could be used to reduce block rotation. Non-routed blocks could be used in AGT designs which were configured with two guardrail bolts or if alternative anti-rotation methods (i.e., toe nails) were utilized.
Date June 8, 2011

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