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I am working on a bridge rehab project that is going to remodel the bridge rail over the Loup River in the Bessey National forest.  Due to the geometrics of the approach roadway we can't attach standard guardrail without encroaching on the driving surface (see third attachment).   Because we can't attach standard guardrail, the speed limit is 35 mph, and this is the off end of the bridge rail, we are proposing to build a tapered bridge rail transition (see first & second attachment).  

Are you aware of any testing or guidance on this subject?  Would this be acceptable?   Thanks for your time.

I need to gather some background info regarding your question.

1. Is the road curving "south" (per drawings you sent and traveling from right to left or "west to east"), does the road continue straight, or both? This would help determine likelihood and possible angle of impacts

2. If road is curving, how tight in the radius?

3. When you state "off end of bridge rail" I'm assuming you mean downstream end for "eastbound" traffic, correct? Can you clarify number of lanes in each direction please?

4. What is the hazard you are protecting? Just the bridge rail end, or is there an non-traversable slope adjacent to the bridge?

5. There appears to be guardrail on the "north and south" sides of the roadway (even though it's drawn with the rail facing away from the road). Why not extend this to the bridge rail? As drawn with the tapered end, there is a blunt end of the guardrail near the downstream (low) end of the concrete which would require a terminal.

6. When stating that the "geometrics of the road" prevent guardrail installation, are you saying that the radius is too tight for a guardrail installation? Or are you saying that the concrete of the roadway was poured wide enough that it covers the area where the guardrail posts would be placed?

7. Is the speed limit 35 only do to the lack of a complete transition, so only near the bridge, or is the road 35 mph for a long distance.

Scott,

                Please see my response to your questions below in red.

1.       Is the road curving "south" (per drawings you sent and traveling from right to left or "west to east"), does the road continue straight, or both? This would help determine likelihood and possible angle of impacts

As you can see from the image above, our spur road, S86B, curves to the west. There is an intersection that continues to the south so the answer would be both.  This is a State/National recreational area with the ranger station being just east of the curve.  

2.       If road is curving, how tight in the radius? The radius for the spur road curve is 205'.

3.       When you state "off end of bridge rail" I'm assuming you mean downstream end for "eastbound" traffic, correct? Can you clarify number of lanes in each direction please? One lane in each direction with the tapered rail proposed for the southwest corner of the bridge which would be the off-end for the southbound traffic.

4.       What is the hazard you are protecting?  Just the bridge rail end, or is there an non-traversable slope adjacent to the bridge?  As you can see from the attached image, looking south from the bridge,  there are trees on a steep bank.  The end of the tapered rail will be 50' beyond the end of the existing concrete bridge rail.  

5.       There appears to be guardrail on the "north and south" sides of the roadway (even though it's drawn with the rail facing away from the road).  Why not extend this to the bridge rail?  As drawn with the tapered end, there is a blunt end of the guardrail near the downstream (low) end of the concrete which would require a terminal. As you can see from the attached image the existing guardrail is constructed out of timber and will be removed.

6.       When stating that the "geometrics of the road" prevent guardrail installation, are you saying that the radius is too tight for a guardrail installation? Or are you saying that the concrete of the roadway was poured wide enough that it covers the area where the guardrail posts would be placed?  The radius is too tight. A standard bridge approach section and end treatment encroach on the driving lane.

7.       Is the speed limit 35 only do to the lack of a complete transition, so only near the bridge, or is the road 35 mph for a long distance.  The entire 7 mile spur is posted at 35 mph.

Are changes also being made to the bridge rail?  I'm asking because the rail shown in your attached photo looks short and the concrete structure you have proposed it 34" and 14" wide.  Seems like a miss-match.

Also, the sketch you had provided showing how the transition would extend in to the roadway appears to be using a TL-3 transition design.  With a posted speed limit of only 35 mph, have you given any thought to using a much shorter (and w-beam only) TL-2 transition? I know of one such TL-2 transition that is only 11'-6" long between bridge rail and standard guardrail. 

See my comments below.

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Are changes also being made to the bridge rail?  I'm asking because the rail shown in your attached photo looks short and the concrete structure you have proposed it 34" and 14" wide.  Seems like a miss-match.

This is a bridge rehab project that will remodel the bridge rail to current standards (See attached preliminary plan, tapered rail not shown).

Also, the sketch you had provided showing how the transition would extend in to the roadway appears to be using a TL-3 transition design.  With a posted speed limit of only 35 mph, have you given any thought to using a much shorter (and w-beam only) TL-2 transition? I know of one such TL-2 transition that is only 11'-6" long between bridge rail and standard guardrail.  The guardrail I showed was a TL-3 design.  The TL-2 transition would be an option if it's non-proprietary.

Seeing that a TL-2 transition is an option for you, I would recommend that you use one of the TL-2 transitions that have been tested down in Texas.   One is a shorter guardrail that utilizes only w-beam and was tested to NCHRP report 350 TL-2, while the other utilizes a short section of thrie beam and an asymmetrical transition piece and was tested to MASH TL-2.

I tried to attach the reports to this e-mail, but the files are too large.  However, you can get them from the Texas Transportation Institute's website (tti.tamu.edu).  Use the publications search and lookup these reports:

1.       report no. 9-1002-8, "Development of a MASH TL-2 Guardrail-to-Bridge Rail Transition Compatible with 31-inch Guardrail", Bligh, R., et. al., December 2011

2.       report no.  4564-1, "Evaluation of Guardrail to Concrete Bridge rail Transitions", Bligh, R., et. al., October 2003

If you have any problems finding the test reports, let me know.