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Adjust Strong Post W-Beam Guardrail Top Rail Height

Question
State IN
Description Text We have existing guardrail with pre-drilled holes to adjust the guardrail top-rail height on Strong Post W-Beam. The hole adjustment heights
varies for different runs of guardrail, 3.5 inches, 5.0 inches, 5.5 inches, and 7.0 inches. Within the Frequently Asked Questions Document
for FHWA #27 they mentioned that the adjustments are normally 3 inches? Can the adjustment be more the 3 inches or should we keep to
3 inches so the blockout is not too far above the post? The FAQ is below. Thank you for your help.

#27 How high do you need to raise guardrail that is lower than 26 ½ inches? What are the best ways to do that?

A. The guardrail should be raised to 29 inches, which represents the target height for new installations of strong post w-beam systems. If
the pavement work requires the barrier to be moved, then the posts should be carefully extracted and, if in good condition, re-driven at the
new location so that the rail will be at 29 inches. If the barrier does not need to be moved, then raising the blockout up to three inches is a
common practice. This will require field drilling or punching of a new hole in the guardrail post.

Keywords
  • Guardrail
Other Keywords none
Date April 22, 2020
Response
Response

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.

 

Date April 22, 2020
Attachment TM602371-P1-P4-1.pdf