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Short-Radius Guardrail

Question
State IA
Description Text

I've got a few questions for you on the short-radius guardrail system that TTI successfully tested at TL-2:

1.      1. It's my understanding that the maximum radius that can be used is 8 feet.  Is that correct?


2.      We may have intersection angles that are less than or greater than 90 degrees.  Can the 12.5-foot rail section be bent to angles other than 90 degrees?  If so, should the number of CRT posts going around the curve remain unchanged?  If bend angles other than 90 degrees are not allowed, how might you suggest we deal with such situations?

3.      I see that the original Yuma County design incorporated a flare on the primary road side, but the TTI-tested version did not.  Do you see any problems using a flare on the primary road side?

4.      I understand that the rail height as tested was 27 inches.  Do you see any issues with raising the rail to the FHWA-recommended minimum of 29 inches?  How about to 31 inches?  If so, do the holes in the CRT posts need to be shifted lower by 2 inches (or 4 inches)?

5.      Any reason why we couldn't install this system with mid-span splices?  How about with 12-inch blockouts? 

Most of our rural sideroad intersections have radii in the 25- to 30-foot range.  Do you have any other suggestions on how we might run guardrail around the corner in a manner that more closely matches the existing radius? 

 

Thanks for your help.

Keywords
  • Bullnose Median Barrier & Short Radius
Other Keywords none
Date June 26, 2012


Response
Response

Ron forwarded me your email to address your short-radius questions. However, before I can address them, I need to clear up which design we are referencing.

 

TTI developed and tested two short-radius designs. One was tested under the TL-3 criteria for NCHRP 230 and one was tested under the TL-3 criteria for NCHRP 350. Neither of these systems met the safety requirements or were implemented.

 

Recently, TTI sought TL-2 approval of the Yuma County short-radius design that was tested in 1988 at SWRI based on their engineering analysis. This system was tested under the PL-1 criteria of the AASHTO Bridge Specifications.

 

No current system has been successfully tested to TL-2. If you can identify which system you are referring to, I will take a shot at answering your questions.

 

Thanks

 

Date July 17, 2012


Response
Response

I was referring to the Yuma County design that " correct me if I'm wrong " has been accepted at NCHRP 350 TL-2.  

Date July 17, 2012


Response
Response

I have looked over your short-radius questions and have comments below in red.

I've got a few questions for you on the short-radius guardrail system that TTI successfully tested at TL-2:

1.      It's my understanding that the maximum radius that can be used is 8 feet.  Is that correct?

The Yuma County system was tested at SwRI with the 8' radius that TTI shows in their details. The performance of larger radii is not fully understood for this particular system as it only underwent limited testing. MwRSF has generally stated that smaller radii are more critical for short-radius designs. A smaller radius size will result in a stiffer curved section, while larger radii will tend to decrease the stiffness of the curved section. Based on the previous research, the use of smaller radii seems to demonstrate more promise for short radius designs. No one has successfully tested any short-radius system radii larger than 16' to either the NCHRP 230 or 350 criteria. As such, we cannot recommend increasing the size of the Yuma County system without further analysis.

FHWA Technical Advisory T5040.32 recommends the use of a short-radius guardrail that was developed by the State of Washington. This design was tested under the impact requirements set forth in NCHRP Report No. 230. The crash testing demonstrated that the system could contain a 1,800-lb small car and a 4,500-lb sedan. However, the testing program was not complete, and the results were marginal in some cases. Guidance for installing the short-radius guardrail is given for systems with radii ranging between 8.5 and 35 ft. The technical memorandum also notes that testing conducted on a 35-ft radius Washington State design did not perform adequately when impacted at 60 mph by a large vehicle (4740 lbs). Satisfactory results were obtained for the 35-ft radius system when a test was performed at a reduced speed of 50 mph with the large vehicle.

We currently have a project with Wisconsin DOT to evaluate the use of the Washington system with larger radii. This work is currently underway and should provide some guidance as to the use of larger radii with short-radius systems.

2.      We may have intersection angles that are less than or greater than 90 degrees.  Can the 12.5-foot rail section be bent to angles other than 90 degrees?  If so, should the number of CRT posts going around the curve remain unchanged?  If bend angles other than 90 degrees are not allowed, how might you suggest we deal with such situations?

It is very difficult for us to make recommendation on the Yuma County system regarding intersection angles other than 90 deg. Small variation in the bend angle should not affect the performance of the system greatly, but it is difficult to define what the magnitude of the acceptable angles would be. The angle of the sides of the system affects performance as the smaller the angle, the stiffer and more energy the system absorbs when vehicles impact on the nose due to the angle that the guardrail is bent during impact. Obviously, as the angles vary a great deal from 90 deg. we begin to approach either a general curved guardrail system or a bullnose system. Thus, it would be possible to employ a bullnose design with flared sides on the very small interior angles or to follow guidance for curved guardrail on very large angles. However, specific guidance on intermediate angles is hard to give without further study, especially on a system where we have only limited test data.

3.      I see that the original Yuma County design incorporated a flare on the primary road side, but the TTI-tested version did not.  Do you see any problems using a flare on the primary road side?

The use of the flare in the system should be acceptable. We actually employed a parabolic flare in the MASH short-radius system we partially developed. The use of the flare helped reduce the potential for the vehicle to be impaled by the guardrail rail if the vehicle impacted directly along one of the sides of the system.

4.      I understand that the rail height as tested was 27 inches.  Do you see any issues with raising the rail to the FHWA-recommended minimum of 29 inches?  How about to 31 inches?  If so, do the holes in the CRT posts need to be shifted lower by 2 inches (or 4 inches)?

We would not recommend changing the rail height of the system. Our experience with testing the small car vehicles with the bullnose and short-radius systems has shown that the small car would be very likely to underride the system if the guardrail height were increased. If you desire to attach the system to a run of 31" high MGS, you can employ a height transition. In the past, our recommendation has been to transition the 3.25" height difference over approximately 50 ft or two 25-ft long sections of W-beam guardrail.

5.      Any reason why we couldn't install this system with mid-span splices?  How about with 12-inch blockouts?

 

We see no issues with using midspan splices or 12" deep blockouts in the system.

 

Most of our rural sideroad intersections have radii in the 25- to 30-foot range.  Do you have any other suggestions on how we might run guardrail around the corner in a manner that more closely matches the existing radius? 

 

As noted in the discussion of larger radii above, there is only limited testing of larger radius systems and that was mostly done under the NCHRP 230 or PL-1 guidance. Thus, we are leery of increasing the radius of the Yuma County system. The best guidance at this time is the FHWA memo noted above. In addition, you may want to contact Roger Bligh at TTI and see if they investigated the use of larger radii with the Yuma County design. Finally, the work we are doing with WisDOT should shed some light on the subject as well. 

Date July 18, 2012
Attachment Project No. 3 SRG w Large Radii LB.pdf


Response
Response

Thanks, Bob.  In your answer to my #2 question, you mention following "guidance for curved guardrail."  May I ask to what guidance you are referring? 

Date July 18, 2012


Response
Response

I saw that coming as soon as I wrote that comment.

 

Currently, the guidance on curved guardrail systems is pretty limited. . One research study regarding vehicle accidents or curved roadways and testing of W-beam guardrail on curves was conducted by ENSCO and sponsored by FHWA in 1989 through 1991. This research study involved the testing and evaluation of strong-post, W-beam guardrail systems that were located on the outer edge of a horizontal curve with a 1,192-ft radius. For this study, the successful safety performance of the curved W-beam barrier system was observed on flat ground for an 1,800-lb small car and a 5,400-lb pickup truck impacting at 60 mph and 20 degrees using flat roadway conditions. However, three subsequent pickup crash tests were unsuccessful (i.e., each resulted in vehicle rollover) when the W-beam guardrail system was installed in combination with a super-elevated, curved roadway. These crash tests were performed using the impact safety standards found in NCHRP Report No. 230 and the AASHTO 1989 Guide Specifications for Bridge Railings. As such, no strong-post, W-beam guardrail systems have been successfully tested for use on super-elevated, curved roadways according to NCHRP Report No. 350 safety performance guidelines or the current Manual for Assessing Safety Hardware (MASH). Because the ENSCO research study is the only available testing of beam guardrail on curved roadways, designers are limited to guidance on the installation of W-beam guardrail on curves based on limited tests of curved guardrail on flat ground and the use of engineering judgment.

 

At this time, NCAC has an NCHRP project, NCHRP 22-29 Performance of Longitudinal Barriers on Curved, Superelevated Roadway Sections, to further investigate those installations. This together with the WisDOT study we are doing should hopefully further our understanding of guardrail on curves. 

Date July 18, 2012


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