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MASH 2016 Bridge Rail Loads

Question
State IL
Description Text I appreciated the opportunity to discuss this subject with you yesterday by phone. Illinois is still attempting to understand all of the issues involved with MASH 2016 so we can meet the deadlines and make some informative decisions moving forward with a standard barrier shape. Currently we use both a 34” and a 42” F Shape barrier and the reinforcement we use is designed per Chapter 13 Appendix of the AASHTO LRFD Bridge Design Specifications. We know that any barriers under MASH 2016 need to be a minimum of 36” tall plus any additional future wearing thickness. In Illinois this means we would need a minimum 39” tall barrier. We were considering just using our 42” F Shape barrier for possibly the MASH 2016 TL-4 load but it would likely require a crash test to assure that it could handle the 57% increase in impact severity. You also brought to my attention that while the TL-5 impact severity did not increase from NCHRP 350 to MASH 2016, it is anticipated that the 124 k and other design values for TL-5 will increase significantly because these design values were apparently calibrated way to low.

Ideally AASHTO will publish a revised load chart soon to replace the current chart shown below with MASH 2016 loads. The 2015 FALL presentation showed the 2nd chart below but there aren’t any values for the other Test Levels as there currently is in AASHTO. You noted that you have a Manitoba 49” TL-5 barrier test coming up next week and that you anticipated learning a lot towards better TL-5 design values from that test. If there are any new developments regarding the TL-5 design loads I would appreciate if you could send me a link to that information.

Illinois is leaning towards switching to a constant 11 degree slope barrier, but we don't know what height yet.For TL-5, it may likely be 45" tall, but we don't have design values for that yet.
Keywords
  • Bridge Rails
Other Keywords none
Date April 6, 2016
Attachment MASH 2016 Bridge Rail Loads-1.jpg
Attachment MASH 2016 Bridge Rail Loads-2.jpg


Response
Response

I have dug out some information from a few quarterly progress reports on NCHRP Project No. 22-20. During that effort as well a few others, TTI researchers utilized modeling to investigate barrier loading. I have provided details in the attached PDF regarding their suggested design forces for analyzing and designing bridge railings. As I recall, existing procedures largely use the application height at the top of the barrier when considering prior 32” TL-4 and 42” TL-5 barrier heights. Now, they have offered revised loads and application heights based on barrier heights. Note that one design load and application height was suggested for TL-4 to cover a range of barrier heights.

 

Next, it is not yet clear as to what the minimum barrier height would be required for all TL-4 rigid barriers. Barrier front face shape and top width may impact the minimum height. TTI had a success at 36 in., while failures were observed at 32”. We know from past experience that rectangular shapes (vertical faces) allowed a lower height parapet to redirect a 8000S vehicle at TL-4 of NCHRP 350 as compared to safety shapes (sloped front faces). We have conducted LS-DYNA modeling around a decade ago that suggested that a 34.5” tall vertical face barrier may actually be capable of containing a MASH TL-4 10000S SUT vehicle. Testing would be needed to confirm. Thus, the 36” minimum is based on a TTI test on a particular barrier shape and top width. Depending on shape, it may be possible to actually use a 37.5” barrier to account for a 3” overlay for later field use of 34.5” (if proven with testing). If overlays are considered, then the barrier and deck design should consider the increased barrier height and load application height from asphalt surfacing.

 

I suspect that we will learn more with the Manitoba TL-5 test next week. We also plan to investigate and evaluate existing guidance for deck design based on barrier capacity at base. We will let you know of those results as they are obtained from the crash testing and follow-on analyses.

 

Finally, several of the topics discussed above are actually addressed in a few proposals under consideration at the April Pooled Fund meeting. 

Date April 7, 2016
Attachment 3072_001.pdf


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