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Steel Specifications for Guardrail Posts

Question
State NE
Description Text MwRSF has received multiple questions referring to the steel specifications and grades for guardrail posts. The Barrier Hardware Guide lists multiple steel specifications for standard guardrail posts: ASTM A36, ASTM A709 grade 36, and A709 grade 50. Additionally, the drawing details for many new systems are specifying the use of ASTM A572 and/or ASTM A992.  Are all of these steel specifications considered equivalent, or which specification should be utilized for guardrail posts?
Keywords
  • Guardrail
Other Keywords steel specification, grade, post
Date December 22, 2016


Response
Response Over the bast few decades the standard steel specifications for rolled structural shapes has changed. The older steels with yield strengths of 36 ksi have been replaced with higher strength materials with Fy = 50 ksi.  Please refer to the attached documents describing this transition. 
There is a lot fo overlap between these material specifications which allows for a particular steel lot/heat/batch to qualify under multiple of these specifications.  For example, A36 requires only a minimum yield and tensile strength, there is no maximum values.  As such, it has been easier to manufacture steel at grade 50 and label it as A992, A572, and A36. It may still be possible to obtain grade 36 steel posts, but it becoming increasingly difficult to find.  Over the past 5-10 years, MwRSF has typically received A572 or A992 when ordering guardrail posts (even when A36 was requested).

The performance of standard guardrail systems should not be negatively affected by this change.  Systems designed and tested with 36 ksi steels should still be crashworthy utilizing 50 ksi steel posts. However, there is a possibility for increased deck damage from bridge rails designed/tested with 36 ksi steel.  The increased strength of the post will transfer more load to the deck. Damage to the deck would also depend upon the deck strength, concrete strength, and rebar configuration, so increased damage is not a certainty.  Increased deck damage is a concern and systems specifying A36 posts should be evaluated on an individual basis.

Within drawing sets and system details, MwRSF has been listing multiple specifications as equivalent steel materials for use as guardrail posts. However, we have made the decision to stop listing grade 36 steels in current and future projects due to a lack of 36 ksi steel availability. MwRSF will only be listing grade 50 steel (A992).  

ASTM 709 is available as a grade 50 steel, but A709 was developed for use on bridges.  Accordingly, standard roadside and median barrier systems will likely not require A709.  Instead, A992 is a better steel specification for these in-ground installations. Agencies may desire to utilize A709 in bridge rails. Thus, it should remain a viable option for use in bridge rail components with attention being given to ensure the proper grade is utilized to match the desired strength of the post/system.

This change is steel materials should also be reflected in the Barrier Hardware Guide.  These recommendations will be given to Task Force 13 in hopes that the specifications within the guide can be updated to reflect current material standards.
Date December 22, 2016
Attachment Steelwise February 2012 (A992) .pdf
Attachment Are You Properly Specifying Materials_ Part 1(2).pdf
Attachment ASTM - A992.pdf


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