The guidance provided by FHWA was generated based on discussions between FHWA, TTI, and MwRSF. Following those discussions, TTI performed some research on raising the blockout height and tested a 28” tall guardrail system with raised blockouts to MASH. See below.
https://www.roadsidepooledfund.org/guidance-for-raising-beam-guardrail-blockout-602371/
http://www.roadsidepooledfund.org/wp-content/uploads/2013/09/RNo12-602371-00001-TX-59.pdf
https://www.roadsidepooledfund.org/guidance-for-raising-beam-guardrail-composite-blockout-for-rail-height-adjustment-605311/
https://www.roadsidepooledfund.org/wp-content/uploads/2015/06/TMNo605311-P1-P9-Final-1.pdf
https://www.roadsidepooledfund.org/608421/
https://www.roadsidepooledfund.org/wp-content/uploads/2017/04/TRNo-608421-1-Finalv2.pdf
The first two studies looked at and evaluated a 3”-4” raise of the blockout to increase rail heights to acceptable levels. As such, we would believe that a 3”-4” increase in blockout height is acceptable for bringing the rail height of reduced height systems up to 31”
The last study tested a reduced height W-beam guardrail system to MASH TL-3 that raised the blockouts 4” to bring the rail height to 28”. This would indicate that systems with rail heights as low as 24” could be raised to a safe height using a 4” blockout raise. We would only recommend this for steel posts systems as would post systems may see increased post fracture when the rail height is increased and the embedment depth is maintained.
Increases in blockout height of more than 4” would likely require further evaluation. There are concerns that further raising of the guardrail height may create issues where the blockout cannot effectively transfer load to the post as designed and may degrade the barrier performance.
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