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Approach Curb with a Type 6A Terminal

Question
State	IL
Description Text	This might be an easy question but wanted to get your input. For the AGT connected to a bridge rail/parapet are you required to have a curb or could one omit it and still have it performance as it was tested. See our standard in the link below. Was is the purpose of the curb besides for drainage? Has there been any testing without the curb that passed either NCHRP 350 or MASH? http://www.idot.illinois.gov/Assets/uploads/files/Doing-Business/Standards/Highway-Standards/PDF/220-631032-09_TrafBarTermType6A.pdf

Question

State

Description Text

This might be an easy question but wanted to get your input. For the AGT connected to a bridge rail/parapet are you required to have a curb or could one omit it and still have it performance as it was tested. See our standard in the link below. Was is the purpose of the curb besides for drainage? Has there been any testing without the curb that passed either NCHRP 350 or MASH?

http://www.idot.illinois.gov/Assets/uploads/files/Doing-Business/Standards/Highway-Standards/PDF/220-631032-09_TrafBarTermType6A.pdf

System Performance Evaluation

System Types
Approach Guardrail Transitions (AGTs)

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System Features

Date May 22, 2018
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Response
Response (active)	I have a few comments. I don’t believe that the curb running up to the bridge rail would be required. TTI tested a similar transition to tubular bridge rail to MASH TL-3. See attached. Note that it is necessary to flare the curb beneath the bridge rail to reduce snag. You could use a flare that provide similar shielding to the TTI test above or base the flaring on the flare we used with the standardized buttress. That buttress flare was designed to work with or without a curb. Note that the curb you have shown appears to be a 6” tall vertical curb. We have typically only tested transitions with 4” wedge-shaped curbs in the past. As such, we have typically recommended that 4” or shorter wedge curbs be used with transitions. While it may be acceptable, we cannot quantify the effect of the taller, vertical curb shown. You show the curb terminated prior to the asymmetric W to Thrie piece. In the past we have allowed this or have recommended extending the curb outside of the transition. t is recommended to transition the curb flush with the ground over a minimum length of 3 ft (0.9 m) rather than leave and exposed end. Thanks

Response

Response
(active)

I have a few comments.

I don’t believe that the curb running up to the bridge rail would be required. TTI tested a similar transition to tubular bridge rail to MASH TL-3. See attached. Note that it is necessary to flare the curb beneath the bridge rail to reduce snag. You could use a flare that provide similar shielding to the TTI test above or base the flaring on the flare we used with the standardized buttress. That buttress flare was designed to work with or without a curb.
Note that the curb you have shown appears to be a 6” tall vertical curb. We have typically only tested transitions with 4” wedge-shaped curbs in the past. As such, we have typically recommended that 4” or shorter wedge curbs be used with transitions. While it may be acceptable, we cannot quantify the effect of the taller, vertical curb shown.
You show the curb terminated prior to the asymmetric W to Thrie piece. In the past we have allowed this or have recommended extending the curb outside of the transition. t is recommended to transition the curb flush with the ground over a minimum length of 3 ft (0.9 m) rather than leave and exposed end.

Thanks

Date May 25, 2018
Previous Views (133) Favorites (0)

Attachment 9-1002-12-4.pdf