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Curbs with Transitions and Guardrail

Question
State IA
Description Text

Somewhat of a follow up question to our conversation regarding curb heights (bottom of Q/A 1079):

We have situations where an existing 6 inch standard curb is adjacent to a lane receiving a 3 inch overlay, ultimately making it a 3 inch curb. We have typically treated curb heights less than 4 inches as a non-issue regardless of their offset to face of rail, but your statement that headwalls (and I’m expanding to curbs here) greater than 2 inches affect vehicle stability brings that into question. What aspect of height is causing the issue; the height itself or the vertical face? To express it in another way, which of the following scenarios should be considered an issue?
1. Two inch or less curb in front of guardrail without a height transition or a transition of less than three feet
2. Two inch or less curb in front of guardrail with a height transition of at least three feet
3. Between two and four inch curb front of guardrail without a height transition or a transition of less than three feet
4. Between two and four inch curb front of guardrail with a height transition of at least three feet
5. Two inch or less obstacle behind face of guardrail but within working width, with a vertical face and without a height transition or a transition of less than three feet
6. Two inch or less obstacle behind face of guardrail but within working width, with a vertical face and with a height transition at least three feet
7. Between two and four inch obstacle behind face of guardrail but within working width, with a vertical face and without a height transition or a transition of less than three feet
8. Between two and four inch obstacle behind face of guardrail but within working width, with a vertical face and with a height transition of at least three feet
9. Four inch obstacle or less behind face of guardrail but within working width, with a height transition of at least three feet and with a sloped face (essentially slope the headwall face and ends to mimic a four inch sloped curb – doesn’t exist but just a thought of how we could make them non-hazardous)
10. Other combinations we should consider as issues…

I’m wondering if your response in Q/A 1079 necessitates a change in how we deal with curbs, such that our guidance (subject to change based on above response) might read:
1. Curbs less than two inches in height
a. Can be ignored and do not need a transition
2. Curbs between two inches and a typical four inch sloped curb
a. Require a transition of at least three feet and that transition cannot occur within:
i. Fifty feet upstream of the end terminal,
ii. Any point within the end terminal, or
iii. Within the nested w-beam and asymmetrical transition piece of a barrier transition section
3. Standard six inch curbs
a. Require a transition of at least three feet down to a four inch sloped curb and that transition cannot occur within:
i. Fifty feet upstream of the end terminal,
ii. Any point within the end terminal, or
iii. Within the nested w-beam and asymmetrical transition piece of a barrier transition section
b. Cannot exist within:
i. Fifty feet upstream of the end terminal,
ii. Any point within the end terminal (Iowa flares all end terminals), or
iii. Within the nested w-beam and asymmetrical transition piece of a barrier transition section
iv. Any location where the offset from gutter line to face of rail is more than six inches
4. Headwalls less than two inches in height
a. Can be ignored and do not need a transition
5. Headwalls greater than two inches
a. Cannot exist within guardrail working width

Thanks for working through my interpretation of your intent, and as always, thank you for your assistance.

Keywords
  • Approach Guardrail Transitions
  • Guardrail
Other Keywords Curbs
Date August 4, 2016


Response
Response

Let me try to answer things in a general sense and then we can work down to specifics.

 

First, the response in Q/A 1079 was specific to the long span system and not intended to be applied to all guardrail in general. The long span system has significantly larger deflections than typical guardrail and the vehicle tends to traverse the headwall and extend over the drop-off. Thus, the height of the head wall become more critical for this application. Additionally, we don’t have data for curbs that first impact guardrail and then impact a curb offset that distance behind the rail. Typically curb studies have focused on placement of the curb at or slightly offset behind the face of the rail or with the curb offset at larger offsets in front of the rail. As such , we limited the headwall height for the long span systems to provide a conservative approach.

 

With respect to the broader aspects of curb and guardrail installations, we believe that the Roadside Design Guide still likely provides the most appropriate guidance based on our current knowledge. Essentially, the RDG states that for high speed facilities guardrail should not be offset from curbs unless crash testing has shown that it is acceptable. If guardrail offset from curbs is needed, it recommends a 1.5” laydown curb that would have minimal effect on the barrier performance. If does note that W-beam can be used with 6” tall curbs if the rail is installed flush with the curb up to 8m km/h and gives guidance for other sloped curbs for at higher speeds. It also notes exception to these guidelines for crash tested systems like the MGS which was tested with a 6” curb and a 6” offset behind the curb. As noted above, the 2” headwall height was specific to the long span and would not supersede the RDG guidance with respect to guardrail systems in general.

 

You note transitioning or tapering of the curbs. Specifically when to do it with respect to AGTs and end terminals.

 

Currently there is no set guidance with respect to curbs and end terminals other than it is not generally recommended due to lack of knowledge and testing of the combination. We currently have a limited study funded by WisDOT to investigate this issue, but it is not yet complete. A previous study by CALTRANS with respect to curbs and inertial sand barrels was conducted in the 60’s and it does provide some guidance w/r/t placement of curbs in front of the barrels, but it was done using sedans and my not be as relevant as it originally was.

 

In terms, of transitions, our recent testing of the MGS stiffness transition with curb (http://mwrsf.unl.edu/researchhub/files/Report295/TRP-03-291-14.pdf) found that a 4” tall sloped curb could be used in the region of the AGT. In order to ensure the safety performance of the MGS stiffness, the 4-in. tall curb should be placed through the entire length of the stiffness transition. Thus, the curb should be extended a minimum of 37.5 ft from the bridge parapet before either being terminated or transitioning to a 6-in. high AASHTO Type B curb. Additionally, it was recommended to utilize a minimum length of 3 ft for any curb shape transitions or terminations (e.g. transitioning from 4-in. curb to no curb).

 

Hopefully this gets us started down the path of answering your questions. Take a look at what is above and see if it addresses the situations you have below. Then if we need to discuss some specific items, we can go over those together.

 

Thanks 

Date November 2, 2016


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