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We are looking for a clarification of the implementation guidelines as stated in report no. TRP-03-243-11 Development of Alternative Wood-Post MGS Approach Guardrail Transition.
From page 90:
"1. A recommended minimum length of 12 ft - 6 in. for standard MGS is to be installed between the upstream end of the asymmetrical W-beam to thrie beam transition section and the interior end of an acceptable TL-3 guardrail end terminal...
2. A recommended minimum barrier length of 46 ft - 10.5 in. is to be installed beyond the upstream end of the asymmetrical W-beam to thrie beam transition section...."
ODOT includes 46 ft - 10.5" as the length of an MSKT end terminal. Can this unit be directly attached to AGT or is a 12.5 ft panel required between the AGT and the end terminal?
|Date||December 5, 2018|
|Attachment||MSKT shop drawing.pdf|
|Response||Based on the FHWA eligibility letter and the MASH 3-31 test (pickup truck at 0 degrees) conducted on the MSKT, the stroke length of the the MSKT was found to be 50.5 ft. If only 46'-10.5" of W-beam were installed upstream of the W-to-thrie transition segment, the impact head of the MSKT would be sliding over the transition segment. This transition segment is made from 10-ga. steel and has an increasing vertical height and bending strength. It is unlikely that any w-beam end terminal head would perform as intended if it had to interact with this larger and stronger transition piece. Thus, MwRSF would recommend that you add an additional W-beam (MGS) between the terminal and the W-to-thrie segment of the transition.
The intent of the first guideline listed in that report and copied in your question was intended to ensure a separation between the terminal and the transition hardware. 12.5' was picked to match standard rail lengths with the understanding that it was likely a conservative length. If this guideline is followed precisely, you would need a minimum of 50.5 + 12.5 = 63 ft of w-beam upstream of the transition. However, a single 12.5-ft long rail would provide enough length to keep the impact head out of the transition hardware - 46.5 + 12.5 = 59 ft, which is greater than the stroke length of 50.5 ft. MwRSF would not have a problem with this installation as it would satisfy the intent. It would just be less conservative than the original guideline.
Note, if you are utilizing nested W-beam guardrail upstream of the W-to-thrie transition segment, the critical rail strength location is moved 12.5-ft upstream to the end of the nested section and your required length upstream of the W-to-thrie transition would need to increase by another 12.5 ft.
|Date||December 6, 2018|
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