Mondo Polymer has asked INDOT accept a 21" composite blockout for the Thrie Beam Approach Guardrail Transition.
(1) I believe it's been determined previously that the composite blockout is an acceptable substitute to timber in this application. Would you please confirm?
(2) Is the 21" blockout acceptable? This height would have the bottom of the blockout even with the bottom of the thrie beam. INDOT's Thrie Beam AGT calls for a 19" blockout. The response to INDOT Q&A for Blockout Dimensions and Material regarding 18” vs. 19” composite blockouts. indicated that both heights had been evaluated previously and either height was considered acceptable.
Blockout heights for all previously MASH TL-3 transitions have used shortened blockouts (18" to 19" tall) that support 17"-18" of the thrie beam height and allow the lower section of the rail to fold under when impacted. This has been done to allow the wheel of the impacting vehicles to get under the thrie beam, which has tended to improve stability.
There would be concern with using a 21" tall blockout as this would fully support the thrie beam element and not allow the folding of the lower portion of the rail during impact.
Composite blockouts would not likely be an issue, but the height may as it has not been evaluated under MASH. There was a TL-2 AGT tested under MASH that used a taller blockout, but that only had a single post in the thrie beam region of the AGT.