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Question  

State  MN 
Description Text  As we discussed in our meeting last Thursday, attached are
Each half of the split median barrier (Fig 5397.131) is I also have a question: Could I get a PDF copy of the Guidelines for Attachments If you need any more information, please let me know. Thanks again for the help, 
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Date  March 6, 2013 
Attachment  fig7131e.pdf 
Attachment  fig7117e.pdf 
Response  

Response 
I conducted a full Yield Line
analysis on one of the bridge rails you sent, and I tried to list / show all of
the assumptions that I made. I attempted to be as detailed as possible in
order to show every step. Take a look at the attached file. Is this
what you were looking for in regards to methodology of calculating barrier
strength? Let me know what questions you
have. 
Date  March 18, 2013 
Attachment  MN bridge rail Fig 5394.117 Analysis.pdf 
Response  

Response 
Sorry it has taken so long to get back to you. We
wanted to arrange a phone conference to discuss the barrier design, but because
of the schedule of the others involved with the barrier design question, we’ve
decided to try and resolve our questions using email. If a phone
conference become necessary, we can try and arrange one. Again, thank you for helping us with our barrier
design. We appreciate your time and expertise in this matter. After reviewing the design you provided, I had some
questions concerning some of your assumptions. First, the design assumes that the rebar in the barrier
yields both longitudinally and vertically, front and back. Because the
bars are so close to the compression face, shouldn’t a yield check be performed
to make sure the bars yield as assumed? Second, the hook on the dowel into the deck does not appear
to be fully developed, should this be accounted for when calculating M_{c}
for both the interior and exterior regions? Third, a f of 0.90
was used to modify the barrier resistance, where does this value come
from? As I read AASHTO f is equal
to 1.0 for extreme event cases. Last, We have always assumed that the longitudinal bars need
to be fully developed on both sides of the yield line. Some of the
longitudinal bars in the barrier are underdeveloped for the end region by this
assumption, should this be considered when calculating the capacity of the end
region? Attached is the original email and pdf of the calculations
you sent me for your reference. Thanks again for your help, 
Date  April 19, 2013 
Response  

Response 
I have answered your questions
in RED below. ______________________________________________________ Sorry it has taken so long to get back to you. We
wanted to arrange a phone conference to discuss the barrier design, but because
of the schedule of the others involved with the barrier design question, we’ve
decided to try and resolve our questions using email. If a phone
conference become necessary, we can try and arrange one. Again, thank you for helping us with our barrier
design. We appreciate your time and expertise in this matter. After reviewing the design you provided, I had some
questions concerning some of your assumptions. First, the design assumes that the rebar in the barrier
yields both longitudinally and vertically, front and back. Because the
bars are so close to the compression face, shouldn’t a yield check be performed
to make sure the bars yield as assumed? I use an Excel spreadsheet program
to calculate the bending strength of reinforced concrete cross sections.
It calculates the strain distribution throughout the entire cross section and
relates that to the estimated stress in the steel (assumes elastic, perfectly
plastic behavior). Thus, the program should have checked for yielding and
calculated all stresses according to strain at each depth. Second, the hook on the dowel into the deck does not appear
to be fully developed, should this be accounted for when calculating M_{c}
for both the interior and exterior regions? I assumed adequate anchorage to
develop the yield strength of each bar. Yield Line Theory requires that
the barrier deflects/bends/yields to absorb energy and balance out the energy
of the impact. If no yielding occurs, the analysis procedure would not be
valid. If a dowel will not develop full
yield strength, I would recommend altering the bar / hook details. Of
course, the development lengths found in ACI 318 are conservative in nature and
designed for static loading. Under dynamic loading, failure stresses are
typically increased. Thus, often times we can rely on field proven or
crash tested embedment/anchorage designs. Third, a f of 0.90
was used to modify the barrier resistance, where does this value come
from? As I read AASHTO f is equal
to 1.0 for extreme event cases. The 0.9 factor comes from ACI 318
for bending strength. We typically use it to give some safety factor to
designs, but you may elect not to. Last, We have always assumed that the longitudinal bars need
to be fully developed on both sides of the yield line. Some of the
longitudinal bars in the barrier are underdeveloped for the end region by this
assumption, should this be considered when calculating the capacity of the end
region? My answer here will mirror what was
said above… yield line requires the full yield strength of the
reinforcement. Thus, it’s easier to just extend longitudinal bars to
obtain the proper development length. When designing end section
reinforcement, we specify longitudinal bar lengths that span the critical
length, an additional foot or two for conservatism, and the required
development length (or splice length if being splice to interior section
reinforcement). Attached is the original email and pdf of the calculations
you sent me for your reference. 
Date  April 29, 2013 
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