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MGS Rail stiffness transition with Curb

Question
State MO
Description Text
In the email below is a question on the amount of curb required along the transition area from a bridge end thru the thrie beam, transition section and into the MGS. Basically, our new bridge end design will likely have curb extended into the transition section area or just past that into the nested MGS. But as we read report TRP-03-291-14, the design has curbing that ends either before post 11 OR extends past the first section of MGS that has been double nested with W beam to between posts 4 and 5 (Fig 54, pg 74). The question comes, is there any way to have a curb from the bridge end that ends within the transition section or within the nested MGS section that has been found as acceptable? This only saves a few feet of curbing at each approach, but multiplied over and over at locations around the state, it adds up.

The recommendations on pages 135 and 136 include nesting the W-beam rail at the upstream end of the W-to-thrie transition AND carries the curb past that nested section into the normal MGS. If that is the option, we will go with it. But, if there is some other design option that allows curbing to end in the thrie-to-W beam transition area, please let us know where to look for recommendations and design guidance. If there is nothing, please confirm that the design in Figure 54 with curb extended into the normal MGS rail in report TRP-03-291-14 is current and the best practice for this transitional area with curb.

Thanks for the help with this question and for providing a written reply that we can share with other staff to assure we have considered the options with lesser curb. Please call or email with questions.


Through my research and understanding of the MGS guardrail system and transitions related to the system, I have ran across report TRP-03-291-14 “Dynamic Evaluation of MGS Stiffness Transition with Curb.” This report details the testing of MGS rail and the stiffness transition to thrie-beam where a mountable curb exists. The report summary indicates the need for nesting of the first 12’-6” section of MGS rail when curb extends upstream of post No. 11 (beginning of the thrie-beam transition section to w beam). The report also goes on to indicate that if curb is present beyond post No. 11 then it should be extended to the end of the 12’-6” stiffness transition. This distance is stated to be approximately 37.5’ from the vertical parapet connection (Bridge end).

My question for MwRSF is – is this still a valid issue? Has there been any additional developments related to stiffness transitions with curb that does not require extending the curb 37.5’ from the bridge end. MoDOT has recently modified our designs for Bridge Approach Slabs and Concrete Approach pavements which will cause the curb to fall within this zone. If we can just double nest the first 12’-6” of w-beam MGS then keep our standard curb lengths that’s great, but if we must extend that curb to the end of the double nested MGS section that is something I would like to see in writing from MwRSF. This will affect every major road we modify if this is the case.
Keywords
  • Approach Guardrail Transitions
Other Keywords none
Date January 26, 2016


Response
Response

The introduction or removal of roadway geometric features has shown to cause critical differences in safety performances for some barrier systems.  Approach guardrail transitions (AGTs) in particular have been shown to fail crash tests after a curb is either introduced or removed from otherwise crashworthy systems, as demonstrated in the noted report (TRP-03-291-14) and in other AGT tests.  In addition, the termination or transition between adjacent features can cause their own issues with system crashworthiness.  Thus, to be conservative and avoid any potential snag and/or stability issues, we have recommended that the curb (or lack thereof) be consistent through the critical area of this MGS stiffness transition – specifically the area under and upstream of the W-to-thrie transition segment.  That has produced the following recommendations:

 

·         If you want to terminate the curb within the AGT system, we recommend doing so prior to the W-to-thrie transition element (or as you stated, before post 11 – from TRP-03-291-14  page 74). 

·          If the curb needs to be carried further than this location, we recommend carrying it past the entire AGT system.  This extends another 18 ft – 9 in. to the upstream end of the nested W-beam rail (or the mid-span between post 6 and 7 from trp-03-291-14 page 74).

 

Terminating or transitioning within this 18.5-ft region of the AGT may be crashworthy, but we simply don’t have the research or testing to confirm that. 

Date January 29, 2016


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