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Guardrail deflection for non-TL-3 situations

Question
State IA
Description Text We’ve had the following issue come up recently and I’m wondering if you can provide some guidance as to what to use for design deflection distances.

The attached Table 5-6 Summary of Maximum Deflections from 2011 Roadside Design Guide provides design deflections for TL-3 impacts. What I’m wondering is what should we be using for deflection distances on a TL-2 situation, commonly an urban 35mph roadway? This question comes up when protecting a railroad lighting/crossbuck pole just off the traveled way, see attached. We currently use the attached BA-204 as the downstream anchor and the attached BA-253 as the general layout. Since the BA-204 utilizes a cable, introducing half post spacing is likely not preferred. That would leave us with nesting the thrie-beam if we felt it was needed.

Are there any reports that might give an indication if we should stay conservative and use the 19.2” number, as I think a 25 impact angle is highly unlikely, or use something more towards the 13.1” number? I’m trying to generate a balance between being far enough away from the railroad pole but not get too close to the roadway to introduce unnecessary impacts.

As always, I appreciate your assistance and insight.
Keywords
  • Guardrail
Other Keywords none
Date September 14, 2015
Attachment eba204.pdf
Attachment eba253.pdf
Attachment Railroad Signal example.jpg
Attachment Table 5-6 2011 Roaside Design Guide.pdf


Response
Response

I have  looked at the details that you sent and have concerns over the function of the shielding of the pole outside of the allowable deflection.

 

As it is laid out in your detail, the signal pole is directly adjacent to the end anchorage for the thrie beam. Vehicle impacts in that area would likely release the end anchor through fracture of the BCT and allow the vehicle to impact the pole with little appreciable drop in velocity. Thus, shielding of the pole in this manner may not be effective.

 

That leaves a variety of options.

1.       Moving the pole upstream of the end of the thrie beam and away from the anchorage may help somewhat, but the longitudinal offset may not be allowable based on where the signal needs to be placed.

2.       Offsetting the pole more laterally may be the simplest answer, but I don’t know what space restrictions you have for lateral offset. A pole with an L-shaped top that allows the pole offset to increase while maintaining signal position might be an option too, but I don’t know if such an option exists.

3.       A more effective post shielding could be done using a short concrete parapet with a TL-2 AGT and end terminal. This may increase the system length, but it would alleviate concerns for interaction with the signal pole.

4.       Install a breakaway base on the pole to make the signal similar to a breakaway luminaire pole.

 

Take a look at these options and let me know what you think. I may have missed something.

 

Date September 14, 2015


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