View Q&A

I have the following questions for your consideration and reply related to recent DOT inquiry on design of

Post and Tube Bridge Rail (see attached typical section). I have also included in attached Figure 1.jpg an excerpt from 2004

AASHTO LRFD Bridge Specifications Section 13, Table A13.2-1: 

Questions:

1. Transverse design forces (F_t) indicate 54kip for TL3. Is this the same for 2270P (MASH)?
2. In addition and in discussions with other test houses, furthered consideration to increase F_t = 78kip

 for TL4 (vs. 54kip shown above). Is this your same understanding?

 - If not, what might be specified for F_t in design of TL4 post and tube bridge rail?

Part 1 - For my quick response, the answer to (1) is no. Historically, I have found the peak lateral impact loads for rigid parapets to range between 60 and 75 kips for a TL-3 2000P impact event. Please note the range is due to barrier shape and height (i.e., climb up barrier and/or roll angle/extend over parapet).

For MASH TL-3 2270 impacts, we have seen the peak lateral impact force on a vertical face with some several inches of minor barrier displacement get up into the high 70s. When displacement is limited on the same vertical face, the peak MASH TL-3 load was in the low to mid 80 kips.

The reason that existing design procedures and a 54-kip load has resulted in acceptable barriers is that the procedures are often conservative in nature and do not consider all terms which add to the barrier capacity. At any rate, the 54-kip no. should be larger for 350 with improvements in design methods. When turning to MASH, the design load should be raised too.

Part 2 " I need much more time to discuss this issue as well as the numbers in the provided table. I do not agree with any of the load numbers other than TL-1 and TL-2 for 350 but not MASH.