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.
Some parts of this site work best with JavaScript enabled.