Bob – I called and left a voicemail earlier today in reference to the following and attached: KDOT has had several discussions on this topic with MwRSF previously and it recently occurred to me there may have been a miscommunication. Previously KDOT has asked whether there is an MGS installation for this type of short radius guardrail installation, which there is not. The MGS terminology is a little misleading. I think what we really wanted to know was; is there a 31” tall version of this type of short radius guardrail installation? I think NDOR has details for this, but I wasn’t able to find them on their website. I did some searching on the Pooled Fund Site, but didn’t find an answer to my specific questions. I know you and Cody Stolle did some investigating on this topic through a Wisconsin funded study in 2014 and found the short radius installation performed acceptably, or maybe even better in some regards, at a height of 31” compared with the 28” height. As a result I wanted to revisit this topic again. I’ve attached several draft standard drawings. RD619 is the current short radius guardrail installation KDOT uses for a mounting height of 28”. RD619A is a modified version of that drawing keeping the hardware through the curved section the same, but transitioning to MGS on either side of the curved portion. To avoid confusion I’m referring to RD619A as a modified short radius guardrail installation mounted at 31”. This is not an MGS installation, but transitions to MGS hardware on either side of the curved section.My goal is to develop something that will allow KDOT to avoid height transitions within an installation and minimize the different types of hardware we are using in our guardrail installations while still maintaining performance comparable to other TL-2 configurations for similar systems. Currently our practice is to use the old 4G1S AGT with the old 4G1S posts and hardware all mounted at 28” (shown on RD619) when a short radius installation cannot be avoided. As a result we have locations where 3 quadrants of the bridge is all MGS hardware and one quadrant that is 4G1S, which KDOT refers to as the Conventional Guardrail System (CGS). The other approach we’ve taken where we have space is to install the MGS and then transition to a lower height of 28” over 50 feet up to the curved section of the short radius installation. At that point the hardware switches back to the short radius hardware for the remainder of the installation unless there is room on the end terminal side (along the minor roadway) to transition the height and hardware back to MGS. For additional information I attached the updated post details and the MGS AGT details, which I reference on RD619A.Just to cover what I found on the Pooled Fund Site during my search I included a short summary below:1. Response to IA dated June 26, 2012 not recommending the short radius mounting height be raised to 31”. A height transition over 50’-0” from 31” to 28” was suggested. The issue we’ve run into at KDOT is these systems are used only when an intersection is in close proximity to a bridge or a needed guardrail installation. In the case of the bridges there often is not enough room (less than 50’-0”) to transition the height so you end up with one quadrant using the old hardware.2. I found a similar response to NDOR dated January 2, 2012 regarding the 31” mounting height.3. I also read through TRP-03-296-14 Extending TL-2 Short-Radius Guardrail to Larger Radii. That report seemed to suggest, if I understood it correctly, mounting the short radius at 31” was acceptable for the configurations shown in the report. The details I’ve attached are yet another variation on the details shown in the report I referenced in number 3 above. I’d like to discuss this over the phone at your earliest convenience before any thorough review is completed. We coordinated the height transition option we currently use when there is space available with MwRSF previously. The new attached proposed drawings would be identical to that type of installation with the height transition omitted since it would not be needed with everything mounted at 31”.
We currently are limited in what recommendations we can provide regarding short-radius type barrier systems. As you are aware, no short-radius system has met the crash testing criteria for MASH or NCHRP 350 at this time. TTI has recently done research on a MASH TL-3 thrie beam short-radius system, but to my knowledge that has not been approved by FHWA and we have some concerns regarding the test matrix and impact points used to evaluate that system. Thus, we are left with trying to make the best of the situation at hand.
The only short-radius system that has met FHWA eligibility is the 27” high TL-2 version of the Yuma county short-radius system that was analyzed by TTI. This is the system that has been implemented most recently by several states. Your system seems to vary from this design somewhat as it included additional cable anchorages near the nose section. I am not sure of the function of these additional anchorages, but you may want to reconsider them as they may not be consistent with any currently approved design.
Subsequent to that research at TTI, we conducted a simulation analysis for WisDOT regarding the performance of short-radius guardrail for larger radii under NCHRP Report 350. This study started with the Yuma county design and extended it to larger radii (over 25’) This study found that the performance of 27” high short-radius guardrail was potentially limited in terms of capturing the 2000P vehicle and that 31” high larger radii short-radius systems had improved potential for pickup truck and high CG vehicle capture. The report also noted that the simulation analysis did not investigate small car interaction with the large radii short-radius systems at either 27” or 31” mounting heights. Passenger cars may underride the rail if a 31-in. mounting height is used despite a beneficial interaction with pickup truck vehicles. Previous thrie beam short-radius systems with 31-in. mounting heights culminated in small car underride and roof or windshield crush. No W-beam short-radius system has been tested and approved with a top mounting height higher than 27 in. Nonetheless, tangent guardrails as tall as 36 in. have redirected small cars at MASH TL-3 impact conditions. Based on these concerns, full-scale testing was highly recommended if a 31-in. (787-mm) tall system is to be used. Thus, while 31” rail height was shown to work acceptably in the study for a limited range of speeds and impact conditions, concerns were noted for small car capture that prevented us from fully endorsing a shift to 31”. We did note that a 29” system may be a compromise between the two alternatives until further research is available.
As you noted in your email, we have made similar responses to Iowa and Nebraska regarding the height issue and have recommended limiting the height to the approved 27” for now even though we have some data that suggests that it may pose problems with high CG vehicle capture. This is due to concerns that the small vehicle capture may suffer. Thus we are currently limited to that guidance until further investigation or crash testing of the increased height short-radius systems are undertaken.
From your email, it appears that this is an issue because you are converting to the MGS system and the 27” height of the Yuma county system likely conflicts with some of the approach transition hardware for the MGS. Unfortunately, for the time being, we can only recommend the TTI/Yuma county system as it was granted eligibility at this time because we do not have sufficient information raise the height based on the concerns noted above. Thus, one may be forced to implement the current Yuma system and keep use older hardware.
Let me know if answers your question. I understand that this may not help much. The short-radius issue has been a big problem for several years and will continue to be until we can resolve it. We currently have and R&D effort with NDOR to evaluate a different treatment for intersecting roadways, but that work is still in the developmental phases.
I will revisit our existing design and compare it to the TTI design you provided in the attachment. Is there anyone at MwRSF who might be able to do a quick review of the details once I have them put together just to get another set of eyes on them in case I missed anything?
Attached are the revised details. I removed all the details for the cable anchor assemblies, soil plates, etc. Since the TTI report noted any tested AGT/End Terminals meeting NCHRP 350 TL-2 or higher could be used outside the curved section I left the 4G1S AGT/End Terminals in the drawing. I attached the original drawing for comparison (RD619_Original). I also attached the other standard drawings I reference on the sheet for information. I did have a couple of questions:
1. I didn’t see this in the TTI report, but I know there was some discussion of this in the work you and Cody did for Wisconsin. Can the STYP posts be used in lieu of the wood CRP posts?
2. I left the mounting height detail showing 28” rather than 27” since that is how tall KDOT’s typical 4G1S w-beam is mounted. Is that acceptable?
3. Are the radii listed shown on KDOT’s version of the drawing ok? I know from the TTI report it had a radius of 8’-0” (shown as 7.96’ on KDOT’s version to give a length of 12’-6” for the rail, which is evenly divisible by the post spacing). From the work you and Cody did it looks like radii of 23’-10.5”, 47’-9”, and 71’-7.5” is also ok. Are radii increments between these radii acceptable? Can those radii be adjusted slightly to give lengths of w-beam rail that are evenly divisible by the typical post spacing (i.e. 23’-10.5” would be ok shown as 25’-0”)?
Thanks for your help on this.
I had another question that crossed my mind this morning after sending this info to you yesterday. Can the MGS guardrail and AGT be used instead of the 4G1S system outside the short radius installation if it is mounted at 27” or 28”?
I have commented to the questions below in red.
Comments are listed here with respect to the details you sent.
1. You details show a 2:1 slope starting 3’ behind the short radius system. While I understand the reality of these slopes, no short-radius system has been successfully tested with these types of slopes and the approval of this system was based on level terrain testing. Based on previous testing and simulation done at MwRSF, we believe that the performance of the system will be degraded significantly with the presence of the steep slope behind the system in terms of vehicle capture and stability.
2. I saw no other significant deviations from the TL-2 approved Yuma County system.
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I don’t see any reason why the MGS cannot be used mounted at the lower height for this application. The midspan splices should improve the performance and the deeper blockouts should aid in vehicle capture as well. Obviously the benefits of the reduced post embedment would not be included as the height would not increase. Cody’s work for WisDOT indicated that the blockouts may improve vehicle capture for the higher CG vehicles. The effect of the blockouts on small car capture is unknown, but TTI noted in the Yuma TL-2 system report that blockouts could be used even in the curved section. However they did not make a recommendation towards the larger blockout in the MGS.
With respect to the AGT, I think you would need to stay with the NCHRP TL-3 approved transitions noted in the TTI report. The MASH tested MGS transition essentially uses NCHRP 350 approved transitions on the downstream end adjacent to the bridge, so that part of the transition would not be different. The upstream end of the transition was designed to convert between the stiffness between the standard MGS and the AGT. It also used a asymmetric W-thrie transition piece. The upstream end of that transition has not been evaluated at the lower height, and if you recall, we experienced a rail rupture of that transition near the asymmetric W-thrie transition piece when we tested it with a curb and a 1100C vehicle, which forced us to nest the w-beam ahead of the asymmetric W-thrie transition piece when the AGT was used with a curb. Thus, a lower system may be sensitive to near the asymmetric W-thrie transition piece. Additionally, the asymmetric W-thrie transition piece would not allow for the correct thrie beam height for the AGT connection at the bridge.
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I am assuming that you are referring to replacement of the timber CRT posts used in the approved system with Steel Yielding Posts (SYP) developed by TTI. We did not comment on this in the WisDOT study, but we do not recommend replacement of the timber CRT posts with any of the steel breakaway posts at this time. The SYP post is a yielding post that bends at a lower load that the standard W6x8.5 section rather than breaking away at the base like a CRT. This behavior may create a ramp for the vehicle to climb in the nose section which could increase the propensity for override of the rail and vehicle instability. The UBSP post that was developed through the Midwest Pooled Fund is likely a better option as it was used successfully in the bullnose and tends to break way at the base. Component testing of that post compared well with CRT’s. However, we have not recommended the use of that section in any system without full-scale testing as it may be sensitive to applications outside of the bullnose. In the case of the Yuma county short-radius system, it is unlikely that it will ever be subjected to a full-scale crash test to evaluate that potential application.
I will leave the mounting height decision up to you and KDOT, because the guidance in this area is mixed. TTI received approval on the Yuma county system based on the 27” height. Thus, from the standpoint of FHWA eligibility, the 27” height has been recommended by both TTI and FHWA. As I noted in the previous email, the simulation effort we did for WisDOT showed found mixed results for the varying rail heights. Cody’s simulations of a 5,000 lb pickup truck on the 27” high Yuma county system with an 8’ radius that were conducted to validate the modeling effort found that the pickup was captured. However, a simulation of the a 4,409 lb pickup truck under the same impact conditions overrode the rail. Additionally, as we simulated larger radii at the 27” height, the 2000P vehicle vaulted over the guardrail in 100, 100, and 80 percent of impact conditions simulated for 24, 48, and 72 ft radii, respectively. Blockouts were added to the CRT posts, and the vaulting override rates were reduced to 80, 36, and 50 percent of simulated impact conditions for 24, 48, and 72 ft radii, respectively. Thus, while the 27” height was listed in the TTI report, our study found that it potentially may have capture issues with the higher CG vehicles. We also simulated 29” and 30” rail heights and found much better capture of the 2000P vehicle. However, increasing the rail height leads to concerns for small car underride which were not investigated in the WisDOT study. Thus, we noted that a 29” rail height might be a compromise, but further study was needed to ensure that small car underride was not an issue.
For you, the decision will be what level of variation from the TTI approved system you can tolerate. The work Cody did seems to suggest that increased rail heights are better for higher CG vehicles, but the concerns for small cars exist. That said, the TTI study was a paper study that did not test the Yuma system under the TL-2 impact conditions. So neither of the current guidance is founded in a solid crash test. I would think that the 28” height you propose is a minimal variation from the TTI system and may improve the high CG vehicle performance based on what we currently know.
In the TTI report on the Yuma county short-radius, they do not note changing of the radius of the system as one of the acceptable system modifications. This is likely because alteration of the radius may affect capture of the vehicle and energy dissipation. As noted above, Cody’s study indicated that larger radii may be an issue as well (however, some of that may have been tied to the height of the rail). Previous recommendations by FHWA have allowed short radius with radii up to 35’. However, that has not been formally verified through a crash test. Thus, it is difficult to recommend larger radii for the Yuma County system. TTI and our research don’t seem to suggest that it should be done, but the previous FHWA memo allowed it, so I am sure many states still have the larger radii in their standard. The best option may be to stick with the 8’ radius and extend the tangent sides of the system. If you chose to allow the larger radii, the intermediate values should be acceptable.
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