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Question | |
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State | |
Description Text | I have been
Based on
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Other Keywords | none |
Date | August 26, 2015 |
Attachment | TRP-30-208-10.png |
Attachment | TRP-30-208-10(2).png |
Response | |
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Response | 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 |
Response | |
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Response | 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 |
Response | |
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Response | 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.
http://mwrsf-qa.unl.edu/view.php?id=676
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.
http://mwrsf.unl.edu/researchhub/files/Report148/TRP-03-175-06.pdf
http://mwrsf.unl.edu/researchhub/files/Report61/TRP-03-69-98.pdf
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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