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Iowa DOT is considering the use of an alternative approach guardrail transition, based on the results of TRP-03-210-10.  We currently use the wood-post version of the "Adapted Iowa Transition" that's shown on page 167 of the report.  However, we were wondering if there might be a simpler design out there that we could use.  Our preference is a design that uses larger post spacing near the bridge end, similar to the layout of the W6x15s used in the report.  We are interested in both steel and wood post designs.

I have attached a drawing that shows the three most common bridge end post shapes that are encountered in Iowa.  If you need exact dimensions on these, let me know.  Type A on page one is the current bridge end post design and is the most common by far.  If possible, we would like to be able to use the same AGT for all three types. 

Our only constraint is that the transition cannot be longer than our current design (25 feet from the bridge to the end of the W-to-Thrie transition piece).

Can you see what other designs are available, if any, that could be adapted to meet our needs?

I have reviewed all of the FHWA accepted transition designs as well as the MwRSF tested transition designs.  From this review, there seems to be two different styles of thrie beam transitions to concrete bridge rail:

1. Use posts similar to the line posts (W6x9 or 6"x8" wood posts) but with increased embedment depths at ¼ post spacing (18.75").  A prime example of this style of transition is the transition you are currently using.
2. Use significantly larger posts (W6x25, W8x24, or 10"x10" wood posts) at 37.5" spacing.  These larger posts  are typically chosen so that the first post adjacent to the bridge rail can be omitted for drainage purposes (see page 165 of the report you noted, TRP-03-210-10).  However, do to the omitted post and the oversized transition posts need to compensate for the gap, the recommended stiffness transition had a length of 31'-6" to the US end of the W-to-thrie transition element.  You expressed that you needed to stay at your current 25' length, so this probably is not the answer you were looking for.

The original approach transition utilized to developed the stiffness transition shown in report TRP-03-210-10 was designed for attachment to a steel post, thrie beam and steel channel bridge rail system.  Thus, I would not recommend utilizing this transition for attachment to concrete barriers until further evaluation/crash testing proves its crashworthiness.

I have attached the recommended wood post alternative to the transition you noted from page 167 of the steel post transition report.  The attached figure comes from the wood post transition report that is currently under in-house review at MwRSF and should be sent to the Midwest Pooled Fund States in the near future.