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Maximum fiber stress in bending for beam guard posts

Question
State WI
Description Text

We have a requirement that our beam guard post have a maximum fiber stress Fb of 1,200.    I believe that AASHTO has a similar requirement (AASHTO M168-6 is below).

Guardrail Posts-Guardrail posts shall be a stress grade of 8.2 MPa (1200 psi) or more,


conforming to the applicable standards contained in AASHTO-ARTBA-AGC, A Standardized


Guide to Highway Barrier Hardware. When a preservative is required, framing and boring shall

be completed prior to treatment in accordance with M 133.

 

What is the Fb of white pine?

If the Fb of white pine is lower than 1,200 do we need to be concerned about this even if there is a passing crash test?

 

 

Keywords
  • Guardrail
Other Keywords none
Date March 5, 2015


Response
Response The fiber stress requirements noted exist to help ensure that timber posts used in guardrail systems possess sufficient capacity to deflect through strong soils and absorb energy during impact as intended. Posts with insufficient capacity pose the risk of fracturing prematurely and degrading the safety performance of the barrier system. 

With respect to the MGS system, MwRSF conducted previous research with the white pine post (http://mwrsf.unl.edu/researchhub/files/Report41/TRP-03-241-11.pdf). In this research, it was noted that there were two approachesto implementing the white pine post, which has lower strength than the typical southern yellow pine post,. One approach was to revise the geometry of the post cross-section to provide similar capacity to the standard 6"x8" SYP post. The other approach was to full-scale crash test the standard post geometry with the white pine post to investigate its performance. The study chose to evaluate the white pine post with the standard geometry. The full-scale test of the white pine MGS system was successful according to MASH. Because the white pine post version of the standard LON system of the MGS was successful, there would be no reason not to use the system with white pine posts, even if the fiber strengths were lower than to specified values. 

There would only be a couple of caveats. 

1. Use of other reduced strength wood species than white pine with the MGS would likely need to be evaluated. MwRSF has done some previous research on alternative species which can be found here. 
http://mwrsf.unl.edu/researchhub/files/Report220/TRP-03-154-04.pdf
http://mwrsf.unl.edu/researchhub/files/Report125/TRP-03-179-07.pdf
2. The use of white pine would be for the MGS system and not previous metric height G4(2W) systems.
3. The report above provides recommendations for the use of white pine posts for special applications of the MGS that should be followed due to these systems potentially being more sensitive to variation of the wood strengh..

Date March 5, 2015


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