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NHSX-52-5(31)--3H-96 Curved Guardrail

State IA
Description Text

As we discussed please see the
curved guardrail layout.  I have include the MicroStation files so you can
do more detailed measurements.   





As I eluded to
below and again on the phone, I think a radial guardrail design needs to be
used for both the Pulpit and Madison roadways. Looking at the attached Google
Earth image, one can see the (no longer) existing layout changes from steel to
wood posts around the curve. We need to effectively mimic that design this time


I contacted
WHKS to get their design file as the plan sheets leave a lot to be desired.
Attached is their reply. Those files and all workup files are available at W:\Highway\Design\MethodsSection\Methods-SubjectAreas\Barriers\_DesignModifications\Guardrail_RadiusGuardail_US52


‘Guardrail Details’ inside 7288.14.dgn shows that the proposed paved
shoulder at Pulpit has an inside radius of 46’ (attached image). Using that as
the design radius (as it is already poured is effectively becomes the radius),
the curved portion of the guardrail layout is shown in attached CurvedDimensions.pdf.


With an Lg of
75, you’ll need (75/6.25) + 3 = 13 CRT posts and (75/12.5) = 6 curved w-beam
pieces. You’ll also need to add a 12.5’ VT section between the end of the last
curved section and the beginning of the End Terminal. The 3 posts that are used
in that section are the 3 added in the sentence above.


Using model
‘Guardrail Detail Shading’, the guardrail splits should look something like

BTS= 28.125

VF = 75

VT1 = 250

Curved VT = 75

VT2 = 12.5

ET = 50

I’m not
entirely confident in the precision of the first VT as the paved shoulder and
guardrail don’t seem to run parallel to each other in the 7288.14.dgn, but
since the paved shoulder is already constructed, drill the holes at 6.25’
increments and begin placing CRT posts at the junction of the ~250’ VT and the
curved VT, continuing through the curve and final VT, ending at the end
terminal. The posts in the BTS and the VT1 may be changed to steel as requested
as I don’t believe that change impacts the radial component.


For the Madison
Road curve, since it is only a partial layout, I can’t really design it. The
same approach would apply though. Basically, the CRT posts begin at the
connection of the VT and the curved w-beam and continue through the curve.
Since the guardrail doesn’t terminate near the intersection, continue the CRT
posts for two posts beyond where the curved pieces flatten out. You’ll have to count
up the number of CRT posts in the field.


US52_CurvedGuardrailAtSideRoad.pdf is a highly modified detail of past
installations. Most of the information has been stripped out as the tables
didn’t cover this large of a radius. The CRT hole spacing is borrowed from the
current BA-211.


You mentioned
over the phone that because of the severe drop off, additional actions were
being added in an attempt to extend the 10:1 grading at least 3’ behind the
face of guardrail, with the potential of 4’ before it broke to 2:1 or steeper.
Normally we would introduce longer posts for that situation, but since these
are CRT posts, I don’t think that makes sense to do so. Brian and Dan may have
a comment on this.


I’m out of the
office tomorrow and on the road Monday, so I’ll be happy to check back in on
Tuesday to see what Brian, Dan, and yourself decided to do for these
installations, including any necessary changes to the curved and VT lengths.




Image 806 is where we plan to
begin the ET,  which is 802 plus 25’ as you note.  The side road this
is at is Pulpit.  This is an area that rock drilling needs to occur, and
Lovewell is marking out the post locations today so yes we need to know shortly
if the post spacing changes.




I would much prefer starting the end terminal from the
end of the rail shown on the attached 806 than the attached 802. It doesn't
appear the end terminal even terminates within the shoulder if starting from
the 802 measuring tape. I would concur that adding the additional 25' from the
802 to end up at the 806 and then start the end terminal would be the preferred


The trailing back around the radius of the side road
throws me a bit here. Looking at page D.3
of the plans
, it appears that they are running normal guardrail around the
corner of Pulpit Rock Road, if that is the location of these images, but I
don't see a detail in the plans for laying out guardrail on that tight of a
radius. We typically use a short radius guardrail detail that has different
posts and spacing at that kind of a radius.


I'm sure you're waiting on an answer, so I'll talk with
Brian this afternoon. In the meantime, can you provide the side road these
pictures were taken at if it isn't Pulpit/Madison, both of which have
potentially the same issue.




The guardrail layout is a long section that extends from
a bridge endpoint on the trailing end back around a radius of a side
road.  The section shown in the attachments is within the VT-2 section
with an additional 25' added.  If we didn't add the 25' the ET section
would end up flaring toward the side road.  Is this acceptable?



These pictures show the last two sections extending the
VT-2 by 25'.The ET would then be in the tangent paved shoulder area. 

  • Bullnose Median Barrier & Short Radius
  • Guardrail
Other Keywords none
Date August 7, 2015
Attachment NHSX-52-5(31)--3H-96.zip


A few years ago, MwRSF conducted a research study for the Wisconsin DOT. This effort explored the performance of W-beam short-radius guardrail systems under TL-2 impact conditions with larger radii. See the link to access a copy of the report. Also, note the Chapters & Sections that are recommended for reading.


See page 199 or PDF page 213!

See Chapter 12 – page 205 – PDF page 219!

See Chapter 13 – page 223 – PDF page 237!

See Chapter 14 – page 227 – PDF page 241!


From this simulation effort, it was determined that rail heights greater than 27 in. and up to 31 in. would improve barrier performance for pickup truck impacts. Although no small car simulations were performed, there is concern that a 31 in. rail height could accentuate small car underride. As such, it was believed that a 29 in. rail height may still provide improved performance for pickup truck impacts but reduce concerns for small car underride. In the absence of an actual crash testing program at TL-2, MwRSF personnel would lean toward the use of a 29 in. rail height versus a 27 in. rail height based on the best available information and results from this study. Of course, the only true evaluation of safety performance would be through full-scale crash testing.


Second, the study noted that blockouts on posts around the radius contributed to improved vehicle capture by better maintaining adequate rail height. Blockouts also showed an ability to reduce vehicle to post contact. Further, CRTs were simulated around the nose through the tangent sections. As such, it would be recommended to maintain the CRTs throughout the entire curve and into tangent for any larger radius system that is implemented.


It should also be noted that the simulation effort was performed with level terrain behind the barrier system. Your real-world scenario will likely feature a gradual slope behind the barrier system for some distance, followed by a steeper slope. Barrier performance can be greatly affected by the presence of various slopes behind the actual barrier. Thus, it is recommended to provide a gentle slope behind the barrier using as much lateral distance as feasibly possible.


Again, these thoughts are provided based on our best available information as well as the research findings from the recent simulation effort. If you have any questions regarding this information, please feel free to contact either myself or my included colleagues at your convenience. Thanks!


Date August 7, 2015

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