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Concrete Barrier Design Guidance with End Sections

Question
State MN
Description Text

As we discussed in our meeting last Thursday, attached are
two barrier details for you review.  As a reminder, we would like to know
what your design methodology is (assumptions and process) and what
recommendations you might have for both the interior and end regions of the
barrier.


Each half of the split median barrier (Fig 5-397.131) is
treated as TL-4.


I also have a question:



Could I get a PDF copy of the Guidelines for Attachments
to Bridge Rails and Median Barriers (February 26,2003)
?


If you need any more information, please let me know.


Thanks again for the help,



 



Keywords
  • Permanent Concrete Barriers
Other Keywords none
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 5-394.117 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 e-mail.  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 Mc 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 e-mail 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 e-mail.  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 Mc 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 e-mail and pdf of the calculations you sent me for your reference.

Date April 29, 2013


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