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|Date||August 26, 2015|
You are correct that in TRP-03-208-10 we utilized a 5:1 vertical taper to go from 32” to 42”. The 5:1 height transition slope that we tested with the PCB transition used a steel cap to create the vertical transition flare. There is some concern that using concrete to create the 5:1 flare may increase friction and or gouging of vehicle components in the flared region. Thus, the 5:1 is an aggressive approach with some concerns for its use. However it has been adopted by many states. Use of an 8:1 slope has been commonly used as a more conservative approach.
Based on the performance of the this 5:1 flared cap, it would seem reasonable to use slopes as high as 5:1 when transitioning from 32" tall barriers up to higher heights. We could not recommend the use of these higher flares for shorter barrier heights below 32" as the potential for the vehicle to climb the flared section may increase if the starting height of the flare is lower.
For transition from heights lower than 32”, the recommendation would be an 8:1 slope. We would recommend this based on the concerns noted above regarding the difference in the slope materials. In addition, we would not want to go to steeper slopes for barrier heights below 32" on the low side due to concerns for increased vehicle exposure to the slope and climb.
|Date||August 26, 2015|
I note in the package you just sent me that the proposed Standardized AGT Buttress uses all W6x8.5 posts (with last 6 posts immediately adjacent to the concrete rail being 78” long and spaced at 18-3/4”). We are currently using the simplified steel post MGS stiffness transition with three W6x15x84” posts immediately adjacent to concrete rail spaced at 37-1/2” with next four W6x9x72” at 14-3/4”, etc (per MWTSP similar to Missouri Transition to Single slope in TRP-03-210-10). Has the post configuration in the proposed Standardized AGT Buttress with all W6x8.5 posts been crash tested before? Were there concerns with the performance of the MWTSP design using the W6x15 posts, or is the proposed change to all W6x9 posts in the proposed AGT buttress being primarily driven by desire to only use one size of posts (albeit two different lengths)?
|Date||August 27, 2015|
I have a some answers to your questions.
First , the upstream stiffness transition that was developed in TRP-03-210-10 was designed to work with a wide range of AGT designs that were tested on only the downstream end in NCHRP 350. When we conducted the crash tests of that upstream transition, we selected a very stiff AGT design adjacent to the bridge rail. The system that was selected was a Missouri AGT to a steel bridge rail with a cap rail because it was a very stiff design that would accentuate any issues with the upstream end of the transition.
That said, that Missouri AGT with W6x15 posts at 37.5” spacing was never tested and evaluated with a concrete parapet. However, you are connecting to a concrete parapet. Thus, you may want to consider designing the downstream end of the transition to comply with a previously tested and approved transition for the downstream end. We provided guidance for adapting existing transitions for use with the upstream end that we developed in the project report (TRP-03-210-10). Thus you could adapt one of those designs, or we could assist you in adapting something else that you prefer to use.
Another potential option would be to use the W6x15 posts at half post spacing that you originally had and connect to a concrete parapet. I went through some old correspondence that we had with Iowa. They requested guidance on using the W6x15 posts at half post spacing and connecting to a vertical concrete parapet. We had replied then that the system was likely to work, but that there were concerns with slightly more thrie beam deflection relative to the more rigid bridge system. However, we believed that the increased deflection would pose minimal risk for wheel snag, excessive barrier deflections, or vehicle pocketing. This design would also require flaring of the end of the parapet to prevent snag. Use of this design would likely require further investigation into the relative deflection and snag potential of similar systems to justify its use. We recommended to Iowa to run this by FHWA as well. The upcoming testing on the standardized parapet may shed light on that as well.
The transition being used for the standardized parapet is a previously NCHRP 350 tested AGT design (with a curb) that was done for Iowa for connection to concrete bridge rail. It was selected for evaluation of the standardized parapet because it is among the least stiff of the approved transitions and it has been shown to be sensitive to use without or without a curb. Thus, we believed it will provide a critical test of the standardized parapet such that we can use it with all previously tested AGT’s. The Iowa transition was also tested to MASH with the curb and passed during NCHRP 22-14.
So the Iowa transition has been around for a while, and the use of the W6x9 posts was not originally intended for simplification of inventories. However, during the development of the upstream stiffness transition, we were asked to considered simplified post configurations to limit inventories, and the Iowa transition was part of that thought process.
|Date||August 28, 2015|
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