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Culvert Top Mounted MGS Steel Question

State WI
Description Text

WisDOT uses the top-mount design that MwRSF crash tested for us. We at WisDOT, are having discussion about what is the appropriate grade steel for the top plate in the top mount MGS.  I’ve taken our standard an put a cloud around the plate we are talking about.

Our structure staff indicate that the steel should be ASTM A709 Grade 50 or 50S.

I indicate that the steel could be: ASTM A36, ASTM A992 50KSI min, ASTM 529 Grade 50, or ASTM 572 Grade 50.


I pulled down two Q/A’s from the MwRSF Question site (see attached). To me it indicates that we can use the steel grades I listed, but I’m not a bridge engineer.

After reviewing the Ohio Q and A the structure staff has the following concerns:

  1. The most current crash test report TRP-03-383-20-R1 is a MASH Test and the Ohio Q/A is for a different for an NCHRP-350 system. So the Q/A may not be applicable.

  2. In component testing in test report TRP-03-278-13, the base plate tore in tests using A36 steel.  When a higher grade of steel was used the base plate did not tear.  Therefore a stronger plate steel is recommended.

  3. The Ohio Q/A is for a different system (i.e. TTI’s) and may not be applicable.

If you could assist I would be appreciative.



Bridge Rails
W-beam Guardrails

Date April 12, 2024
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Attachment View Q&A _ University of Nebraska–Lincoln.pdf Attachment View Q&A _ University of Nebraska–Lincoln v1.pdf Attachment sd-14b51.pdf

A few thoughts on the steel specification for the culvert mounted post base plate comments you are running into.


  1. The original NCHRP 350 culvert mounted strong post guardrail system used A36 posts and base plates in its design.
  2. We have been using grade 50 material in the last 10 or so years to be compliant with preferred structural steel grades from AISC. Thus, we have been specifying A992 for steel post materials and A572 Grade 50 for steel plate in most cases. We do get standard W6x8.5 guardrail posts in A36 in some cases. However, most of the A36 material we receive has a yield strength near 45 ksi rather than the nominal 36 ksi value.
  3. The increase in the post grade to A992 Grade 50 was observed to increase the loading slightly on the base plate in TRP-03-278-13 leading to base plate rupture. Thus, it made sense to have the post grade and base plate grade be more consistent. As such, in the MASH TL-3 tested system we specified A572 grade 50 plate. This worked well in the MASH full-scale testing. Note that the energy absorbed by the A36 and A572 plates in the component testing were very similar, so either base plate grade was deemed allowable. We selected A572 Grade 50 for the MASH TL-3 system because it generally made more sense to have similar grades for all the parts of the post.
  4. A709 Grade 50 and A572 Grade 50 are basically identical in terms of yield strength, ultimate strength, and % elongation. Thus, there would not be any issues with using A709 Grade 50 if desired. The A709 specification does have requirements for notch testing and temperature effects. However, we have not required the use of the A709 grade. I would assume that your structures staff may be requesting this grade because they consider this system somewhat of a bridge railing.
  5. The Q&A response below deals with a different base plate design used in a similar application developed at TTI. The base plate in this system is much thicker and was not designed to deform and dissipate energy during the post impact and loading. In that case, switching to a higher grade material would not be expected to change the post behavior.



Date April 13, 2024
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