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.
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.