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MGS Spacing Guidelines

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Please review the attached excerpt from the draft Tollway Traffic Barrier Guidelines manual that we are working on.


What I am proposing does not use special posts and does not eliminate any posts, but limits the maximum and minimum post spacing to try to control the changing rigidity.  Let me know what you think.  The max and min values are just numbers I made up for discussion purposes.  They could be more or less if you are comfortable with the idea.  Maybe my idea works up to a certain size drainage structure and then we go to CRT posts???


Is the purpose of the CRT to prevent pocketing?

  • Guardrail
Other Keywords none
Date May 6, 2010
Attachment MGS usage guide - portion for Ron 05062010.docx


I believe that establishing a maximum and minimum post spacing for these special applications without the need for CRT post (or other specialized posts) has merit.  The minimum spacing you proposed is very near a ½-post spacing (3' vs. 3'-1.5"), thus it seems reasonable.  Along the same lines, I would consider 150% of standard post spacing as an acceptable maximum spacing limit.  Your proposed limit is very close to this value (9'-6" vs. 9'-4.5"), and thus also seems reasonable.


I also agree with and encourage your statement to keep the post spacing as uniform as possible in these situations in order to prevent large variations in stiffness that cause pocketing.


One important thing to note here, these post spacing variations should only be applied to standard segments of the guardrail system.   Guardrail transitions and terminals are carefully designed to accommodate increases in stiffness along the system. Therefore, these general rules do not apply and any variations to a transition or terminal need to be individually analyzed.


To answer your question about the use of CRT posts in the Long-span system " They reduce the affects of vehicle snag on the posts.  CRT posts are designed to maintain bending strength about the string axis (laterally) but are substantially weaker about the longitudinal direction.  Thus, when a vehicle contacts a CRT post, it will fracture or break away and consequences of vehicle snag are minimized.


I like both transitions (to ½ post and to ¼ post spacing) and only have a few comments.


 First, the distances shown from the hazard/obstruction to the beginning of the transition segments should be shown as minimums.  There hazards may not line up as nicely as shown in your drawings, making these exact distances not possible.  By stating the minimum lengths required you cover all situations.


Second, in circumstances where the hazard/rail is susceptible to impacts from vehicle traveling in the opposing traffic direction, you would want to make the transition symmetric about the hazard (i.e., change the downstream portion to match the upstream. 


Third, the length of ¼ post spacing prior to the hazard must be greater than 7.5 ft (as determined from the full-scale test NPG-6 as the distance from contact to maximum deflection).  You currently have 12.5 ft listed for this distance, thus it could be shortened slightly.  I would recommend a minimum of 9 ft  of ¼ post spacing prior to the hazard.  However, 12'-6" is conservative and could still be used.

Date July 1, 2010

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