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MGS transition to quarter post spacing

State IA
Description Text  

For MGS side obstacle protection on a divided highway, what is your suggested practice for transitioning from standard post spacing to quarter post spacing?  Would you mirror the transition on the downstream side, even though this is a one-way situation?



  • Guardrail
Other Keywords none
Date December 14, 2011


As noted below, you inquired as to whether a stiffness transition should be utilized when connecting full-post spacing MGS to quarter-post spacing MGS.

Recall, the standard MGS is configured with guardrail splices located at a mid-span location of a 6 ft " 3 in. wide span. Further, we know that barrier stiffness increases with the use of reduced post spacing. Thus, dynamic barrier deflections would be expected to decrease when considering comparable vehicular impacts into the MGS with standard, half-, and quarter-post spacing variations.

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.

Date December 15, 2011


Could you clarify the sentence highlighted below?  I'm having trouble following.  Also, how far upstream of the obstacle would you recommend the quarter-post spacing begin?


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.


Date December 15, 2011




The quarter-post spacing MGS should begin a minimum longitudinal distance of approximately 12 ft " 6 in. in advance of the hazard.


In terms of rail splices locations, numerous rail splices occur at mid-span locations within the full-post spacing MGS. However, there will be a post where full-post spacing ends and half-post spacing begins. We are suggesting a preference to not locate a rail splice at this transition post. Instead, we would prefer that the first rail splice within the half-post spacing region occur either 1 or 2 posts away from where full-post spacing ends. I hope that this helps to clarify the issue.  

Date December 15, 2011

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