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MGS Stiffening

Question
State WV
Description Text Do you know of any research and results of stiffening MGS with double nested beams or reduced post spacing.

Of course with added post it becomes a splice on the post system, but I was interested in the transition from MGS to the changed section. I was looking at the RDG, Table 5-6 on Page 5-34 and the maximum deflection of a Double W-Beam and 38 in post spacing looks desirable.

I am protecting High Mast lighting in the median that has a 4’ dia. foundation 2’ above grade and a poles that will not break. I really want some solid protection and bullnose can’t be graded in due to the bifurcated typical. 
Keywords
  • Guardrail
Other Keywords none
Date September 18, 2015


Response
Response

We have looked at stiffening methods for  the MGS system in the past. We have not evaluated nested rail applied to the standard MGS system at this time. We have used nesting in a couple of special applications for transitions, but we have not done it for a standard length of need system.

 

That said, we have looked at the use of reduced post spacing for the MGS. The report for this work can be found at the link below. In the report, we tested ¼ post spacing and developed guidance for ½ post spacing deflections as well.

 

http://mwrsf.unl.edu/researchhub/files/Report162/TRP-03-139-04.pdf

 

Historically, common W-beam guardrail systems have been easily transitioned between full and half-post spacing variations as well as half- and quarter-post spacing configurations without changes to post lengths or rail configurations. When the MGS with quarter-post spacing is deemed necessary to shield hazards closer to the traveled way, the needs exists to connect full-post spacing MGS to quarter-post spacing MGS. Under this scenario, MwRSF has previously suggested that an intermediate stiffness transition be utilized to more gradually blend the varied lateral stiffness of the two systems. More specifically, MwRSF suggested that a 12-ft 6-in. long MGS segment with half-post spacing be used to gradually transition the lateral barrier stiffness and strength, thus resulting in four spans of half-post spacing between the two systems.

 

Although the standard MGS utilized mid-span locations for rail splices, it would be expected that rail splices would occur at post locations for the MGS variations which utilized a reduced post spacing. Thus, MwRSF has suggested that rail splices be configured to occur a minimum of 1 reduced span (3 ft - 1½-in.), and preferably 2 reduced spans (6 ft " 3in.), beyond the last or first MGS full-post spacing.

 

The stiffness transition noted above is suggested for situations where impacting vehicles first contact the full-post spacing MGS and subsequently engage the quarter-post spacing MGS. Therefore, it would not be necessary to apply a similar stiffness transition to the downstream ends of quarter-post spacing MGS unless prone to reverse-direction impacts.

 

As a side note, we currently have a research project with the Illinois Tollway to evaluate the minimum offset for luminaire poles behind the standard MGS system. We can keep you up to date on the outcomes of that study if you are interested.

 

Date September 22, 2015


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